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Study Unit

ManufacturingProcesses, Part 3By

Thomas Gregory

In this unit we’ll focus on the actual manufacturing tech-niques and processes used to make parts and assembliesfrom raw materials to finished products. The choice of materials, the processes used, the performance goals, andthe aesthetic goals all are significant factors in the productdesign process, which starts with the development of a performance specification, or “spec.” The spec can be firm,complete with dimensions, weights, performance limits, andcosts; or it can be a simple goal of providing a product tomeet a need identified by marketing surveys, with minimumperformance goals and some desirable features. The productdesign process is a continuous analysis. It consists of evalu-ating possible combinations of material selection, processselection, design for processing, part count analysis, designfor ease of assembly, assembly costs, and part costs. Thegoal is to produce the product efficiently for the highest pos-sible value at the lowest possible cost. Very often, advancesin materials will make changes in processes and assemblymethods desirable, to lower the finished product’s cost orraise its performance level.

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When you complete this study unit, you’ll beable to

• Identify and describe the general classifications of manufacturing systems that provide us with product

• Identify important processes used to manufacture parts from different materials, and select appropriatetechniques to produce finished parts

• Understand and describe the fundamental cutting andforming processes used to make products or parts

• Understand and describe the basic technical aspects ofnew manufacturing processes for high-tech applications

• List the advantages and disadvantages of manufacturingmethods used to process various materials and understand why each of these processes may be used in different applications

• Identify and describe the most important methods of joining components or subassemblies to make completed parts

• Understand and select appropriate finishing methods formanufactured parts

• Understand and describe the basic manufacturing systemsused to assemble parts into finished products

DEFINITIONS AND CLASSIFICATIONS 1

Primary and Secondary Processes 1Discrete and Continuous Processes 2Types of Processes 4

MANUFACTURING PROCESSES 8

Casting and Molding 10Plastic Processes 21

FORMING PROCESSES 29

Forging 31Rolling 35Spinning 38Extrusion 39Drawing 41Powder Metallurgy Processes 41

MATERIAL REMOVAL PROCESSES 45

Introduction to Cutting Processes 46Conventional Machining Processes 54Unconventional Machining Processes 67Process Comparisons 75

JOINING PROCESSES 78

Welding 79Brazing and Soldering 88Adhesive Bonding 92Mechanical Fastening 93Additional Processes 96Assembly Processes 102

SELF-CHECK ANSWERS 111

EXAMINATION 113

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DEFINITIONS AND CLASSIFICATIONS

Primary and Secondary Processes

Primary manufacturing processes are those that producematerials used to make other parts or products. For example,a steel mill is a place where the raw materials of iron ore,coke, limestone, and alloying ingredients are gathered andput through a primary process that results in steel ingots.These in turn are reheated and processed into shapes suchas billets, rods, bars, tubing, and plate. These manufacturedproducts serve as the raw material for other manufacturingprocesses that turn them into auto parts, golf clubs, ormachine tools. Primary manufacturing industries generallyuse naturally occurring raw materials as input to theirprocesses, and include such diverse industries as farming,mining, and wood processing.

Another example of primary manufacturing is the productionof plastics from petroleum. These materials contain organic(carbon-based) molecules that are polymerized to form longchains of monomers linked together. As you know, thermo-plastic materials can be melted and molded into variousshapes. The raw material for these processes often is shippedfrom the primary manufacturer in the form of containers ofsmall plastic pellets.

Manufacturing Processes,Part 3

Manufacturing Processes, Part 32

An important primary manufacturing business is the produc-tion and fabrication of building materials such as framinglumber and sheathing materials including oriented-strandboard and plywood. Producing these wood products involvesharvesting suitable trees, debarking and sawing them intoshapes, drying the boards in ovens until they have a specificmoisture content, and sawing and planing them into the finished shape, such as the common 2 � 4. These in turn are used in other manufacturing processes such as the construction of houses, garages, and items such as tables or furniture.

Secondary manufacturing processes are those that take the output of a primary process and make it into finishedparts or products. Common examples of secondary manufac-turers would be modular home manufacturers (who buildresidential houses), appliance manufacturers (stoves anddishwashers), toy manufacturers (children’s plastic figurines),or auto manufacturers (cars and trucks). All these industriesuse raw materials that were prepared for their use by a primary manufacturer, which processed naturally occurringraw materials into usable forms.

Discrete and Continuous Processes

Manufacturing processes can be further classified as discreteor continuous. Discrete manufacturing processes are thosethat make individual parts or assemblies. They may be pro-duced by the thousands every hour, 24 hours a day, but the method always produces individual parts. Continuousmanufacturing, on the other hand, produces a continuousflow of product. Common examples include manufacturingwire or cable, and chemical processes such as petroleumrefining.

Aluminum wire for electrical installations is produced in acontinuous process that starts with large individual billets ofaluminum. These billets are heated and rolled into increasinglysmaller shapes until they can be drawn through dies and madeinto wire. The wire in turn is drawn through a series of diesuntil the desired diameter is achieved. When the materialavailable from one billet has been completely processed,

Manufacturing Processes, Part 3 3

another section of wire is welded onto the end of the precedingone, making a continuous length of wire as long as desired.The wire “flows” through the factory as a never-ending piece.As it moves through the transport machines, it’s heat-treatedand processed into its final form. Many applications call formetal cable, which is produced from smaller strands of wirethat are twisted together in winding machines. Often plasticinsulation material is applied before the cable is wound ontoa large reel and prepared for shipment.

Wire rope is made in a similar fashion from many strands of smaller steel wires that are twisted together into largercables; these larger cables are twisted together to make evenlarger ones. Wire rope is a key feature of suspension bridgesand elevators.

Many types of chemical manufacturing are also continuousprocesses. For example, crude oil is refined into gasoline andother products in a process called fractional distillation. Asthe crude is heated, various components, called fractions,vaporize at successively higher temperatures based on theirmolecular weight. Gasoline has a low molecular weight, so itvaporizes at a temperature just below the boiling point ofwater, while the other components are still liquid. Furtherincreases in temperature allow the distillation and recovery of other products such as diesel fuel and kerosene, waxes,lubricants, greases, and even asphalt.

Modern refining processes produce about 21 gallons of gaso-line from every barrel of oil (42 gallons), while also producingabout 3 gallons of jet fuel, 9 gallons of oil and distillates, alittle less than 4 gallons of lubricants, and about 3 gallons ofheavier residues (Figure 1). Catalytic processes allow the pro-duction of other types of chemicals, generally classified aspetrochemicals, from which we derive primary materials suchas alcohols, detergents, synthetic rubber, glycerin, fertilizers,solvents, and the raw materials for the manufacture of drugs,plastics and polymers, paints, polyesters, explosives, dyes,and insulating materials. The petrochemical industry con-sumes about 5 percent of the total supply of oil and gas inthe United States.


Types of Processes

Manufacturing businesses supply almost all the durable and nondurable goods we use every day. Durable goods likeappliances, automobiles, and airplanes are meant to last along time. Nondurable goods are those consumed quickly,such as paper, food, clothing, and calculators. Subsets ofthese very broad classifications of manufactured productsinclude several major industries that have large impacts onour economy. Changes in the conditions, resources, methods,or markets in these industries will have major effects on ourlives and jobs. These are

• Iron and steel manufacturing—the production of steelmaterial such as rod, plate, billets, bar, and beams usedin many other manufacturing industries. Iron and steelmanufacturing require large energy inputs due to theuse of blast furnaces, and ready access to iron ore and acarbon source such as charcoal.

20 C Petroleum Gas

Gasoline

Kerosene

Diesel

Industrial Fuel Oil

Lubricating Oil, Paraffin Wax, and Asphalt

Furnace

150 C

200 C

300 C

370 C

400 CCrude Oil In

FIGURE 1—Productionof gasoline is a continuous chemicalmanufacturing processdone in a distillationtower that also pro-duces many other useful petroleum by-products.


• Textile manufacturing—the production of raw fibers of wool,rayon, linen, cotton or other polymers, and turning thesematerials into fabrics, and eventually clothing. Textilemanufacturing can be highly automated, with high-speedweaving machines making thousands of square yards offabric and clothing daily.

• Lumber and paper manufacturing—the harvesting of treesand producing the raw materials such as boards, sheetmaterials, and paneling, and the pulp used for makingpaper. Harvesting trees is one of the most dangerousjobs in the United States. The mills that cut and grindup the wood require large, expensive equipment. Papermaking uses wood chips chemically processed into apulp that’s flattened, rolled, pressed, dried, and cut into paper products.

• Automobile manufacturing—a worldwide, multinationalbusiness that employs hundreds of thousands of workersto manufacture complete cars and trucks as well as spareparts. If you consider all of the peripheral industries suchas car washes, auto body repair shops, gas stations,auto accessories, and service centers, you can see whythe auto industry is a major component of our economy.

• Aerospace—manufacturing airplanes, spacecraft, and components, as well as the research activities thatdrive innovations in air transportation. The aerospaceindustry has developed relatively recently, with majorchanges occurring with the development of civilian airtravel after World War II and the regular use of spacetravel and rockets for commercial purposes such assatellite communications.

• Electronics manufacturing—the design, manufacture, and distribution of consumer, industrial, and militarycommunications and computing devices. Since 1994, the Internet has dramatically affected our ability to communicate with people all over the globe, and satel-lites are now routinely used for worldwide cell phonecommunications. Future developments will blend com-puting and communications devices, and consumermanufacturing will use more computers, software, and Internet methodology for daily tasks.


• Petrochemical manufacturing—the locating, accessing,and developing of crude oil into raw materials for manu-facturing processes based on organic carbon compounds.The petroleum industry produces oil, gasoline, and theraw materials for plastics and some pharmaceuticalmanufacturing businesses. Petroleum is used to producematerials for adhesives, textile fibers, coatings, drugs,pesticides, fertilizers, and other fuel products such asmethane and propane.

• Agricultural manufacturing—the process of growing, harvesting, transporting, processing, packaging, and distributing food products. Most people don’t think offood or agriculture as a manufacturing business, butagricultural and food production businesses are one ofthe largest manufacturing sectors in the United States.Many factories are highly automated, especially in theirprocessing and packaging operations.

These manufacturing processes are all extremely importantto our national economy and our daily lives: it’s hard toimagine one of these manufacturing sectors undergoing a sig-nificant change without it affecting all the others. In fact, youcan probably see many ways in which all of these types ofmanufacturing are interrelated. Future developments arelikely to be in the areas of energy consumption, conservationof resources, and elimination of hazardous by-products fromthe environment. There’ll also be a continued transfer of low-skilled jobs to less-developed countries, while our economyrelies on the innovation, creation, and high-tech skills of aneducated workforce. This is one reason why you should planon continuing your education for a long time in the future.Manufacturing processes will certainly change in the future,as will the products demanded by consumers here and allover the world.


Self-Check 1

At the end of each section of Manufacturing Processes, Part 3, you’ll be asked to pause and

check your understanding of what you’ve just read by completing a “Self-Check” exercise.

Answering these questions will help you review what you’ve studied so far. Please complete

Self-Check 1 now.

Complete the following statements.

1. Manufacturing processes can be classified into two types, depending on the source of the rawmaterials: _______ and _______.

2. Production of plywood from harvested logs is an example of _______ manufacturing.

3. Manufacturing of gasoline can also be classified as a _______ manufacturing process, whileproduction of bicycles can be classified as _______ manufacturing.

4. Automobiles are an example of _______ goods, while cell phones are a kind of _______ product.

5. Raw materials for plastics are derived from _______ products.

Check your answers with those on page 111.


MANUFACTURING PROCESSES

The techniques used to form raw materials into useful parts orassemblies fall under the general classification of manufac-turing processes. Some of these processes, such as drilling,sawing, shaping, or gluing and welding, can be performed bymachine tools available to nonmanufacturing people such ashobbyists or homeowners. However, the range of availablemanufacturing processes is much more extensive, and choos-ing the proper process depends on many factors. One of themost critical aspects of product design is, in fact, determiningthe best possible combination of materials and methods thatwill produce the product or part to the desired specifications.Some of the factors that an engineer or technician must consider in the selection of the best process include

• The number of parts to produce

• Equipment and tooling costs

• Costs of labor and supervision

• Costs of energy and waste disposal

• Maintenance requirements

• Material cost and availability

• Material and process compatibility

• Quality requirements such as surface finish and dimensional tolerances

These decisions are often developed and evolved during thedesign cycle and are firmly established by the time the full-blown manufacturing cycle begins. However, the success ofthe product may in the end depend on how well the materialsand processes were selected originally.

Manufacturing processes can be grouped into four generalareas, depending on how the material is made into a part orproduct: casting (die casting, centrifugal casting, injectionmolding, sand casting); material removal (mechanical machining, jet machining, ultrasonic machining, chemicalmachining); joining (welding, brazing, adhesives); and materialforming (forging, rolling, extruding, slip casting, vacuum forming).


Figure 2 shows a diagram illustrating the classifications ofthese methods as well as examples of each. In the next severalsections, we’ll discuss each of these major categories as well assome aspects of each that you should know as you continueyour studies in manufacturing.

Continuous Casting

Gravity Die CastingPressure Die Casting

Centrifugal CastingInjectionReaction InjectionRotationalCompression

Permanent Mold

Sand MoldShell Mold

Permanent Pattern

Expendable Mold Investment CastingCeramic CastingEvaporative Pattern Casting

Water Jet MachiningAbrasive Jet Machining

Jet Machining

Chemical MachiningElectrochemical MachiningElectrical-Discharge Machining

Mechanical MachiningSingle-Point Cutting (Turning/Shaping)Multiple-Point Cutting (Drilling/Milling)Grinding/Honing/Lapping

Electron BeamLaserPlasma

Material Removal

Powder ProcessesSlip Casting

Pressing and Sintering

Sheet-Metal FormingVacuum FormingBlow Molding

ForgingRollingExtrusion

Forming

Drawing

Joining Brazing/SolderingWelding

AdhesivesFasteners

Bulk

Sheet

Electrical/Chemical

High-Energy Machining

Casting

FIGURE 2—Classifications ofManufacturingProcesses


Casting and Molding

Casting is the process of creating a part by pushing or pour-ing liquid metal or plastic into a mold, a cavity that has thesame shape as the final part. The process is generally similarfor both metal casting and plastic molding, with two maindifferences. Liquid metal is much less viscous than moltenplastic, and flows much more easily into cavities. And tem-peratures for processing plastics are much lower, meaning thatgreater control of the temperature of the mold is necessary tomake sure the plastic flows into all parts of the mold before it hardens.

Molds for casting metal objects can be made of several different types of material, including sand and ceramics. The liquid metal solidifies as it cools below its melting point.When it reaches a manageable temperature, the part isremoved from the mold for finishing operations such asmachining or polishing. Casting processes can also utilizeother raw materials, such as ceramics, glasses, and someplastics, but most of the techniques and processes we’ll discuss apply to metals.

The design and construction of molds is fairly complex, sincethey’re exposed to hot metals, high forces, and rapid, extremetemperature changes. The mold design must allow the moldto fill completely without any voids, which means that gasesmust be allowed to vent properly as the molten metal ispoured or forced in. The dimensions of the mold must allowfor the contraction of metal upon cooling, typically 1.25 per-cent for steel and 2 percent for aluminum. The mold mustalso prevent too-rapid cooling that could restrict furthermetal flow or cause cracking as the larger sections of thematerial cool. Finally, you must be able to remove the moldafter the solidification is complete. Molds and mold designwere once done by experienced craftsmen, but modern manufacturing design relies more heavily on computer-drivenadvanced modeling techniques and finite-element analysis todetermine optimum mold designs to match the materialsused and the part production requirements.


Molds can be either permanent or nonpermanent. Permanentmolds can be reused many times, whereas nonpermanentmolds can be used only once. The application will dictatewhich type of mold is used, but generally, permanent moldsare used for large quantities of medium-sized or small parts.

Some of the different types of casting that are used in manufacturing include

• Sand casting

• Shell casting

• Die casting, by gravity or pressure

• Centrifugal casting

• Investment casting

In this section we’ll discuss important considerations of someof these processes as they’re used in manufacturing, includ-ing typical applications, advantages and disadvantages, anddesign aspects.

Sand Casting

In the sand-casting process, moist sand is packed around apattern, or representation of the part, made of material such asmetal, wood, or hard plastic. Often a top and bottom portionof the mold, called a cope and drag respectively, will containhalf of the desired part shape. The cope (top) will have risersand runners (sprues) to allow the molten metal to flow in andgases to escape as the mold is filled. The cope and drag aremated together and hot metal is poured in, then allowed tocool. The cope and drag are then split along a parting line,and the solid part is removed. Figure 3 shows a simple exam-ple of a sand-casting mold. Most metals, especially aluminumand ferrous materials, can be sand cast. Lead, tin, and zinc,and refractory alloys, and beryllium, titanium, and zirconiumare difficult to sand cast.

Sand casting is used for large steel or cast-iron parts, such asengine blocks and cylinder heads, manifolds, pump housings,machine tool bases, valve bodies, and machine shafts.


Sand casting is desirable for many applications because it’seffective for both high-volume production (50 parts per hour)and single-design prototypes. The patterns are generally easyto make, and the complexity of the part is limited only by theability to make the pattern. Tooling and equipment costs arelow and patterns are easy to reuse. Disadvantages include thefact that material is lost in the runners and risers (up to 50percent), though a lot of it can often be recovered for reuse.Also, labor costs can be quite high since sand casting is laborintensive. Finishing costs can also be high, with secondarymachining or grinding operations necessary to make mount-ing surfaces, remove parting lines, or improve the exteriorfinish. For critical applications such as valves and pressurevessels, voids, cracks, and other defects inherent in castingprocesses must be found with nondestructive testing techniquessuch as radiography or ultrasonic testing, then ground out orfilled with weld material.

Sand-cast parts can be quite complex and can range in sizefrom ounces to several hundred tons! However, design of thegating systems to allow proper flow of molten metal is critical.The mold must have special designed-in features that allowquality parts to be produced: sharp corners must be avoided

Liquid Metal Enters Here

Parting Line

Drag

Cope

Sand

Mold Cavity

Riser

Vent

Sprue/Runner

Gate

FIGURE 3—Sand casting isone of the oldest metal-forming techniques, andwas probably first usedfor jewelry.


to prevent cracking; placement of parting lines must be considered; small holes (less than 1/4 inch) are difficult toincorporate; the pattern must have a draft angle of 1º to 5º toallow the part to be easily removed from the mold; and thinsections (less than 1/4 inch for steel, 1/8 inch for nonferrousmaterials) should be avoided. Draft angle is incorporated intothe side of the mold, reducing friction between the cast ormolded part and the mold, to allow for easier removal.

Parts made from sand-casting processes will have rough surfaces and must be inspected carefully for cracks.Secondary operations are often necessary to add requiredpart features such as holes, mounting surfaces, bearing surfaces, or locating dimensions. As mentioned before, cast-ings are inherently prone to inclusions (contaminants andimpurities trapped in the solidified metal) and voids, whichoften must be filled to maintain required strength levels.

Shell Casting

The sand in contact with the hot metal is the only part of themold that determines the part shape, and the bulk of the sandin the mold is only there for support. One way to speed theproduction process is to use shell casting for smaller parts(less than 50 pounds). In this process, a pattern is used toproduce a thin sand shell that’s used instead of a bulky copeand drag. Figure 4 shows a simplified shell casting process:

1. A metal pattern (usually half of the part shape) is heatedto about 200ºC.

2. The heated pattern is placed in a dump box.

3. The box is inverted to allow special sand coated with thermosetting plastic to attach and form around the pattern.

4. The dump box is turned over to allow the unmelted coated sand to fall back to the bottom. The pattern isremoved from the dump box with the shell still attached.

5. The pattern and attached shell are placed into an ovenfor further curing at about 400ºC.

6. The shell is removed from the pattern (shown still inplace) and mated (sometimes wired together) with othermatching shells.


7–8. The shell pattern is installed in a flask. It’s then used tocast metal much like the sand-cast process, although it’ssometimes supported by a sand or steel shot backingfor strength.

When the hot metal comes in contact with the sand shell, thethermoset binder vaporizes from the shell sand. Shell castingcan also be used with most metals, with the same exceptionsas sand casting.

Because of the need for precision, shell casting is used forsmaller castings such as small cylinder heads, gears, hous-ings for machines or tools, and support brackets.

It wastes less material than sand casting, even with productionrates of up to 200 parts an hour. Shell casting lends itself toautomated systems, which cuts the labor required by high-volume manufacturing. The resin-coated sand costs slightlymore, but only about 5 percent of the amount of sand is used,compared with conventional sand casting. Disadvantages ofshell casting include higher equipment costs and more difficultyin changing part designs during production.

Metal Shot

Flask Shell

Casting

Ejector Pin

Sand with Thermoset

Heated Pattern

Dump Box

(1)

(2)

Pattern in Position for Coating

(3)

Shell Mold Removed

(4)

Shell Cured in Oven(5)

Release Shell(6)

Ready for Pouring

(7)

Finished Casting

(8)

Two-Part Shell Mold

FIGURE 4—Shell casting can save time and effort over bulky sand-casting processes.


Shell casting allows for more complex shapes and greater varia-tion of section thicknesses than conventional sand casting,with smaller radii and thinner sections (minimum 1/16 inch).A part’s maximum section can be as large as 1/4 inch. As withsand casting, a draft angle must be added to parts: 0.25º – 1ºdepending on the thickness of the section. Shell casting can beused for parts up to about 50 pounds, but is best for partsup to 10 pounds. Cored holes must be greater than about 1/8

inch. Placing the parting line is important, and it shouldn’tcross critical dimension.

Quality concerns are similar for shell casting and sand casting. Surface finish is better, with fewer occurrences of voids and inclusions.

Die Casting

Gravity die casting, as you can guess from its name, usesgravity to make molten metal flow into a die, which gives the part its shape. Dies are usually made of steel, graphite,or refractory materials such as ceramics, molybdenum, ortungsten. They’re machined to the desired shape, and mayhave multiple sections or parts that allow complex shapes to be produced. Die casting typically is used for nonferrous materials such as aluminum, zinc, copper, magnesium, andnickel. Gravity die casting is used to produce precision partsfor a wide variety of products such as gears, engine connect-ing rods, wheels, pistons gear housings, and appliance parts(Figure 5). Die-cast parts can be quite complex, limited by theability to make the die halves into a desired configuration.Parts may weigh up to 150 pounds, but are usually limited to 5 pounds or less. Maximum section thicknesses are on theorder of 1/4 inch while the minimum can be as low as 1/16 inch.

Producing dies can take many weeks, but die casting is very adaptable to automated manufacturing techniques.Production rates can be quite high, up to about 50 parts per hour, depending on their size, and costs for tooling andequipment are moderate. Because of the high-quality finishespossible with die casting, finishing costs are usually low.


In pressure die casting, molten metal is forced into a die undervery high pressure. As illustrated in Figure 6, it’s pressurizedby using a cylinder containing molten metal. The cylinder isoperated by either a hydraulic or pneumatic actuator outsidethe container. When the metal has solidified in the mold, thedie is opened and the part is ejected. Molds are usually hard-ened and tempered alloy steel. Pressure die casting can beautomated easily because of the equipment necessary to per-form the basic motions. A variation of pressure die casting isthe addition of a vacuum within the die cavity that helps fillthe mold and eliminate voids or porosities. Pressure die cast-ing is used primarily to cast low-melting-point materials suchas aluminum, magnesium, zinc, tin, lead, and copper alloys,although copper alloys reduce die life because of their highermelting point.

Mold Half

Mold Half

Core Pin

Core

Mold Halves Clamped

Molten Metal Added Here

FIGURE 5—Gravity diecasting is used exten-sively in the automotiveindustry to produce alu-minum engine parts.


Fixed Die

Hydraulic Actuator

Cylinder

Molten Metal

Core Pin

Clamping

Gate

Piston

Force

Shot Sleeve

Casting

Bolster

Fixed Die

Clamping

Platen

Ejectors

Cold-Chamber Casting(B)

Hot-Chamber Casting (A)

FIGURE 6—Pressure DieCasting


Pressure die casting is used to produce thin-shelled housingsfor product such as small motor or pump housings, pumpcomponents such as impellers, appliance components, andtoy bodies. It uses material efficiently and can be used forvery high production rates of up to 200 pieces per hour, andcan produce up to 100,000 pieces for aluminum parts foreach die set. In addition, full automation of the process ispossible, which significantly reduces direct labor input.Finish of the parts is excellent, and they can often be usedwithout secondary finishing operations in some applications,other than removing any flash, gates, and sprues. However,die design is complex and it can take several months to pro-duce a set for prototypes or manufacture. Equipment andtooling costs are also high, and pressure-die-cast part designconsiderations are similar to gravity-die-cast considerations.

Pressure die casting produces high-quality castings with lowporosity, and is good for applications that have high strengthrequirements. Larger parts can run into problems with poros-ity because of gas being trapped in the mold as liquid metalis injected. Surface finish of parts is particularly good, oftenrequiring little finishing.

Centrifugal Casting

Centrifugal casting is a process where molten metal is pouredinto a mold rotating at high speeds (300–3000 revolutions perminute). As the metal flows into the mold, it’s thrown by cen-trifugal force onto the mold walls where it solidifies (Figure 7).Because of the rotation of the mold, centrifugal casting issuitable only for round parts such as pipe or hollow cylinders,

Pouring Basin

300 to 3000 rpm

Liquid Metal Flows to Outer Diameters

Due to Centrifugal Force

FIGURE 7—Centrifugalcasting is efficient butcan be used only forparts with rotationalsymmetry.


or other parts with cylindrical symmetry. The molds are usu-ally arranged horizontally, but can also be vertical for shortparts. Most metals can be centrifugally cast, as can materialssuch as glass, ceramics, composites, and thermoplastics.

Tooling, equipment, labor, and finishing costs for centrifugalcasting are relatively low and finishing costs are reasonable.Production rates can be up to about 50 pieces per hourdepending on the size of the parts, but this process can alsobe used for prototyping when needed. Material usage is veryhigh because there are no sprues or risers.

Though centrifugally cast parts are by nature axially symmetric,contoured surfaces are possible. It’s possible to cast parts withtwo metals, combining the different properties of two metalsinto one part. Draft angles must be on the order of 1º andcored holes must be greater than about 1 inch. Maximumsection thicknesses can be up to 5 inches, with the minimumattainable thicknesses as low as 0.1 inch, depending onmaterials. Parts as large as several tons have been producedby centrifugal casting.

Rotational forces often push impurities to the outside of thecasting, and these must be machined off as a finishing step.Also, properties of the casting may be different at differentdistances from the center of rotation. Castings are usuallyfree of shrinkage, and the mechanical properties of densecastings can approach those of forgings.

Investment Casting

Investment casing, which is sometimes called the lost-waxprocess, is a casting process that uses a wax pattern in theshape of the desired part as a form to make the actual mold.A ceramic slurry or other refractory material is applied to theoutside of the wax pattern, and it’s then fired in an oven tocure. As it heats up, the wax melts and flows out of the mold.The mold is then filled with liquid metal, which solidifies intoone or more finished parts (Figure 8). All metals can be castusing this process, including refractory (high-melting-point)metals and precious metals. Sometimes the pattern can be


made from thermoplastic resin or water-soluble cores insteadof wax. More complex patterns can be made by making waxpatterns of small portions of the desired part and then attach-ing them to a larger pattern. Larger, complex parts or “trees”containing hundreds of smaller parts can then be cast in asingle operation. After the metal is poured into the refractoryinvestment mold and allowed to harden, the investment matteris broken away from the part.

Wax Pattern(A)

Wax PatternSprayed with

Refractory Slurry(B)

Molten Wax Forms Cavity

Furnace

(C)

Set Slurry Case

Molten Metal Fills Cavity

(D)

Finished Part(E)

FIGURE 8—Investment castings can be used to make precision parts such as medical implants, but arealso popular for jewelry.


Investment casting is particularly suitable for complex or preci-sion parts such as jewelry, figurines, automotive components,gear blanks, turbine blades, and internal mechanical parts of assemblies ranging from small arms to automobiles ormachine tools.

Investment casting works best with metals that have highmelting temperatures or those that are difficult to machine,such as the refractory metals used for turbine blades.Production rates can be very high—up to over a thousandparts per hour depending on the size, although it’s an overallslow process because of the many steps involved. Wax andplastic patterns can be produced by injection molding tospeed the process.

Very complex patterns can be made with investment casting,but the cast object(s) must be easily removed from the mold.Complex shapes can be made from simple shapes that arebuilt up. Sharp corners should be avoided, and uniform sec-tions are best. Maximum section thicknesses can be up toabout 3 inches and minimum sections can be as thin as 1/16

inch. Maximum part size can be up to 200 pounds, but theprocess is best for parts under 10 pounds. Investment castings have good surface characteristics and detail, andhigh-strength casting can be produced even though moderateporosity is sometimes unavoidable. The overall quality of thecasting depends upon the quality of the wax pattern.

Plastic Processes

Injection Molding

Injection molding of plastics is done by filling a mold withmelted plastic materials under pressure, as illustrated inFigure 9. In an injection molding machine, plastic pellets arepoured into a hopper, which feeds them into a heated tubewith a large metal screw in the center. A motor turns thescrew, which moves the pellets farther down the heated tubeuntil they eventually melt. The screw also pressurizes theplastic and forces it into an assembly of die blocks. The dieblocks cool the plastic. When the plastic is completely solid,


the blocks are separated, the part is ejected, and the cycle is repeated. Injection molding usually uses thermoplastic materials, although thermosetting plastics, composites, andelastomers can be used with some modifications to the process.

Production rates for injection molding can be extremely high,up to thousands of parts per hour. Material utilization ishigh, and thermoplastic scrap generated in risers and spruescan be recycled for reuse. The dies can be complex and takea long time to manufacture, but can be used for large pro-duction runs. Full automation can also be achieved with theuse of computer-controlled machines and robotics. Toolingcosts are high, as are the machines to produce and handlethe parts, but direct labor costs are low.

Injection-molded plastic parts have an unlimited number ofapplications, from toys to aerospace components, and common-ly include containers and lids, electrical parts, tool handles,and plumbing fittings.

Reaction injection molding is similar to conventional injectionmolding except that it’s used for thermosets with two partsthat must be mixed. The two parts of the thermoset resin areinjected into a mixing chamber, and then injected into themold, where polymerization and hardening takes place. Thepart is then ejected and the cycle repeated. Reinforcementmaterials such as fiberglass or carbon fibers can be added tothe mixing chamber to add strength and stiffness to the finalparts. Figure 10 shows a diagram of a typical reaction injectionmolding machine. As you can see, it’s more complicated than

Screw-Feed Injection Molding

Two-Part Die

Injection-Molded Component

Injector

Screw-Feeder Motor

Hopper

Material from Hopper

FIGURE 9—Plastic injec-tion molding is a versatileand popular method ofproducing plastic parts.Many types of thermo-plastics can be formed inthis way.


simple injection molding. Because of the hardening timerequired, production rates for reaction injection molding are only on the order of one to ten parts per hour.

Reaction injection molding is used to produce such things asautomobile bumpers, light machine housings, outdoor furniture,and plastic panels. Very complex shapes and intricate detailsare possible, and the surface finish of the parts is a functionof the mold quality. Molds are usually made from aluminum,although hardened steel molds are used for large production runs.

Compression Molding

Compression molding, shown in Figure 11, is another processused primarily for thermosets and, to a lesser degree, compositesand elastomers. In the compression molding process, a precisequantity of unpolymerized material is placed into a heated moldthat’s closed under pressure while the plastic melts. As it melts,the material is forced into all of the mold areas, as in diecasting with metals.

Polyol Tank Isocyanate Tank

Heat Exchanger

Heat Exchanger

Metering Pumps

Circulation Pumps

Mold

Mixing Head

FIGURE 10—Reactioninjection moldingprocesses produce plasticparts strengthened byfiller materials such ascarbon or fiberglassfibers.


Production rates can be quite high, up to 150 pieces perhour, but each mold must be loaded individually, and thegreater the part thickness, the longer the curing time.Because of the use of measured material in individual molds,waste is very low. Equipment costs are moderate and laborcosts are low, with some automation of the process possible.

Some of the typical applications of compression moldinginclude dishes, automotive parts, appliances, and smallmachine housings, flat panels, handles for tools and utensils,and electrical components.

Blow Molding

Blow molding is a high-volume production method used toproduce hollow plastic parts with thin walls such as bottlesor plastic ducting. In blow molding, an injection molder feedsa head that extrudes a hollow tube of thermoplastic materialcalled a parison into a die, as shown in Figure 12. The pari-son is clamped by the die halves and air is blown into the topof the parison, inflating the soft walls into the die. The die iscooled and opened, and the part is ejected.

Ejectors

Heating Heating

Die Plunger

Heating Heating

Component

Die

Preheated Plastic

FIGURE 11—Compressionmolding is an efficientmeans for high-speedproduction of plasticcomponents.


Blow molding is capable of fully automated production ratesof several thousand parts per hour, thus offsetting somewhatmore costly dies and equipment costs. Blow-molding processescan use very high atmospheric pressures to form the plastic,allowing great surface detail, including thread formations andletterings. Very little finishing is necessary except for minortrimming. Blow molding is limited to parts that are hollowwith large, rounded corners, and thin walls.

Blow Pin

Plunger

Molten Plastic Parison

Bottle Die-Half

Die Open–Parison Extruded

Extended Plastic Tube (Still Soft)

Die Closed–Bottle Formed

Die Opened–Bottle Complete

Parison

FIGURE 12—Blow molding is one of the main ways plastic water bottles, baby bottles, and small medi-cine containers are made.


Vacuum Forming

In vacuum forming, sheets of thermoplastic material are heated by ovens, or infrared or electrical heaters, until theysoften. A vacuum under the sheet pulls the softened materialuniformly into the mold, where it’s cooled by the mold andsometimes cold air. Where the sheets are thick, compressed airis sometimes used to ensure the sheet fills the mold completely.The mold is then separated from the part and the excessmaterial trimmed off (Figure 13).

Supporting the Sheet(A)

Vacuum Forming(B)

ClampPlastic Sheet

Formed Part

VacuumAir

Removed

Heat Applied

Part-Forming Progression(C)

FIGURE 13—Vacuum forming is a popular way to make food containers, especially the disposable kinds.


Cold working doesn’t necessarily mean the metal is cold whenit’s deformed. In fact, many cold-working operations occur atmoderately high temperatures (for steel, up to about 300ºC).Cold working causes significant work hardening, that is, anincrease in the strength and hardness due to the plasticdeformation of the grains in the metal. This work hardeninglimits the amount of deformation that can occur in one step,so forming may take place in several steps with an annealingprocedure in between steps to restore ductility to the metal.Cold working is desirable because it leaves the part in a high-strength condition, with good surface finish characteristics.Greater dimensional control of the finished part is also possiblewith cold-forming processes.

Table 1

APPROXIMATE MELTING POINTS ANDRECRYSTALLIZATION TEMPERATURES FOR SOME METALS

Metal Melting Point ºCRecrystallization

Temperature ºC

Tin 230 –50

Lead 325 –50

Zinc 420 75

Aluminum 660 150

Copper 1080 250

Cast Iron 1175 540

Steel 1525 600

Titanium 1700 600

Tantalum 3000 1000


Vacuum forming is used to produce such diverse items as plasticcontainers and lids, cups and storage containers, automotiveparts, decorative panels, bathtubs, shower stalls—even smallboat hulls. Production rates can be extremely high, up to sev-eral hundred parts per hour (3600 cups an hour can be made),and complete automation of the process is possible, makingdirect labor inputs very low. Equipment costs can be quitehigh if the process is fully automated.

The principal disadvantages of vacuum forming are that the rawmaterial must be available in sheets, which is more expensivethan the pelletized forms of thermoplastics; shapes are limitedto those that can be formed from a single plane of material;and there’s a lot of waste due to necessary trimming of thepart after removing it from the mold.


Self-Check 2

Please complete these statements.

1. Three classifications of manufacturing processes include _______, _______, and _______.

2. Mold design for cast metals must take into account _______ of the metal when it cools.

3. The model of the part that’s used to make a sand mold is the _______.

4. Production can be increased for small castings by using the method of _______ casting, which requires less material for the mold than sand casting.

5. A mold made out of metal, ceramic, or a refractory material is called a _______, and theprocess of casting by pouring liquid metal into the mold is called _______.

6. A casting method for producing symmetrical round products is _______ casting.

7. One of the most popular methods of producing thermoplastic parts by pressurizing moltenplastic is _______.

8. A molding process for thermosets that uses a two-step process, and which can be used to addreinforcing materials, is _______ molding.

9. _______ forming is a method of producing plastic containers and lids from thin sheet material.



FORMING PROCESSES

Casting processes take raw material in a fluid form andcause it to fill a container whose boundaries are the shape of the desired product or part. In forming, solid raw materialis shaped mechanically—often by applying high forces—to adesired shape. As you learned in the previous study unit, whenthe applied forces create a stress in the material greater thanthe yield strength of material, the material undergoes plasticdeformation, and is permanently deformed. Most metals aresomewhat ductile and are easily shaped without cracking orfracturing by the forming processes. As with castings, addi-tional operations such as machining, polishing, drilling, orgrinding may be necessary to make a finished part. Somecommon examples of forming processes are

• Forging

• Rolling

• Spinning

• Extrusion

• Drawing

The two general methods of forming metals are hot working andcold working. Hot working occurs at temperatures above themetal’s recrystalization temperature, which is the temperatureabove which grain boundaries in a plastically deformed metalwill begin to reform into larger grains (Table 1). A disadvantageis that at high temperatures, oxide scale will often form onthe surface, causing material losses and poor surface finishes.Hot-worked materials are also softer and not as strong ascold-worked metals.

Recrystallization is asoftening process usedto restore ductility tometals after mechanicalworking. Forming metals above this temperature uses lessenergy because lessforce is required. Muchlarger deformations arealso possible withoutthe metal tearing orcracking.


Forging

Forging is a process that shapes raw material by using pressure,impact forces from hammers and presses, or both, to producea final or near-final form. Metals are easiest to form at highertemperatures, but cold forging is also frequently used. Forgingcan be done in a single step or in a series of steps with inter-mediate shapes that approach the final form, usually withheat treatment between steps to restore ductility.

Forging is generally classified into two types: closed die andopen die. To create the part, closed-die forging uses one ormore impressions that progressively approach the finaldesired shape. As shown in Figure 14, heavy-duty pressespowered by hydraulic cylinders provide the many tons offorce necessary to deform the metal. After the final shape isreached, the part is trimmed to remove any excess metal orsupport structures at the die closure points. Open-die forginguses a flat or shaped punch and die set to generate shapessuch as round, square, or polygons. The initial raw material isusually round or square bars, and the press operator controlsthe part’s final dimensions.

Many metals can be forged, as indicated in Table 2, and themost important factor for selecting forging as a manufacturingtechnique is the metal’s ductility at the forging temperature.Carbon, low-alloy, and stainless steels are the most commonmaterials used for forged parts, although aluminum is popularfor products in both the automotive and aircraft industriesbecause of its high strength-to-weight ratio.


Hydraulic Cylinder

Drop Hammer

Die

Metal

Die

Metal

Die FlashRemoved during Finishing Process

Head

Flat Punch

Flat DieBed

Open-Die Forging

Closed-Die Forging

FIGURE 14—Forging requires very high forces to deform the metals into desired shapes, and is usuallydone in hydraulically powered presses.


Forging makes parts stronger and harder butless ductile, because the grains of the metalare plastically deformed. The main advantageof forging as a manufacturing process is thatthe parts produced are in a high-strengthcondition and often don’t require any addi-tional finishing operations. Figure 15 shows a comparison of grain structures of forging,casting, and machining a part. Note that in aforged part the grains are aligned with theshape of the part, whereas they’re randomlyoriented in a cast part, and unidirectional ina machined part.

Typical applications include piston rods and crankshafts for aircraft and automotivecomponents; gears, shafts, hubs, and axlesfor machines; tools, generators, and turbines;frames and brackets for airframe parts; andmotor mounts, spring hangers, and other crit-ical mounting and support components. In animportant variation of forging called upsetforging, a heated component is gripped bydies with an end protruding, and a hammeror press forges the exposed end into the desiredshape. High-strength fasteners such as bolts,cap screws, and rivets are commonly produced

Table 2

RELATIVE FORGING EASE FOR COMMON METALS

Material Ease of Forging

Aluminum alloys Easiest

Hardest

Magnesium alloys

Copper alloys

Carbon steels

Stainless steels

Titanium alloys

High-alloy steels

Refractory metals and nickel alloys

Grain Flows in Part Direction

Forging

Machined fromSolid Stock

Casting

Grain Oriented in Single Direction

Random Grain Orientation

FIGURE 15—Forging makes the metal grainsalign in the direction of the part, which makesthe part much stronger than a comparablemachined or cast part.


by upset forging. Forged heads on these fasteners are verystrong and less likely to fail because the head doesn’t havesharp radii or corners from machining operations. (Parts withsharp corners subjected to high stresses and fatigue loadsoften fail at the point where the geometry of the part changesradically.) Upset forging is also sometimes called cold-heading,and can be used to produce a variety of semi-complex shapes,such as spark plug bodies (Figure 16).

Some parts today are still hand-forged in ways a blacksmithwould recognize. These techniques are primarily used for decorative and architectural work that doesn’t require highstrength or low cost.

As a manufacturing technique, forging is critical in producinghigh-strength and high-quality parts, but it can be expensive.Part design should allow for the largest possible radii for metalflow, and other aspects of part geometry are critical to preventcracking, internal ruptures, and irregular grain flow. Finishedparts have low porosity and inclusions, high strength andtoughness, and a good surface finish. Residual stressesresulting from the material deformations can be relieved byheat treating. Equipment costs are high and skilled labor isoften needed for critical operations. Some types of forgingrequire additional finishing operations, and material usage cansometimes result in scrap and waste rates approaching 25percent. However, production rates can be up to 300 parts

Die Shapes Head

Punch Shapes Head

Center Portion Upset

Punch and DieShape Head

Examples of some finished head shapes

Die

Ejector

Punch

FIGURE 16—Different types of heads can be fabricated on fasteners or other parts. Upset-forgedheads are much stronger and less likely to fail than machined heads.


per hour depending on the forging process used, and forgingcan be combined with other operations (machining, welding,surface treatments) to optimize the manufacturing processfor specific applications.

Rolling

Rolling is really a specialized form of forging (as are all of theprocesses in this section except for powder metallurgy) in whichmetal is continuously deformed as it’s pushed or drawn betweenflat or shaped rollers, as shown in Figure 17.

Coarse, Random Grains

Rolling Direction

Elongated and Aligned Grains

Metal Being Squeezed

Mill Frame

Backup Roll

Work Roll

Backup Roll

Work Roll

FIGURE 17—Rollingdeforms metal by com-pressing it betweenrollers. The grainsbecome flattened in thedirection of the rolling,giving the metal greaterstrength.


Rolling can be done in single or multiple passes, with variousconfigurations of rollers, and with the shape or height adjust-ed incrementally to achieve a desired finished shape. Rollingcan be done with the raw material heated, hot rolled, or atambient temperatures, in which case we say it’s cold rolled.Hot-rolled materials are easier to form, but often leave anoxide scale on the parts that must be removed later.

Rolling is often the first operation performed on the as-cast rawmaterial to allow it to be further processed. Primary rollingproduces slabs (thick, flat plates), billets (large square bars),blooms (large rectangular bars), and some structural sectionssuch as beams and rails. Secondary rolling as shown inFigure 18 is used to produce shapes that are used for bothfinished parts and for subsequent manufacturing operations,such as plate, sheet, or strip; rods, bars, and tubing; and otherstructural shapes such as channels, beams, and angles.Almost 90 percent of all raw material product used for othermanufacturing operations, such as machining, welding, orstamping, is produced by rolling.

Rolling mills are the various possible arrangements of thecylindrical forms that give the part its shape. Mills can be“two high,” “three high,” “four high,” or arranged in clustermills that have many rollers arranged to support the finishrollers and minimize deformation of the rollers themselves,thus maintaining tight dimensional tolerances. Thread rollingis done with two flat plates with thread forms engraved ontheir surfaces. Rolled threads on fasteners are much strongerand less likely to fail in critical applications, as well as beingrelatively easy to form using a rolling process.

Production rates for rolling are extremely high. Some processesand equipment can produce several thousand linear feet perminute of finished material. Rolled threads can be made on upto 30,000 pieces per hour. Finishing costs of rolled productsare quite low, and direct labor costs are minimal. However,lead times to produce rolled parts can be many months dueto the high cost and complexity of the equipment, and thecomplexity of the shaped rollers, which must be produced by forging, heat treating, and finishing.


Plates Pipe Products

Hot-Rolled SheetsHot Strip Mill

Pickling Line Pickled and Oiled Coils

Cold-Rolled Coils and SheetsCold Mill

Heat Treating and Finishing Lines

Slabs

Structural Shapes

Seamless Tubes

Bars and Rods

Blooms and Billets

Reheat Furnace

Raw MaterialBlooms, Billets, and Slabs Rolling Mills

FIGURE 18—Secondary rolling is used to make raw materials for other manufacturing operations.Slabs, blooms, and billets made from primary rolling processes are reheated to soften the metalbefore it’s rolled again into smaller shapes.


Spinning

Spinning is the method of forming sheet or tubing into thincylindrical or circular sections by pushing the metal with aroller against a spinning core called a mandrel. The sheet ortube, known as the blank, takes the shape of the mandrel’sexterior surface. Spinning can be used to form conical, spherical,or flanged parts that would otherwise be difficult to manufac-ture by conventional machining. Again, because of the plasticdeformation that takes place, finished parts are left withhigher strength than the original material. Figure 19 showshow spinning is done using a blank that’s mounted betweena turning spindle and a freely rotating tailstock holder. As themetal spins, a roller tool is pressed into the blank, pushing itinto shape against the mandrel.

Spinning is used to produce products such as cooking utensils,reflectors for lights, missile nose cones, funnels, and bells.Production rates are low, but tooling and equipment costs arealso low. Skilled labor is necessary to control the formingprocess, although future robotics applications may reducethe required labor input.

Lathe SpindleMandrel

Mandrel Blank

Live Center Turns with PartRest

Backup Lever

Lever

Tailstock

Roller Tool

FIGURE 19—Spinning isused to produce thin, hollow shapes that wouldbe difficult or expensiveto machine out of solidstock.


Extrusion

In extrusion processes, a billet of raw material is placed into a lined container and is pressed through an opening bya hydraulic ram that exerts a high force against the material,as shown in Figure 20. The opening contains a precision,high-strength die that forms the cross section of the emergingmaterial. After the material has left the chamber, it’s cut tolength for the desired applications. The process may be donehot or cold. Cold extrusion requires much higher forces dueto friction, but results in products with better dimensional tolerances. Most ductile metals such as copper, aluminum,and magnesium can be extruded easily, but extrusion canalso be done to some extent with low-alloy steel, stainlesssteel, and nickel and titanium alloys. Hollow parts such astubing can be extruded with the use of a mandrel to form theinner dimensions, but it’s much more complex and costly.

Container

Raw Material

Die

Backer

Platen

Extruded Section

Liner

Hydraulic Ram

FIGURE 20—Extrusion can be used to make fairly complex cross sections in different metals such assteel I-beams or aluminum heat sinks.


Extrusion is a very efficient process and can achieve highproduction rates, but the dies are expensive, especially forcomplex shapes. Extrusion leaves little waste, and finishingcosts are low. Equipment costs are also high, but once themachines are in place, very little labor is necessary.

Extrusion is used to make many common parts such asmoldings and decorative trim, window frames, structural,edge, and corner sections, railings, gears, and heat sinks forelectronic products (Figure 21). Aluminum parts are oftenanodized after extrusion, giving them a hard surface coatingthat can be colored for decorative uses.

FIGURE 21—Extruded aluminum heat sinks are common in electronic products such as this power supply. Large heat-dissipating surface areas can be quickly produced.


Drawing

Drawing is a process where long lengths of tubing, wire, or rodare pulled or drawn through a series of progressively smallerdies, reducing the cross section until the final desired shape isreached (Figure 22). The process is usually performed cold, andis best suited to materials that are ductile at room temperature.Like rolling, drawing is an important process for producingraw material for other manufacturing processes, and is usedto make materials such as spring wire, welding wire, and var-ious sizes of rods, bars, and tubes. Drawing results in strainhardening from the deformation process, and the productformed has high strength.

Shapes produced by drawing must have rotational symmetry.Production rates can be as high as 30 feet per minute fortubing and 6500 feet per minute for wire. Equipment costsare low, and lead time to produce drawn products is usuallylow. Drawing processes can be highly automated, allowinghigh production with little direct labor.

Powder Metallurgy Processes

In a way, powder metallurgy (PM) processes are like castingwithout liquid metal. Manufacture of a PM part starts withthe production of a metallic powder. Several methods areused involving chemical, electrical, and mechanical means to take solid metals and break them into small, powder-sizedpieces. Liquid metal can also be atomized by spraying a jet of hot metal into a stream of water or gas, resulting in fine,

Casing

Drawn-Wire Diameter

Starting Diameter

Wire

FIGURE 22—Wire ofdifferent sizes is easilymade for many materi-als by drawing a largerwire through a carbidedie that sets the finalsize.


irregular-shaped particles. However produced, the mostimportant characteristics of the powdered-metal raw materialare the size and shape of the grains, the size variation amongthe grains, and the powder’s purity and apparent density. Allthese affect the ability of the material to be compressed.

Metal powder is placed into amold and pressed into a “green”shape in a way similar to pres-sure die casting. After the greenshape is removed from the mold,it’s heated in an oven wherethe metallic particles sintertogether. As an alternative, theparts can be heated and sin-tered while being compacted.After sintering, parts are some-times mechanically sized byagain placing them in a fixtureand compressing them, whichis called coining. This results ina more dimensionally precisepart. Other operations are alsosometimes performed to achievedesired properties or dimen-sions (Figure 23). At this point,some parts are also impregnatedby oil and low-friction plasticssuch as Teflon. The oil-saturatedparts are used to make self-lubricating bearings and bushings. The plastic-filledmaterials are used for bearingmaterials and for parts that

will be plated or coated after finishing. The plastic materialmakes them easier to machine and prevents coating productsfrom entering the base material.

The main advantages of PM technology are the ability to con-trol a material’s weight (density) and mechanical properties.It’s possible to mix two or more materials that won’t alloytogether and even to mix metals with such nonmetals asceramic powders. The ease of control for metallurgical and

Raw Materials

Mixing

Metal PowdersAdditives Such as

Graphite and Lubricants

Sintering during Hot Compression

Sintering in Atmosphere or Vacuum

Manufacturing

CoiningSizing

Resintering

ForgingRerolling

Metal InfiltrationRepressing

Additional Operations(Optional)

Secondary OperationsMachining

Heat TreatingSteam Treating

Plastic Impregnation

PlatingOil Impregnation

Shot Peening Tumbling

Finished Product

Hot CompactionForming

Warm CompactionForming

Cold CompactionForming

FIGURE 23—The powder metallurgy process is able to controlmany physical properties of the parts.


physical properties makes PM parts attractive for many applications. More than 70 percent of all PM parts are manufactured for the automotive and oil industries for suchapplications as small gears, self-lubricating bushings, pulleys,motor and alternator brushes, internal transmission linkages,valve guides, camshafts, and connecting rods. Another impor-tant use of PM technology is in the production of carbideinserts for metal-cutting operations. Mixtures of cobalt andcarbides of tungsten and titanium are sintered at tempera-tures slightly higher than the melting point of cobalt, whichis 1500ºC. In this operation, called liquid-phase sintering, themolten cobalt flows between the carbide grains to act as abinder, resulting in exceptionally high densities and strengths.Liquid-phase sintering can be used for other applications as well.

The advantages of PM result directly from the ability to formmetal parts without melting the raw material to producethem. This gives the designer many options for materialselection and processing. High production rates are possible(up to 1800 per hour), and although equipment and powdercosts are high, direct labor and finishing costs are low. Themain disadvantages of PM technology are the costly produc-tion dies, which require large production runs before PM canbe considered in place of conventional casting or machiningfrom solid stock. Other disadvantages include the maximumsize of PM parts, which is limited by the maximum pressureavailable to compress the green shapes.


Self-Check 3


1. Two general methods of forming metals are _______ and _______, depending on the temperature at which the material is formed.

2. A disadvantage of forming materials at high temperatures is the _______ that forms on theoutside that must be removed afterwards.

3. The temperature at which the metal grains tend to reform into larger, softer grains is the _______ temperature.

4. Of the two metals, 1020 carbon steel or 304 stainless steel, _______ would be easier to usein forging.

5. _______ can produce fasteners such as bolts and screws that have stronger heads thanmachined fasteners.

6. The process of producing lengths of material with constant cross sections is _______.

7. Springs are formed from wire that has been _______ to the correct size, which makes it harder and stronger.

8. Powder metal (PM) forming is a two-step process of molding and then _______ to make theparticles stick together.

9. An advantage of PM parts is that they can be impregnated with _______ to make bushings for motors.

10. The process of sizing a PM part after sintering is called _______.



MATERIAL REMOVAL PROCESSES

So far, we’ve discussed ways of producing a part by startingwith an empty space (such as the inside of a mold or die) andputting material into it. In this section, we’ll discuss manu-facturing methods that start with a solid material and “putspaces” into it, by using various cutting processes. Many ofthe raw materials used for material removal processes areproduced by casting, forging, or rolling; however, in this sec-tion our focus will be on the ways we can remove portions of this material to make useful parts. All of these removalmethods involve the use of some type of cutting tool and away to position the material so that the solid material can be removed effectively. The machines that cut and remove materials are called machine tools, which can be divided intotwo broad categories, conventional and unconventional.Conventional machine tools and cutting methods use sometype of mechanical force to remove metal, such as drillingand boring, turning, milling, shaping, and planing. Thesemachines use what are called single-point cutting tools andmultiple-point cutting tools. Also included in the conventionalgroup are the tools that use grinding, lapping, and honing,which remove small amounts of material to achieve precisedimensions or smooth finishes. Unconventional machine toolsare relatively recent developments using technologies presentafter the beginning of the twentieth century. These include toolsdeveloped to use abrasive jets (water or abrasive material);lasers; electrical-discharge machining (EDM); chemical andelectrochemical machining relying on chemical reactions;ultrasonic; and electron beam methods. Of this latter group,EDM is probably the most economically important technique,although the other technologies were developed to solve particularly difficult design and manufacturing problems(Table 3).


Introduction to Cutting Processes

Most conventional machine tools remove material with a cutting tool that’s said to have a single point or multiplepoints, although in fact, these tools cut with a single edge or multiple edges. The term cutting tool can refer to the entire assembly, consisting of a holder, or shank, and a hard cutting tip, usually a carbide alloy. A removable cuttingtip called an insert is attached with a clamping mechanism(Figures 24 and 25). Some cutting tools have a carbide cut-ting tip brazed to the end of the shank. The shank of the tool is usually a type of tool steel that has high strength andtoughness; the insert is the cutting point, which is muchharder than the part material. The geometry of the cuttingedge against the part determines the effectiveness of themachining operation.

The physics of the process is very complex, and cuttingprocesses have been studied for years to improve machiningmethods. Note that the cutting action described below is thesame whether it’s the material or the cutting tool that’s moving.

Table 3

CATEGORIES OF MATERIAL REMOVAL PROCESSES

FundamentalEnergySource

Mechanical Thermoelectric Electrochemical

MaterialRemoval Techniques

Shear Abrasion Vaporization Ion displacement

Mediumused formaterialremoval

Cutting tools,single- andmulti-point

Abrasive particles athigh speeds

Electrons –High-voltagedischarge

Light – high-energyphotons

High-densityelectrical current in electrolyte

Process Conventionalmachining

Ultrasonicmachining

Electrical-dischargemachining

Laser Electrochemicalmachining


Cutting Edge

Holder

Locking Pin

FIGURE 24—Removableinserts can be replacedquickly when they’reworn, and they’re avail-able in a wide variety ofshapes. These inserts canbe rotated to expose anew cutting edge.

FIGURE 25—An indexable insert used for threading a rod can be removed or rotated to expose a newcutting surface.


When the cutting tool’s edge is brought to and forced into thepart, often called the workpiece, it generates a shearing stressin the material that causes a layer to “peel” off the surface ofthe part, as shown in Figure 26. This layer of peeled material,or chip, shows an experienced machinist how well the tool iscutting and whether the cutting action needs to be adjusted.This could be by changing the depth of the cut, the rotationalspeed of the workpiece, the speed of the tool as it’s fed intothe part, or all three. A chip that curls continuously off thework across the cutting tool indicates that as much as 30percent of the cutting power is being wasted as friction, andtends to wear the top face of the tool. Cutting power is alsobeing wasted when the tool compresses the chip until it’sthicker than the depth of cut. Improper feed, speed, anddepth of cut for a given cutting tool may cause an excessivebuilt-up edge (BUE) to develop, which actually changes thegeometry of the cutting edge. This occurs as the chip slidesover the top edge of the tool and adheres to the insert due tothe heat generated in the cutting action. Figure 27 illustratessome of the types of chips that are produced by cutting toolsfor various materials and different feeds and speeds.

Part Motion

Workpiece Texture

Shear ZoneChip Texture

Melting Zone

Cutting Tool

Tool Face

FlankMachined Surface

FIGURE 26—The basic cutting action of a tool isshearing a layer of materi-al away from the surfaceof the workpiece.


The cutting tool itself is the most important factor in the efficiency of the machining process. Thin edges cut effi-ciently because the friction is lower. However, thin edgesdon’t conduct heat away from the cutting zone, so they wear too fast to be very useful. The best cutting edge is the thinnest possible edge that offers a reasonable tool life.

The rake angle is the angle at which the chip leaves the topface of the tool. Tool edges are usually ground with additionalangles called side rake and back rake angles (Figure 28) thatcombine geometrically to make an effective rake angle. Thisdetermines the efficiency of the machining. The optimumrake angle will vary with different materials. Different insert

Shear Zone

Chip

ShearZone

Tool

Continuous Chip–Narrow Shear Zone

Secondary Shear Zone

Interrupted Chips–Two Shear Zone

Discontinous Chip Segmented Chip

Continuous Chip–Built-Up Edge

Continuous Chip–Large Shear Zone

ShearZone

Part

Molten Material

FIGURE 27—Experienced machinists can tell a lot about the effectiveness of the operation by the typeof chip produced by the cutting tool.


geometries are optimized for different feed rates, depths of cut,and material types. Some of the factors that insert designersattempt to optimize are

• Chip control

• Surface texture

• Accuracy

• Cutting-edge strength

• Tool life

• Metal removal rates

Modern machine tools use an indexable insert that can berotated to expose a new cutting edge when one edge becomesworn. The inserts are complex combinations of angles, flats,and radii that are designed to control chip formation, contactlength, and chip breaking. Most new inserts have positiverake angles and are inclined at negative angles in the holderto promote positive cutting action. The design of inserts forspecific machining conditions is a complex technology.

Side-Rake Angle

EndFace

Side-Relief Angle

FlankCutting Edge

Work Rotation Top Face

Direction of Feed

SideCutting-Edge Angle

Back-Rake Angle

ShankFlank

End-Relief Angle

End Cutting-Edge Angle

Top Face

CuttingEdge

Nose Radius

FIGURE 28—Tool geometries are important in establishing maximum cutting speeds for a given sur-face. The angle at which the chip leaves the top face of the tool is the rake angle. Tool edges are alsoground with side rake and back rake angles.


You should consult insert manufacturers’ catalogs and engi-neering data to learn more about the proper selection and application of cutting tools.

Because of the friction involved in cutting materials withmachine tools, any cutting edge or insert will wear out overtime. A tool is said to be worn out when it can no longer create the desired surface texture at the intended conditions.However, some operations, like rough cutting, can toleratemore insert wear than, say, a finish cutting operation. In thepast, a cutting-edge life of 15 minutes was standard. Today,although tool life depends on conditions, it’s dramaticallylonger than what was once standard.

Tool wear is a result of various load factors on the cuttingedge, which determine the life of the edge. These are

• Mechanical loads

• Thermal loads

• Chemical loads

• Abrasive loads

Mechanical loads result from the forces of the edge cutting intothe workpiece, which can break the thin edges of the insert.Mechanical loads can be dynamic as well, with impact loadsoccurring due to variation in the hardness of the materials.Thermal loads occur because of friction, and increased tem-peratures can lower the strength of the insert material as wellas promote undesirable chemical reactions. These reactionscan diffuse contaminants into the insert material and oxidizethe surfaces; the oxidation must then be removed by friction.Abrasion can occur for a number of reasons: direct abrasionwith hard particles within the workpiece, fatigue loading andsubsequent fretting of the insert surfaces, and adhesion oflayers of the chips that become welded to the insert surface,becoming part of the edge.

Fretting occurs when a material’s surface wears through a combination of corrosion and mechanical impact. Metal bearings are particularly susceptible to fretting.


Tool life can be defined as the period of time that the cuttingtool performs efficiently. It can be very hard to determine,because of all the variables: the material to be machined,cutting-tool material, cutting-tool geometry, machine condi-tion, cutting-tool clamping, cutting speed, feed, and depth ofcut. Frederick W. Taylor, famous as the “father of scientificmanagement,” published studies on tool life in 1907 thatbecame the foundation of later studies in this area. Taylorwas able to show empirically that the relationship betweencutting speed and tool life was

VT n = C

where:

V = cutting speed, in feet per minute (FPM)

T = tool life, in minutes

C = a constant depending on work material, tool material,and other machine variables. Numerically it’s the cuttingspeed that would give 1 minute of tool life.

n = a constant that depends on work and tool materials

Empirical relationships are those determined based on actualobservation and experience rather than theory.

The exponent n has values ranging from 0.125 for high-speedsteel tools, to 0.70 for ceramic tools. This relationship isimportant because cutting-tool life is significantly affected bythe cutting speed, which is often set as high as possible toincrease manufacturing speeds. The higher cutting speedswill result in higher costs due to shortened tool life, and needto be balanced with decreased manufacturing and labor costs.

A final factor in both tool life and surface finish is lubrication.Few machining processes are done without some type oflubrication using a liquid called a cutting fluid or cutting oil.The purpose of a cutting fluid is to lubricate and cool the tooland the workpiece, and carry chips away from the cuttingarea. In a typical machine setup, a nozzle is positioned closeto the cutting tool edge and the fluid is directed onto theworkpiece and the cutting tool. Cutting fluids are producedfor specific applications and materials, and may be waterbased or oil based. For example, a cutting fluid chosen forrough-cutting carbon steel will be different for one needed


to finish machine magnesium for an aircraft application.Water-based fluids remove heat better, but oils are better for lubrication, so oils are generally used for low-speed, low-machinability materials, or difficult cutting operations suchas threading. Water-based fluids (commonly referred to ascoolants) are used for high-speed cutting, easy-to-machineoperations, or wear problems where built-up edges areencountered. Cutting fluids must be maintained properly by preventing contamination with other liquids as well asparticulates, and need to be periodically replaced. Cuttingfluids can also be hazardous and must be handled with care to prevent accidents.

Over the years, machinists and engineers have attempted to establish guidelines to represent the ability to machinevarious materials, which has only been partially successful.Machinability is a combination of five criteria:

• Wear resistance

• Specific cutting pressure

• Chip breaking

• Built-up-edge formation

• Tool coating character

One measure of machinabilty is a numerical scale based on a standard value of 100 for plain carbon steel, AISI B1112.Materials with a higher numerical rating are easier to machine,and materials with a lower rating are more difficult. These rat-ings are based on cutting tests by the ASME. Its Manual onCutting Metals has a lot of useful information about machiningand machinabilty of metals, but their quantitative measure-ments should be used with care. Numerical scales alone don’taccount for variations in alloy compositions, heat treatment,tool conditions, machine setup conditions, and variations inmicrostructures that affect chip formations. Table 4 below listssome of the ratings for various metals you may encounter inmanufacturing processes.


Conventional Machining Processes

A drill is a rotary cutting tool for making cylindrical holes.Cutting occurs at the edges on the end of shaft while grooves,called flutes, cut in a spiral fashion along the sides of the shaft.These grooves allow chips to be removed from the cutting edgesand cutting fluids to reach the cutting edge and the workpiece.Figure 29 shows some of the parts and terminology of a com-mon drill, as well as some specialized drill shapes and reamers.Reaming is like drilling except that the reamers have nopoints because they’re used to machine a finished diameterin a hole that’s predrilled. Drills can range in diameters froma few thousandths of an inch to several inches and can haveseveral different shapes for the shanks, flutes, and cutting tips.Drills are usually made of high-speed steel (HSS), and areavailable with a very hard surface coating of titanium nitride(TiN) for added life. The TiN coating is only about 0.0001inches thick and gives tools’ cutting edges and flutes a yellow coloring.

Table 4

RELATIVE MACHINABILITY OF METALS

Material Hardness Machinability Rating

6061-T Aluminum 7075-T

— 190%

Aluminum — 120%

B1112 Steel 160 BHN 100%

416 Stainless Steel 200 BHN 90%

1120 Steel 160 BHN 80%

1020 Steel 148 BHN 65%

8620 Steel 194 BHN 60%

304 Stainless Steel 160 BHN 40%

Iconel X 360 BHN 15%

Rene 41 215 BHN 15%

Waspalloy 270 BHN 12%

Hastelloy X 197 BHN 9%


Drilling is done with a variety of machine tools, and the most common is the drill press. Drill presses can be manu-ally operated or automated, and come in many sizes and configurations, from heavy industrial to hobby types. Drillingcan also be done on a lathe, where a workpiece is drilled onits turning axis by a stationary drill mounted in the tailstock.Drilling can also be done in CNC (computer numerical control)lathes, milling machines, or machining centers where thedrill is one of several different tools indexed in sequences tomake many different operations without changing the setup.

FluteShank

Diameter

Multidiameter Bit

Counterbore Bit

Conventional Bit

Chamfer AngleChamferLengthReamer

Shank

FIGURE 29—Drilling isone of the most commonmachining operations.


Other processes similar to drilling are spot facing, counterboring,and countersinking. Spot facing provides a flat surface fromwhich to start other machining operations (such as drilling).Counterboring is drilling a larger hole around a smaller onefor a limited depth in order to provide a below-the-surfacerelief for a hex, socket, or button-head screw or bolt. Drillingis often combined with countersinking to make holes in theends of a workpiece tool to hold it between machine centers,or to allow flat-head screws to be inserted with their headsbelow the surface of the part.

Drilling should always enter and exit a surface perpendicularto the hole to prevent the drill from being pushed off center.Large holes should be started with a center drill to make surethe larger drill doesn’t start off center. A center drill has athick shank with a small drill tip on the end, which is usedto drill a small indentation in the workpiece as a startingplace for the larger drill. Long, small-diameter holes with alength-to-diameter ratio greater than 70:1 are extremely diffi-cult to produce and are prone to veering off center as well asbreaking drills, and thus should be avoided in the design ofthe products.

Turning and boring are two similar types of operations doneon a lathe. Turning operations produce rotationally symmet-ric shapes by holding the workpiece in a chuck or collet. Achuck is a mechanical head with three or four adjustablejaws that clamp the work. The chuck is fastened to the endof the lathe and locked so that it doesn’t fall off. Chucks canhold round shapes such as bars and tubing. A four-jaw chuckcan be used to hold irregular, or non-round, parts so thatdiameters can be turned or holes drilled. A collet is a roundtube, threaded on the back with a low-angle taper on the out-side front and slots cut into the taper. A collet is threadedinto the lathe head and a mechanical lever is used to pull the collet into the lathe, slightly squeezing the taper togetherto pinch and clamp round parts. Figure 30 shows a photo of some of the many workpiece holders, with collets shown in the front. Figure 31 shows the basic setup for turningoperations in a chuck-type head, including illustrations ofsome common turning operations.


Once mounted in the lathe, the workpiece is rotated and astationary cutting tool is moved to begin the cutting process.Cutting tools for lathes are usually single-point tools. Theyuse a shaped and hardened cutting edge mounted on a bar,or shank, that’s in turn clamped to a tool holder on the lathe.The position of the tool holder is controlled by the operatormaking the cut on the part. Turning usually refers to makingoutside diameters or radii. Boring refers to turning insidediameters. Other operations done on a lathe include facing,or turning a flat end on a part; parting, running a thin-bladed cutting tool all the way through a rotating part toremove a portion; and threading, using a V-shaped or otherspecial cutting tool to cut internal or external threads on arotating part (Figure 31).

FIGURE 30—Many different tools are used to mount the workpiece properly during turning and boringapplications. These are just a few available to machine operators.


Cutting metals places large forces on both the cutting toolsand the parts being machined. The magnitude of these forcesdetermines how fast the material can be machined for a givensurface finish. The cutting requirements for most applica-tions have been analyzed and documented by an engineerskilled at mechanical design. Available reference tables willgive you a range of machining feeds and speeds derived fromthese calculations, but they’re really a starting point for youto evaluate the best conditions for your specific application.

Tool Bit

Tool

Workpiece

FIGURE 31—Turningprocesses on lathes arevery common in machineshops. Parts must haverotational symmetry, butturning can be highlyautomated.


Many factors will influence your final selection of inserts andtoolholders, cutting fluids, sequences of operations, and feedsand speeds.

Control of the lathe can be done manually, as was commonin the past with engine lathes, or it can be done with com-puter controls. Modern lathes often come in the form of computer numerical control (CNC) machining centers, whichcan perform multiple operations on a part without changingsetups. Actuators in the machine move the cutting tools,index cutting tool heads, set the correct feeds and speeds ofthe cutting process, and remove and insert workpieces forcontinuous manufacturing operations. Robots around thesemachining centers bring raw material to the machine andmove finished parts to other parts of the factory. The skillspreviously required by operators in controlling the cuttingprocesses have been effectively replaced by the skills requiredto program the machine and the robots!

As you may guess, CNC lathes and machining centers likethe one shown in Figure 32B are very expensive to buy, setup, and program. Manual engine lathes, on the other hand,are much cheaper and flexible in what they can do, especiallyfor prototyping and low-quantity manufacturing runs. This isone reason why most manufacturing shops have a variety oflathes available. For high volumes, however, the efficiency ofthe CNC machines will offset their additional cost.

Milling is a process that uses rotating multiple-point toolsmounted in a stationary head, while the part is clamped in amovable holder that’s brought into the cutting zone. Millingmachines can be horizontal or vertical, depending on the orientation of the cutting tool. Vertical mills are probablymore common and have the cutting tool shaft in a verticalorientation, while the parts are mounted on a horizontal tablethat can be controlled in the X and Y directions. The heightof the table can also be controlled, allowing variation in the Z direction. Milling can generate flat surfaces, slots, profilessuch as dovetails or grooves, and can also be used for drillingwhere necessary. Figure 33 shows facing operations per-formed on each type, and Figure 34 shows different cutterarrangements possible.


(A)

(B)

FIGURE 32—(A) Enginelathes such as thisrequire high levels ofoperator skill but areadaptable to a wide vari-ety of jobs. Lathes similarto these have beenaround for hundreds ofyears. (B) Modern CNClathes require operatorsto program, monitor, and sometimes adjustpart-specific computerprograms.


Overarm

Table

SaddleColumn

Base Base

Knee

ColumnHead

Table

Knee

Saddle

Horizontal-Knee Type Vertical-Knee Type

Transverse MotionLongitudinal

Motion

FIGURE 33—Milling is a versatile operation that can be used to cut complex shapes in materials.


Table Feed Table Feed

Conventional (Up) Milling Climb (Down) Milling

End Milling

FIGURE 34—End milling is useful for cutting slots, edges, lips, and even holes in non-round work-pieces. Up-milling and down-milling place different forces on the cutting tools and workpieces, oftenresulting in different surface finishes for the same feeds and speeds.


Milling machines can also be computer controlled, with multiple indexed heads that can perform several differentoperations in sequence without removing the workpiece, asshown in Figure 35. Again, conventional milling machinessuch as the one shown in Figure 36 are cheaper and moreflexible in their capabilities, but CNC mills are ideally suited tohigh-volume manufacturing shops. Skilled labor is necessarywhether using conventional or CNC mills.

FIGURE 35—ModernCNC mills can beextremely efficient,making hundreds ofcomplex parts per day.Operator skills mustinclude programmingskills in addition tomachining knowledge.


Planing and shaping are similar, and both produce variousshapes in metals with single-point cutting tools that move instraight lines. In planing, the workpiece moves back and forth,while in shaping, the cutting tool reciprocates. Figure 37Ashows some typical planing and shaping tools. Shaping isone of the simplest and oldest machining methods, and prob-ably an evolution of hand shapers used to carve wood. Earlyrifle barrels were rifled by moving a single-point cutter alongthe inside of a barrel while the tool (or the barrel) was rotatedas the tool progressed along the length of the barrel.

FIGURE 36—Millingmachines such as thesehave been around sinceearly in the twentiethcentury, but are still valuable for producingprototypes and smallmanufacturing runs.


Shaping and planing can also be used to make slots in shafts,grooves and dovetails in flat plates or shafts, and gear teeth.Production capabilities for planing and shaping are very low.

Broaching is similar to planing and shaping, except that amultiple-point tool is pushed or pulled through the part. Thetool shape, as shown in Figure 37B, is such that deeper cutsare taken as the tool progresses through the part, generatingthe final contour. Broaching is used to produce speciallyshaped contours on inside diameters of parts such as hubs,pulleys, or gears. Double and single keyways, square drives,rectangles, hexagons, and splines can all be made on theinside diameters of these common parts.

Finish Teeth

Semi-Finish Teeth

Rough Teeth

Pull EndCutting Motion

Round Square RectangularHexagonal Keyway DoubleKeyway

Spline

Broaching Produces Internal Shapes(B)

Slotting Side DovetailRoughing

Planing and Shaping Produce External Forms(A)

Finishing

FIGURE 37—Planing and shaping (A) can produce linear cuts with fairly complex cross sections, butthe process is relatively slow. They’re useful for producing odd shapes such as keyways and otherexternal forms. Broaching tools (B) often have multiple points for making deeper cuts as theyprogress through the workpiece, and are used to create internal forms.


Grinding is the removal of small layers of material by a rotating wheel composed of hard particles bonded together.Grinding is suitable for hard materials but doesn’t work wellfor soft metals or plastics. The grinding wheel can be consid-ered to be a mass of randomly oriented cutting tools becauseof the irregular edges of the carbide, aluminum-oxide, andcertain other types of particles. Grinders can be both hand-controlled as well as arranged as machine tools. Hand grindersare often made of small, hand-held electric motors driving flat disks of abrasive material, and are often used for quickremoval of material in preparation for welding or other finish-ing or sharpening. Machine grinders have a variety of usesbut are very important for finishing products to precisedimensions. Precision grinders can finish parts to within a fewmillionths of an inch tolerance for such critical applicationsas gage blocks, camshafts, valves, and ball bearings.

Surface grinding and centerless grinding are two types ofgrinding processes used to make precision flat plates andround shapes respectively (Figure 38). In centerless grinding,the round workpiece is held between two rotating grindingwheels turning at different rates. One of these wheels is

Grinding Wheel

Motion

Wheel Spindle

Workpiece Rotates

Grinding Face

Cylindrical Grinding

Surface Grinding

Grinding Wheel

Workpiece

Magnetic BaseReciprocating Table

Grinding Wheel

Feed WheelWorkpiece

Rest

Centerless GrindingTable Motion

FIGURE 38—Grinding is a precision operation used to finish parts to critical dimensions and surfacefinish requirements.


actually grinding while the other serves only as a positioningdevice. Mounted grinding wheels are also used to sharpencutting tools such as knives, drills, and lathe tools.

Other types of machining operations use grinding tools forfinishing parts. Honing is used to finish the inside diametersof parts such as engine bores, which have been machinedfirst by boring. Lapping is used to finish parts such as valveseats, and the process uses an abrasive powder mixed in apaste. The paste is placed in between mating parts that aremoved relative to each other, making each surface conform tothe other in a slow, labor-intensive process. Lapping can alsofinish flat parts in the same manner by moving them on a flatsurface with the paste between the part and the lapping surface.

Unconventional Machining Processes

Unconventional machining processes are the result of newtechnologies developed to solve manufacturing and machin-ing problems. Most of the processes discussed in this sectiondidn’t exist before the twentieth century, and many havebecome practical and effective only as the result of improve-ments in other technologies. For example, lasers have beenknown for many years, but only improvements in laser materials and technology have made them available at powerlevels that can be used for metal (or material) removal. Youcan expect these processes to become more important tomanufacturing as products become more complex and moreautomated in their manufacture.

Electrical-discharge machining (EDM) is probably one of themost common and cost-effective processes in this group. Likeelectron beam machining (EBM) and laser beam machining (LBM),it transforms electrical energy into thermal energy that heatsand vaporizes small sections of the workpiece. In the case ofEDM, the energy comes from a controlled spark generatedfrom electrical pulse-generating circuits. Material removalrates are quite small, especially when compared to conven-tional machining methods, but the precision of the cuts andthe complex shapes that are possible make EDM attractivefor many unusual applications. A modern EDM machine isshown in Figure 39.


EDM cutting tools, or electrodes, are made by conventionalmachining methods from materials that have a high meltingpoint and good mechanical strength, and are good conduc-tors of electricity. Modern EDM tools are usually graphite ortungsten. Tungsten wire is the material of choice for wireEDM, a process that cuts using a thin wire as an electrode.

The basic setup of an EDM machine is shown in Figure 40.In the EDM cutting process, the electrode is positioned with a small gap between the tool and the workpiece, in the rangeof 0.0005 to 0.020 inches. The tool and the workpiece aresubmerged in a dielectric fluid such as paraffin or light oil. A dielectric material is one that doesn’t conduct electricity easily. A power supply applies a voltage (150–250 V), whichcauses an arc, or discharge, between the tool and the workpiece.

FIGURE 39—An EDM machine is an important addition to a modern machine shop. EDM machines areuseful for making one-of-a-kind parts for prototypes and special projects.


Ram

Wire

Pump

Ram Head

Servomechanism

Workpiece

Dielectric Fluid

Electrode

DC PowerSupply

Filter

FIGURE 40—EDM machines can make cuts in any electrically conductive material regardless of hardness,and can make shapes that can’t be machined by conventional cutting tools.


A large electrical current flow ionizes and raises the tempera-ture of the dielectric fluid to 8000–12,000ºC, vaporizing metalfrom the workpiece. The electrical pulses last from 2 to 1600microseconds, and the quality of the surface finish and thematerial removal rate is determined by the machine settingsfor voltage, current, and pulse duration. After the arc stops,the melted metal resolidifies in the fluid and is carried awayfrom the gap. The cutting fluid is constantly circulated tobring fresh fluid to the working gap and to remove debris.

The main advantage of EDM machining is its ability to cutshapes or features in metals that would be difficult or impos-sible with conventional machining. The EDM simply burnsaway material, without placing any forces on the part. EDMcan also cut any material regardless of its hardness, whichmakes it useful for machining difficult materials such as therefractory metals used in jet engines or turbines.

Electrochemical machining (ECM) is a metal-removal processthat’s the reverse of the electroplating process, in which ametal coating is deposited on a part. As shown in Figure 41,both the workpiece and the ECM cutting-tool electrode aresubmerged in an electrolytic fluid, and a direct-current (DC)power source is connected to them such that the workpiece isthe anode and the tool is the cathode. Metal is stripped away

Contains Sealed Electrode Pressurized

ElectrolyteUsed Electrolyte

and Sludge

Workpiece Anode

Electrode-Cathode

DC Power Supply+

–

Pump and Filter

Gap

FIGURE 41—ECM is a slow process but can produce finished parts from difficult-to-machine materialssuch as refractory or hardened metals.


from the surface of the workpiece and travels towards thetool. The metal ions are carried away by the fluid movingthrough the gap to prevent them from plating the tool.

The tool and the part can be thought of as positive and negative (core and cavity) versions of the same image, andmachining can actually take place around the entire part atone time. As a result, complex parts can be machined withan excellent surface finish. Machining can be done on materials that are difficult to machine, such as hard or toughmetals like tungsten, or metals that have been heat treatedor surface hardened. Material removal rates are unaffectedby the type of material, hardness, or complexity.

Electron beam machining (EBM) uses a beam of electrons pro-duced by a cathode material, focused into a thin beam andaccelerated toward the workpiece target by a high voltage (upto 150 kV). All of this requires the parts and equipment to bein a good vacuum, since the electrons won’t readily travelthrough air. The process uses pulsed beams of up to 1000pulses per second, and the part is moved while the electronbeam equipment remains fixed (Figure 42).

A principal use of EBM is drilling, and complex patterns ofthousands of holes can be quickly produced with automatedcontrols. Any material can be cut, regardless of conductivity,hardness, or shape. The production of clean, burr-free holesrequires a backing material as shown schematically in Figure 42.This cleans out melted metal from the holes when the electronbeam reaches and vaporizes the backing material. The maximumthickness of materials can be up to 6 inches, and the mini-mum hole diameter is about 0.0005 inch. Length-to-diameterratios can be up to 100:1.

EBM is typically used to produce small-diameter holes in relatively thick materials, such as injector nozzle holes andextrusion die holes; irregular-shaped holes; engraving; andfeatures on silicon (and other) wafer materials for electronicsand nanotechnology industries.


The laser (from “light amplification by stimulated emission ofradiation”) was discovered around 1960. A laser first uses elec-trical energy to generate light, and then a special chamber ofsolid, liquid, or gaseous material causes the light to becomeamplified and coherent, so that every photon is synchronizedwith the others, like soldiers marching in step. The energyfrom these concentrated light beams, which are usually gen-erated in short pulses, is concentrated on a target. The colorof the laser light depends on its materials and design.

Anode

VoltageCathode

Vacuum PumpWorkpiece

Electron Stream

Magnetic Focusing

E-Beam Machining

Backing Plate

Workpiece

Molten Metal Coating

Metal Removedfrom Workpiece

E-Beam Drilling

FIGURE 42—EBM can drill thousands of holes in a short time with automated controls. These machinesare becoming important in nanotechnology manufacturing because of their ability to fabricate intricatedetails in virtually any material.


There are two general categories of lasers used for manufac-turing processes: gas lasers and solid-state lasers. Gas lasersinclude helium-neon (He-Ne), argon (Ar), carbon dioxide (CO2),and excimers, which use a gas such as fluorine. Solid-statelasers include neodymium (Nd) and YAG (yttrium-aluminum-garnet) types. These lasers can be operated continuously withpower outputs to over 100 watts, or pulsed with peak poweroutputs such as 1 megawatt for 50 nanoseconds (Table 5).

Laser machining is complex and expensive, but its exceptionaladvantages will increase its use for manufacturing applications.Laser output can be varied to use for heat treatment, welding,or machining. Lasers can vaporize any known material,including diamonds and ceramics. The beams can be precise-ly controlled with optical components such as mirrors andlenses, and can produce extremely small details, as in thecase of etching silicon wafers for microchips and nanotech-nology fabrication. The laser equipment itself is fairly rugged andcan be used by semiskilled labor in production applications.Lasers are becoming less expensive, with wider applicationswith every passing day. However, the overall efficiency of alaser is quite low and it therefore isn’t suitable as a tool forhigh-production material removal.

Table 5

CHARACTERISTICS OF LASERS USED IN MANUFACTURING

TypeElectrical

Efficiency

Wavelength,

nm

Continuous

Power, Watts

Peak Power,

WattsApplications

CO2Greater than

10% 10600 Up to 1000 108Cutting, drilling,heat treatment,welding, scribing

He-Ne 0.1% 633 0.001 0.002 Alignment, measurement

Nd in glass Greater than1% 1060 Up to 1000 109 Drilling, welding

An excimer is a moleculethat bonds two mole-cules in an electronicexcited state.


Chemical machining is the selective dissolving of metal areasof a workpiece by an acid or alkali solution. Areas that aren’tto be removed are masked with tapes, plastic, or other coat-ings that prevent the chemical etching of the material. It’spossible to machine thin parts without forces that would dis-tort or break them. Because the material removal rates aresmall and the masking process can be done photographicallyon flat parts, the process is particularly useful for producingcomplex flat parts such as special springs, decorative panels,honeycombed parts, printed-circuit boards, parts with irregu-lar contours and stepped cavities, and silicon wafers for usein electronics products. It’s also used in the aircraft industryto reduce the weight of parts.

Chemical machining isn’t used for high-volume productionbecause of the low material removal rates and the dangerouschemicals used. Prototypes can be produced quickly, however,and tooling and setup costs are low.

Abrasive jet machining (AJM) is a cutting process that uses anabrasive powder mixed with water and fired at the workpieceat high speed. Normal machining processes generate lots ofheat, but the AJM fluid keeps the workpiece cool. A hydraulicoil system generates a high-pressure water jet with an outputpressure of up to 4000 bar, and a precise mixture of abrasiveparticles is added in this jet at the nozzle (Figure 43). The AJMprocess can cut materials, shapes, and thickness that wouldn’tbe otherwise possible, such as thin sheets, laminated materials,composites, printed-circuit boards, and honeycombs. AJMcan also cut refractory materials such as ceramics and glass,leather, plastics and foams, and silicon. Materials are cut with-out leaving a heat-affected zone that can require additionalfinishing, and cuts can be made without burrs in very complexpatterns or arrangements.


Process Comparisons

Manufacturing processes are selected to balance performancewith cost. Figure 44 shows some of the variables consideredwhen selecting a basic process. As you can see, different process-es are more cost effective for high-volume production runsthan for small ones. Each process has inherent capabilitiessuch as precision, appearance of finished product, toolingcosts, and additional finishing requirements. Some processes

Mixing Chamber

Water Jet Nozzle

Abrasive Head

Abrasive Material

Abrasive Nozzle

Water/Abrasive Jet

Water Hydraulic Equipment

Pressure Converter

Low-Pressure Filter

Return Valves

CNC Controler

Return Valves

Pulsation Damper

High-Pressure Filter

Cutting Head

FIGURE 43—Abrasive jetmachining has manyapplications, thanks to its ability to cut materials without heat-ing them, such as foams,plastics, ceramics, andglass.


are suited for prototyping for proof of concept, or one-offs, asthey’re called. Others are practical only for high-volume pro-duction runs because of the cost of equipment or operator skillor training requirements. Expertise in selecting appropriatemethods will develop as your experience in manufacturingincreases.

InvestmentCast andMachine

Machine from Solid

PermanentMold andMachine

Sand Castand

Machine

Shell Castand

Machine

Forgeand

Machine

Material SelectionComplexDesign Freedom

Tolerance CapabilityLead-Time Prototype

Very High High

Medium

Low

High

Wide

Short

Very High

Very Wide

Medium

Medium

Narrow

Long

Medium

Medium

Medium

Medium

Medium

Medium

Medium

Medium

Low

Medium

Long

Relative Costs - High Volume - 10,000 Pieces or More per Year versus Low Volume

Process Piece Price

Tooling Cost

Secondary Machining Cost

HighHighHighHighHighHigh

HighHighLow

MediumLowLow

MediumHigh

MediumHigh

MediumMedium

High

LowLowLowLow

High

MediumMediumMedium

HighMediumMedium

LowMediumMedium

HighMedium

High

MediumHighHighHighLow

Medium

Fabrication

Medium

High

Narrow

Long

Process

Feature

FIGURE 44—Process used in manufacturing can be compared over many variables of effective performance.


Self-Check 4


1. _______ use single- or multiple-point cutting tools to remove material from the surface of a part.

2. Two mechanical means of removing material are _______ and _______.

3. _______ inserts have multiple cutting edges.

4. Tool wear is the result of _______, _______, _______, and _______ loads.

5. _______ carry away heat, lubricate, and flush away chips from the cutting area.

6. Of the two materials, 304 stainless or 416 stainless steel, which is the easier to machine?_______

7. Many turning and boring operations are done on a _______.

8. _______ is a machining process that can be considered to be using a mass of randomly oriented cutting tools.

9. The most common unconventional machining process is _______, which uses electrical energyto vaporize small portions of the workpiece.

10. _______ is the reverse process of electroplating.

11. An advantage of _______ is that it can cut without generating heat on the workpiece.



JOINING PROCESSESIn the last section we discussed the differenttypes of systems used to manufacture parts.These parts must be joined into assembliesand subassemblies. However, this assemblyprocess implies some method of physicallyholding these parts together either perma-nently or semipermanently with the ability to disassemble them for service or repair.Figure 45 shows how various methods ofjoining can be classified by method. Thereare four broad categories of joining, most ofwhich are permanent:

• Welding—a high-temperature processthat involves melting the base partmaterials together with or without anintermediate filler material to aid thejoining process. Welding involves fusionof the base and filler metals.

• Brazing and soldering—a lower-temperature process that uses an intermediate low-melting-point materialthat’s melted and used to hold the basepart materials together.

• Adhesive bonding—a low- or room-temperature bonding process using aglue or adhesive that cures and bondsthe materials together.

• Mechanical fastening—a method of join-ing materials with mechanical parts suchas screws, bolts rivets, staples, nails,snapfits, press fits, shrink fits, or pins.

Welding

Soldering and Brazing

Solid StateExplosive Welding (EXW)Friction Welding (FRW)Ultrasonic Welding (USW)

Thermochemical Gas Welding (GW)

Resistance

Seam Welding (RSEW)Flash Welding (FW)Electro-Slag Welding (ESW)Projection Welding (RPW)Spot Welding (RSW)

Arc

Metal Inert-Gas Welding (MIG)Tungsten Inert-Gas Welding (TIG)Manual Metal Arc Welding (MMA)Submerged Arc Welding (SAW)Flux Cored Arc Welding (FCAW)Plasma Arc Welding (PAW)

Radiant Energy

Laser Beam Welding (LBW)

Electron Beam Welding (EBW)

Brazing

Gas BrazingFurnace BrazingInduction BrazingResistance BrazingDip BrazingInfrared BrazingDiffusion Brazing

Soldering

Gas SolderingFurnace SolderingInduction SolderingResistance SolderingDip SolderingInfrared SolderingIron SolderingWave Soldering

Mechanical Fastening

Snap FitPress FitShrink/Expansion FitBlind Rivet

Permanent

RivetingFlangingStakingStapling/StitchingCrimpingSeamingNailing

Semi-Permanent

Retaining RingSelf Tapping ScrewQuick Release DevicesPinsTapered KeyMagnetic DevicesThreaded Fasteners

Non-Permanent

Adhesive Bonding

AnaerobicCyanoacrylateEmulsionEpoxy ResinHot MeltPhenolicPolyurethaneSolvent-born RubberTapeToughened AdhesivePolymide

FIGURE 45—Various Joining Methods


Welding

When two materials need to be permanently joined together, oneof the first choices is welding. Welding has been around formore than a hundred years, and while many welding proce-dures in manufacturing are still done by hand, many othersare automated to save time and labor, ensuring high-quality,uniform welds in high-volume production systems. Welding canjoin materials, particularly metals, with a resulting strengthin the joint that’s equal to the base materials’ strength.

There are many different types of welding, but most of the common processes used in manufacturing fall into fivemain categories:

• Electric-arc welding—generating an arc between two adjacent metals that causes them to melt

• Electric-resistance welding—passing an electric currentthrough two materials in contact with each other will meltthe materials at the junction. Heat is generated at theinterface because of resistance to the electrical current.

• Gas welding—the use of an oxygen-acetylene flame as aheat source to melt materials

• Solid-state welding—joining two materials with frictiongenerated by relative motion of materials, usually bymotion or ultrasound

• Radiant energy—laser beams or electron beams are theheat source to melt materials

Welding can be used to join parts with many different config-urations (Figure 46), but it can also be used to repair metalswith defects by grinding out around the defect and then fillingthe void with weld filler metal.

Welding processes apply mostly to metals, although thermo-plastics may be joined by similar methods. Thermoplasticssoften with the application of heat, and the typical process isto heat the materials until they soften, and hold them firmlytogether until they cool. Heating can be done with either


ultrasonic sources or radio-frequency sources (27 MHz energy source) for thin sections of up to 0.020 inch. This isthe preferred method for blister packs of items such as toys,hardware, or tools sold in retail stores. Thermoset plasticsdon’t soften, and must be joined with adhesives.

Metals have relatively high melting points that require a lot of energy to reach. To join two pieces of metal, an electric arccan melt the materials in a precise location to make a strongbond. The metals melt together and then resolidify into a uniform joint. In most welding processes, a filler metal is

Joint

Joint

Butt Joint(A)

Corner Joint(B)

Joint

T-Joint(C)

Lap Joint(D)

Joint

Joint

Edge Joint(E)

FIGURE 46—Welding has many different configurations.


required to fill in a specially designed gap between the parts.A filler metal is metallurgically compatible with the base met-als and aids in forming a homogenous weld zone between thetwo parts.

The arc is generated between the workpiece (the parent metal)and the welding electrode, which is often a wire or rod made offiller metal. The resulting current ionizes the air and adjacentmetals into a gaseous material called a plasma. The electricalarc currents can be over 1000 amperes, even though the voltagesused are less than 100 volts. The resulting plasma tempera-ture reaches more than 15,000ºC, high enough to melt anymaterial. When the electrode is part of the electrical circuitand is melted as a filler metal as the weld progresses alongthe joint, we call it a consumable electrode, and it’s dispensedeither as a rod or a coil of wire that unwraps as needed. Witha rod, an operator holds the tip of the rod next to the desiredjoint area and gradually feeds rod from the electrode holderwhile welding along the joint. When the rod gets too short, heor she stops the weld and inserts another rod into the holder.When a coil of filler metal is used, the wire is automaticallyfed at a constant rate to the electrode holder until the coil isfinished. With a coil, welding is interrupted less frequently.

One of the main problems in welding is that as the materialsare melted, air can contaminate the molten mixture. Air cancause oxidation of the metals, which will cause them not tosolidify into a homogenous mixture. To avoid this, most weld-ing processes use some method to shield the molten metalsfrom the atmosphere during the weld. The simplest form ofarc welding uses a filler metal rod covered with a baked-onpowered flux material (Figure 47). As the rod melts, the pow-der also melts and releases an inert gas and liquid that keepsair from contaminating and oxidizing the molten metals untilthey’ve solidified. The flux mixture also solidifies as a slagmaterial that’s on top of the weld, and which must be removedmechanically after the weld is finished. This method is knownas manual metal arc welding (MMAW) or stick welding. It’s oneof the most versatile and common of all welding processes,but requires great skill to produce reliable welds.


Another variation of protecting the weld with an inert gas is a welding process called submerged arc welding (SMAW orSAW). Here, the welding is usually done with a mechanicallycontrolled welding head that feeds a coil of filler metal wire to the welding area. The gap between the filler metal and theworkpiece is controlled by machine rather than by hand. Whenthe weld is started, powdered flux is fed to the weld area froma hopper through a tube in front of the moving tip, and thepowdered flux keeps the weld covered. Unmelted flux is vacuumed up after the tip passes; melted flux forms slag.

Both SMAW and MMAW leave a slag that must be removedmechanically after the weld is complete. An improvement ofthese methods for some applications uses an inert gas insteadof a powdered flux. These methods are called tungsten inertgas welding (TIGW or TIG), and metal inert gas welding (MIGWor MIG), and the processes are identical except that TIG usesa tungsten electrode that’s not consumed during the weld,whereas MIG uses a metal filler metal that’s consumed. BothMIG and TIG welds have the electrode mounted in a weldinghead that has a tube where an inert gas can flow around theelectrode during the weld (Figure 48). The flow of gas forms a“blanket” around the molten metal that keeps out air. Gasesused in these processes include mixtures of CO2 and argon forlow-carbon steels, and helium or argon for stainless steels,aluminum, and nonferrous metals. In MIG welding, the elec-trode is the filler metal and is fed from a reel as the weld isdone. In TIG welding, the filler metal is fed from the side, asin manual welding. Because filler metal isn’t moved across agap, it produces high-quality, slag-free welds that are popular

Core Wire

Flux Covering

Parent Material

Weld Pool

Consumable Electrode

Slag on Surface

Weld Material

Gas Shield

FIGURE 47—Stick weldingis simple and easy to setup, and produces reliableweld joints.


for precision welding in aircraft, nuclear, electronics, chemical,and food industries. TIG welding can be highly automated, as shown in Figure 49, which shows a setup for producingpipe welds.

Electron beam welding (EBW) is very similar to electron beammachining, and uses a high-velocity stream of electrons focusedon a workpiece whose movement is controlled automatically.Electrons from a high-voltage source—up to 150 kV—generatea beam hot enough to melt any material. Since electrons don’ttravel well in air, the entire process must be done in a highvacuum. This limits EBW to specialized applications like

Nonconsumable Electrode

Filler WireGas Shield

Parent MaterialWeld Material

Weld Pool

TIG Welding(B)

Gas Shield

Parent MaterialWeld Material

Weld Pool

MIG Welding(A)

Contact Tube

Consumable Electrode

FIGURE 48—Automatedwelding can producehigh-quality weld quicklyin specialized setups thatare often used for criticalwelds in chemical ornuclear power applications.


welding refractory metals, vacuum tube assembly, and spe-cial drilling. An advantage of EBW is that it can join even dissimilar metals, something most other processes can’t.

Lasers can generate light beams with very high energy levelsthat can be used for both material removal as well as welding.Most laser-beam welding (LBW) is done by a neodymium/yttrium-aluminum-garnet laser similar to those used for cutting,and the light beam is directed to the weld joint by a fiber-opticcable. Laser beam welding doesn’t normally require filler metals,and doesn’t need to be done in a vacuum, but it does requirean inert gas shield to protect the weld joint from oxidation.

FIGURE 49—Pipe Welding


LBW equipment is very expensive, and normally used only for high-volume production of critical quality joints or forunique applications.

While metals are usually good conductors of electricity, there’salways some resistance to any electric current that passesthrough them. When two pieces of metal are held firmlytogether, there’s also a significant resistance at the gap wherethey meet. If an electric current is passed through this gap,heat is generated according to the well-known principle ofOhm’s law:

H = I 2 � R � t

Where H = the heat produced in joules, R is the resistance ofthe joint in ohms, I is the electric current in amperes, and tis the time in seconds.

Resistance welding takes advantage of the fact that moreresistance occurs in the joint than in the base metals, and ifa high electric current is passed through the joint, heat willbe developed everywhere the current flows (Figure 50). As anaid to the welding process, the metal parts are held together bya mechanical pressure that squeezes the molten metal areas,producing a uniform weld. Because the current requirementsare so high, however, the welding current can be maintainedfor only a few milliseconds, limiting the types of welds thatcan be done with resistance methods.

One of the most popular uses of resistance welding is for spotwelding, used mostly to join sheet materials. The metals areplaced between a set of electrodes that works as a press, anda pulse of current is passed through the electrodes either bya manual trigger or by a predetermined force setting. Spotwelding is easy to automate, and robots are often used inautomobile assembly plants to join sheet-metal parts togetheror to attach them to frame members. Spot welding can alsobe done by hand, and without access to both sides of thesheets, with hand welders.


Another type of resistance welding is seam welding, where theelectrodes are made from wheels that press the metal sheetstogether and periodically pass a current through them to makea continuous seam. Seam welding techniques can be made toproduce air- and liquid-tight joints in cans and drums at relative-ly high speeds. Seam welding is mostly used in manufacturingcontainers, radiators, and heat exchangers.

Force

Force

Electrode

Current Source

Spot Welding(A)

Electrode Wheel

Drive Wheel

Workpiece

Electrode Wheel

Seam Welding(B)

FIGURE 50—Intense heatis generated as currentpasses through the resist-ance of the gap betweenthe two metals. Spotwelding is a commonmethod of joining sheetmetals in many indus-tries. Seam welding usesconductive rollers thatpinch the seam betweenthem as the current pass-es through the metals.


In projection welding, another variation of spot welding, thesheets have raised dimples and are pressed between two flatelectrodes. When a current passes through the electrodes,the sheets are welded together at the raised points. Stillanother variation is called flash butt welding. Two wires arebutt welded together by connecting them to a high-currentsupply, which generates a high-current arc and jams the endstogether as the metal is melted. This technique is suitable formaking continuous long lengths of wire rope and cable.

Gas welding refers to the use of oxygen and acetylene (aflammable gas) to make a very hot flame for welding and cutting. Varying the mix of oxygen and acetylene makesflames of different temperatures that can be used for welding,cutting, soldering, and brazing. Different nozzles can change thetip temperatures and flame patterns for different applications.Most gas welding is done by hand, and it’s best suited to general-purpose and maintenance welding, as it’s seldomsuitable for higher-volume manufacturing operations. Gaswelding is very portable and doesn’t require a power source, soit’s used extensively in automobile repair and other mechanicalassembly jobs.

Friction welding and ultrasonic welding are the two mainexamples of solid-state welding, in which no plasma formationoccurs from arcs or flames. Friction welding generates theheat required to melt the base part materials by mechanicalmotion, usually rotation. One of the parts is rotated at a veryhigh speed, and it’s then jammed into the stationary part,generating large amounts of heat from the friction betweenthe relative motions. As the rotation slows, the parts are heldtogether and a solid joint forms as the friction melts the basemetals and resolidifies. Rotational speeds for small parts maybe up to 80,000 rpm, but can be much less for larger parts.Thermoplastics can also be friction welded.

Ultrasonic welding uses a high-frequency sound to vibratemolecules of the base materials at the weld gap. An electricaltransducer causes the vibration of the parts at frequencies ofup to 200 kHz, and the relative motions of the parts generatea mechanical friction that in turn generates enough heat tomelt and fuse the materials. Ultrasonic welding is very fastand can join dissimilar materials easily. Thermoplastics canbe welded ultrasonically with good results.


Brazing and Soldering

Brazing and soldering are joining techniques that use anintermediate material with a lower melting point than thematerials being joined. The braze or solder material can be a pure substance such as copper, gold, or silver, or it can bean alloy of two or more substances, such as copper and gold,or tin and lead. Braze materials are usually supplied as thinsheets or wire of various diameters. Technicians who designbraze and solder joints often must calculate the volume ofthe braze material so that there’s just enough to fill the gapwith no excess.

Brazing and soldering can be done by placing the parts in an oven to uniformly heat the braze assembly. Sometimes asimple flame from a torch can be used. Manufacturing opera-tions requiring consistent, reliable brazes are most likely touse a gas or electric oven for controlled heating and cooling.Parts should be brought slowly to a temperature just below themelting point of the braze alloy, allowed to stabilize briefly, thenraised to a point above the braze material melting point forlong enough to melt all of the material (usually no more thana few minutes). The melted braze material is drawn into thegap by capillary action. The parts are then quickly lowered toa temperature below the melting point of the braze material,and then slow cooled to room temperature.

Brazing is a relatively slow joining process, but yields extremelystrong, vacuum-tight joints between many different types ofmaterials (Figure 51). Metals and ceramics can be easily joinedby nickel coating the ceramic at the joint area, where the brazematerial will “wet” the plating on the ceramic. This is an impor-tant process in producing vacuum tubes and high-voltageelectrical equipment. Brazing allows two or more incompatiblematerials that can’t be welded, such as copper and steel, tobe reliably bonded.

When two like materials are joined, design of the joints canusually ignore temperature changes of the parts, since bothwill expand and contract at the same rate. However, whenthe two parts are made from different materials with differentcoefficients of expansion, the design of the joints must accountfor the dimensional changes of each material when it reaches


Induction Brazing Unit

Brazed Joint

Part to BeBrazed

Support (Jig) to Maintain Workpiece in Correct

Position

Coil

Shaft with Collar to Be Brazed

Joint to Be Brazed

Electric Power Connection

Insulating Shell

Molten Chemical (Flux) Bath

Electrodesfor Heating

Bath

QuartzLamp

Brazing Atmosphere

Reflector

Assemblyto Be

Brazed

PreplacedBrazingFillerMetal

Retort or Furnace

FIGURE 51—Brazing pro-vides strong bonds formany different types ofjoint configurations.Where materials havedifferent coefficients of expansion, care isneeded in the design ofholding fixtures.


the braze temperature. There must be a uniform gap whichthe braze alloy can fill when it melts, typically on the order of 0.001 to 0.002 inches. The room-temperature dimensionsof each part must be calculated to allow for dimensionalchanges at the temperature of the braze, preserving the nec-essary gap. Sometimes fixtures must hold the parts to fixeddimensions while being heated, and the dimensional changesof the fixtures must also be taken into account! If there arelarge differences in the expansion rates of two materials beingjoined with brazing, one of the materials must be reduced toa thin section where they’re joined in order to allow one to“give” slightly when it’s cooled back to room temperature.Otherwise, as the parts shrink on cooling, thermal stressescould tear the braze joint apart.

Figure 52 shows a vacuum-tube assembly for a night visiondevice that has been assembled using both MIG welding andbrazing. The ceramic tubes were nickel plated and then brazedto Kovar (metallic material that’s often bonded to ceramics)flanges. The Kovar end flanges were welded on for a vacuum-tight seal prior to processing.

FIGURE 52—Brazing and welding were used to assemblethis low-production, high-performance device. Braze jointdesign included allowances fordifferences in expansionbetween the ceramic and theKovar.


Soldering is similar to brazing except that soldering is generallydone at temperatures below 500ºC (Figure 53). The heat isoften manually applied by an electric heater or gas flame torch.The heated parts melt the applied solder, which is drawn intothe joint by capillary action. A paste flux is often applied to theparts to prevent oxidation prior to the soldering, or the soldermaterial itself may have a flux core within the wire. Solderingis used extensively in electrical-assembly manufacturing, wherethe electrical parts and the copper etchings on printed-circuitboards are soldered together by wave soldering. In wave soldering,the board with the components is held just above a liquidreservoir of molten solder and the tank is agitated to producea small wave that travels from one end of the tank to the other.The solder that washes onto the bottom of the board quicklycools, fixing the components in place. Common solder materialsinclude alloys of tin and lead, tin and zinc, lead and silver, zincand aluminum, and cadmium silver. Lead solder can’t be usedin cooking utensils or potable water pipes, where people mightingest it.

Simple Lap

Joggle Lap

Double Strap

Seam

Electrical Connections

LeadSolderCopper

Sheet Metal

Flanged T

Joggle Pipe Joint

End Cap

Tube Joints

Base

FIGURE 53—Solder joint configurations are similar to braze joint designs. Since the temperatures arelower, differences in expansions of the materials aren’t as great a concern.


Adhesive Bonding

Adhesive bonding is an alternative to joining materials bybrazing or welding, especially with materials that can’t beheated, such as thermoset plastics and some types of coated orpainted materials. Most materials, including metals, plastics,composites, wood, glass, paper, and ceramics, can be suc-cessfully bonded by carefully designing the bond joint andselecting the adhesive.

Adhesives can be manually applied, but high-volume manu-facturing operations often require a system of automaticallyapplying adhesives with dispensers or robotic arms. Adhesivebonding is a very flexible process that can simplify assemblyprocesses, reducing overall costs and the total number ofparts. The curing rate of the adhesive will determine manu-facturing rates, and fixtures are recommended for holdingparts until fully cured.

Adhesive can be of many different materials, such as

• Epoxy resins—two-part mixtures of resin and hardener,cured with heat

• Anaerobics—adhesives that cure without oxygen, such asthread lockers that mate close-tolerance machined parts

• Cyanoacrylates—the so-called “super glues” that cureinstantly between flat parts, using the surface moistureas the hardener

• Hot melts—thermoplastics that melt and bond upon cooling

• Phenolics—based on phenol formaldehyde thermosettingresins, which cure with heat or pressure, for strongstructural bonds

• Polyurethanes—similar to epoxies, fast-acting, and usedfor low-temperature, low-load applications


Other types of adhesives are also used for special applications,and new types of adhesives are constantly being developed.The key to successful adhesive joining is using the properadhesive in a well-designed joint (Figure 54). Adhesive jointswork best when external forces are spread out over a largearea, and lap joints tend to work best. Adhesive joints don’twork well with cleavage or peeling forces. Surface preparationis critical to a good bond, and the surfaces should be pre-pared by degreasing, degreasing and sanding, or by chemicalcleaning. Lightly sanded joints are stronger than those betweenpolished surfaces.

Mechanical Fastening

Mechanical fastening refers to a tremendous variety of methodsto join two or more objects, and include screws, bolts and nuts,rivets, pins, clips, and even such methods as press fits orshrink fits (Figure 55). One general classification separatespermanent from nonpermanent mechanical fastening methods.

ForceAdhesive

Butt

Force

Incr

easi

ng S

tren

gth

Simple Lap

Joggle Lap

Stepped Lap

Tapered Lap

Scarf

Strap

Double Lap

Joggle with Strap

Double Strap

Tapered Double Strap

FIGURE 54—Good strength in adhesive bonding requires proper joint design to compensate for adhesive’s relatively low shear strength.


Clearance Hole

Threaded Hole

Machine Screw

Side View

RivetPlatePlate

Parts Being Joined

Grooved Pin

Grooved Pinin Position

FIGURE 55—There are many ways to join parts with mechanical fasten-ers, either permanently or temporarily.


Permanent mechanical fastening can be done in several ways:

• Press fits and shrink fits—this process involves matingtwo parts whose dimensions at room temperature inter-fere with each other, usually on a diameter. For example,metal pins are often used in jigs and fixtures to locateparts for assembly. Pins with a slightly larger diameterthan the hole are pushed into the hole by means of apress. The pin and hole are deformed when the pin ispressed into place. Shrink fits do much the same thingexcept that the material containing the hole is raised intemperature until the inside diameter of the hole willallow the pin to slide in easily. When the part cools, itshrinks until it grips the pin firmly. Shrink fits can bedone on some parts other than pins and holes.

• Interlocking joints—permanent assembly can be done withparts that have flat edges or sheet components by plac-ing two thin edges together and rolling together. Metaltabs can also be used by cutting a tab in one sheet, aslot in the mating part, and bending and inserting thetabs into slots. This type can be disassembled severaltimes, if necessary.

• Rivets—riveting can be used when at least one of theparts is sheet material and matching holes can be lacedin both parts. Holes are drilled in both pieces and ametal pin is placed in the hole where it’s upset to makeits ends or diameter larger, gripping the parts and holdingthem together.

Nonpermanent fasteners have an extremely large number of variations that can hold two or more parts together.Nonpermanent fastening can be divided into two categories,semipermanent and temporary. Temporary fasteners allow foreasy disassembly, whenever repairs or required maintenanceis necessary. These types of fasteners include the familiarfamily of threaded fasteners such as screws, nuts, and boltsas well as other methods such as internal and external snaprings or clips, keyways on shafts, taper and roll pins (springpins), hose clamps, U-bolts, and many others too numerousto list. Threaded components, as you’ve learned from previ-ous units, have standardized thread forms and dimensionsso that parts can be easily interchanged.


Additional Processes

In many cases, after a part or assembly is manufactured,additional processes are required to make it functional orattractive. Material properties can be optimized for differentcharacteristics such as strength or hardness, or the part canbe altered for aesthetic purposes by adding decorative coatingsor paint. The coatings can serve a dual purpose by changingsurface physical properties and providing a more attractiveappearance. Anodizing aluminum is one such treatment. It canbe done in several different colors, and it makes the surfaceof the part extremely hard and electrically nonconductive.These alterations to a finished or near-finished part are clas-sified as surface coatings, bulk material heat treatments, orsurface treatments. Figure 56 shows how these processes canbe classified, and how many variations of each are possible.

Heat Treatments

As we mentioned before, processes such as forging, extruding,or stamping change the properties of metals by work hardening.Work hardening occurs when the grain structure of the as-castmaterial is deformed, breaking the soft, large grains into smaller,stronger grains. This makes the material much harder andtherefore more difficult to do subsequent required operations.It also leaves residual stresses in the material that make itsusceptible to cracking and to corrosion if left as a finishedproduct.

Heat treatment relieves internal stresses, to make the grainslarger so that more manufacturing processes can occur without fracturing the part, and also to make the materialstronger or tougher.

Different metals and alloys react differently to combinationsof heating, cooling, and rates of temperature change, andmuch information is published by organizations such as theASME, ASTM, AISI, and the CDA. Consult their technicalinformation for specific alloys when you need to learn moreabout the effects of heat treatment.


A type of heat treatment for low-carbon steels that’s actuallya form of surface treatment is commonly known as casehardening or carburizing. Low-carbon steel is inexpensive andcan be machined easily, but isn’t suitable for many applica-tions because it’s relatively soft and wears quickly. In casehardening, the part is heated to 800–900ºC in charcoal forseveral hours to several days. The charcoal supplies a largeamount of carbon that readily diffuses into the surface of thesteel to form a layer of high-carbon steel. The part is thenquenched in water or oil. High-carbon steel, when quenched,is extremely hard and wear resistant. Case hardening is usedto harden surface layers of knives, cutting utensils, gear teeth,bearings, axles, shafts, and bushings.

Bulk Treatments

Softening (Annealing/Tempering)

Hardening (Aging/Full Quench/Step Quench)

Stabilizing (Freezing/Normalizing/Stress Relieving)

Surface Coatings

Cladding (Adhesive/Braze/Diffusion/Explosive/Roll)

Oraganic (Elastomeric/Metal-Loaded/Plastic)

Inorganic Chemical (Electroless Nickel/Oxide/Stove Enamel)

Electrochemical (Electroplating/Electrophoresis)

Hot Dip (Zinc/Aluminum/Lead/Tin Alloys)

Thermal Sprayed (Carbides/Nitrides/Polymers/Oxides/Metals)

Weld Coatings (Cobalt/Carbides/Nickel Alloys/Copper Alloys/Steels)

Vapor Deposition

Surface Treatments

Thermal Beam or Laser Hardening

Thermochemical

Mechanical (Shot Peening/Rolling)

Passivation

Diffusion

Ion Implantation

(Anodizing Phosphates)

FIGURE 56—Once a part is manufactured, additional processes are often necessary to make the partusable or attractive. A wide variety of finish treatments are available.


Bulk and Surface Processes

After a part is manufactured, additional steps are often need-ed to finish the product or make it suitable for its intendedapplication. Many times, this involves some type of surfacefinishing or treatment. There are three main reasons fortreating the surfaces of manufactured parts or assemblies:

• Cleaning

• Improving the surface finish by polishing or smoothing

• Adding protective or decorative coatings

Surface cleaning can be done mechanically or chemically.Mechanical processes include wire brushing and sandblasting,but some manufacturing operations use processes such astumbling and vibratory finishing. Tumbling is simply placingthe parts inside a slowly rotating container that allows theparts to randomly hit each other, removing burrs, sharpedges, and excess materials. Vibratory finishing is similarexcept that the parts are immersed in an abrasive materialand vibrated as they’re moved around in the container. Forexample, brass casings for bullets are prepared by vibratingthem in finely broken walnut shells. This makes them cleanand shiny, and removes burrs and sharp edges. The abra-siveness and particle size of the cleaning material is easilycontrolled to produce the desired finish, and vibratory clean-ing is easier to automate, and thus has become more popularover the years.

Chemical cleaning is an important way to remove surfacedebris and contamination, and is often used immediatelyafter machining operations. Degreasing is another term oftenused for chemical cleaning, and the methods often uses asolvent such as trichloroethylene heated to about 100ºC toremove oils, cutting fluids, and corrosion inhibitors that maybe on the parts. Parts are usually placed in a basket, immersedin the hot solvent for a few minutes, and then left to soak inthe vapors for several minutes. As with any process that uses asolvent, appropriate safety procedures must be used to preventinjuries from exposure to solvent fumes and liquids.


Another process becoming more popular, especially for smaller parts is ultrasonic cleaning. Parts are immersed in aliquid—sometimes a solvent such as alcohol—and placed in a tank that vibrates them at ultrasonic frequencies of 25–40kHz. The vibration causes cavitation, which is when largeamounts of bubbles form and collapse rapidly. This quicklyremoves unwanted surface particles. Ultrasonic cleaning isalso used to clean delicate parts and jewelry.

Surface-smoothing processes can also use mechanical andchemical methods. Grinding is the most severe, with bothexternal debris as well as base materials easily removed bycoarse grinding wheels. Grinding is often used as a prepara-tion for welding, especially for filling defects in castings.Somewhat milder in effect is sanding, with either handsanders (pneumatic or electric) or belt sanders. Sanding isoften done in a sequence from coarse grits such as #24 tovery fine grits such as #400. Sanded surfaces can be primedand painted with durable coatings of enamel, lacquer, andepoxy paints, forming an attractive, long-lasting protectivecoating. Polishing uses an even finer abrasive than sanding,usually with a type of cloth wheel to rub the paste onto thesurface to be smoothed. With enough effort, polished sur-faces can be made almost mirror-like in appearance.

Electropolishing is a chemical means of surface smoothing,and is the reverse of electroplating. Where electroplatingdeposits a smooth coating on the surface, electropolishingremoves a thin layer of material. This has the effect of elimi-nating small scratches and machining marks, making thesurface shiny and mirror-like. Bacteria don’t grow well onelectropolished surfaces, making this finishing method excel-lent for products used in food, drug, and medical products.

Coatings include all methods of placing a thin layer of another material over the base part to provide protectionagainst an environment, increased functionality (such as adecrease in wear rates), or a more attractive surface.

One major use of coatings is to provide sacrificial protectionin a severe environment such as parts or products exposed to weather. You may be familiar with the zinc-coated screwsand nails used in building construction. These fasteners aremade from steel, and a zinc coating will actually corrode


before the steel, leaving the fastener intact and strong. Thistype of coating provides cathodic protection against corrosiondue to the preferential attack of the environment against thecoating instead of the base part. Galvanizing is the term usedto apply this same type of zinc coating to nails as well assheet steel that’s used as inexpensive roofing material andpanels used in some type of vehicle construction.

When applied, conversion coatings actually change the outerlayer of the part into a different material, which then acts asa protective layer. Probably the most common conversioncoating is anodizing. Anodizing is used on aluminum partsand the process converts the surface layer of about 0.0005inches up to about 0.002 inches from aluminum to alu-minum oxide, which is actually a type of ceramic. Anodizingis extremely hard—over RC 60—and is an electrical insulatoras well. Anodizing can be done in several different colors, soit can be a decorative coating, but it’s applied in a thinnerlayer. Figure 57 shows how the oxide layer is built up fromthe metal surface, with small pores in the center of each cell.The coloring agents are added to the pores. Teflon and otherlubricating materials can also be impregnated, making thismaterial suitable for low-friction bushings. Anodizing can beused to make a hard surface that’s wear or weather resistant,such as for bearing or bushings, and may also be applied tomagnesium and its alloys in addition to aluminum.

Coloring Agents

Aluminum

Oxide Layer

Anodic CellPore

FIGURE 57—Aluminum oxide is the second-hardest material known, next to diamond, and it gives analuminum surface a hard protective coating that can be colored for decorative purposes.


Another conversion coating is phosphating, widely used to treatsteel prior to painting. A dilute solution of phosphoric acid isused to convert about 75 �m (1 �m is approximately 40 mil-lionths of an inch) of the base material into a phosphate layer,which is corrosion resistant. Phosphating also works withiron, aluminum, or zinc. Chromating is a similar process butyields a much thinner coating—about 0.01 �m—and is usedprimarily with aluminum, zinc, magnesium, and cadmiumand their alloys.

Another method of coating parts is by electroplating. In thiscoating method, the parts are immersed in an electrolyte bathalong with the metal to be plated onto the part. The part isconnected as cathode (negative terminal of the voltage source),and the metal to be deposited is connected to the positive terminal. The amount of voltage and current determines therate at which metal is plated onto the part.

Table 6 shows some common plating materials and typical thicknesses.

Table 6

METALS USED FOR PLATING

Plating Material Coating Thickness, ��m

Cadmium 5–10

Chromium—decorative uses 0.2–1.0

Chromium—industrial uses 75–500

Copper 10–50

Nickel 8–500

Silver, gold, platinum 0.5–100

Tin 0.1–2

Zinc 2.5–25


An exotic method of coating is now available that can apply spe-cialty materials for unique applications, especially in cuttingtool applications. Chemical vapor deposition (CVD) is a way toapply extremely hard coatings such as titanium carbide (TiC),titanium nitride (TiN), aluminum oxide (Al2O3), and titaniumcarbonitride (TiCN). These coatings provide an extremely wear-resistant coating to the cemented carbides used for cutting toolsfor machining processes and have revolutionized cutting-tooltechnology. In CVD, the parts are heated to about 1000ºC ina controlled, chemically reactive atmosphere containing thetitanium, carbon, and other elements. CVD will deposit coatingsof up to 12 �m. A variation called physical vapor deposition(PVD) is done at about half that temperature, and is used pri-marily for coating very sharp tools such as drill and end mills.Drills, end mills, taps, and dies often have a yellow color; thisis a thin coating of TiN. These processes are mostly automat-ed today, and are able to produce the millions of cutting toolsdemanded by modern manufacturing businesses.

Assembly Processes

Up to now, we’ve discussed manufacture of individual parts;however, a product is usually a group of mating parts that areassembled into useful device. Almost any product you can thinkof is an assembly, and a great deal of product design andmanufacturing effort centers on how to collect the individualparts in their proper places for assembly and packaging.Assembly operations in the past always involved the manuallabor of people, although in the future more and more assem-bly will be done partly or entirely by automated systems. Thereare distinct advantages to having people assemble productsas opposed to machines. But as technology improves, lesshuman involvement will be necessary.

All methods of assembly must address these basic concepts:

• Feeding—the presentation and orientation of the parts andtools to the assemblers (whether people or equipment)


• Handling—the movement and positioning of parts orcomponents to the place where assembly takes place

• Fitting—putting the components in place with jigs, fixtures, and pins, where they’re joined by welds, adhesives, and fasteners, using appropriate tools

• Checking—detecting and correcting errors due to missing parts, defects, false presentations, breakages,and foreign objects

• Transfer—moving the completed assembly from the workstation to subsequent workstations for additional processing

All of these steps may be done by people, by machines, or acombination of the two. Henry Ford is famed for having usedassembly lines to break a complex assembly into a series ofsimple sequence of low-skill steps performed on the parts onmoving conveyor lines. In fact, he simply applied the knowl-edge and concepts of other scientists and engineers, such asFrederick Taylor. Mechanization developed over the decadesas tools became better and the use of jigs and fixturesbecame commonplace, ensuring reliability and repeatability.From mechanization, the idea of automation developed:replacing the “unreliable” human operator with machinesthat could tirelessly perform precise, repetitive motions. Fullautomation, however, requires extremely accurate parts andaccurate positioning and placement capability on the part ofrobots, which may be extremely expensive.

Problems with assembly operations are mostly the result offactors related to

• Components out of tolerance

• Gross misalignment or adjustment errors

• Malformed parts, missing features, damage

• Foreign matter causing contamination, jamming, or blockages

• Absence of parts due to exhausted supply lines or inefficient feeding

• Incorrect parts for different models or variations


• Improper or inadequate joining methods

• Lack of adequate operator skill

Any consideration of assembly methods should address all ofthese factors when analyzing the true costs of manual versusautomated methods in manufacturing processes.

The trade-offs between using people and machines involveanalysis of the requirements of production volumes, equipmentcosts versus labor costs, quality of parts and subassemblies,and safety hazards in the manufacturing environment. Inthis section we’ll look at several different ways of performingassembly operations, although you may see other differentclassifications as you continue your studies.

Figure 58 shows three classifications of assembly systems,manual assembly, flexible assembly, and dedicated assembly.Many factories operate with considerable overlap of these systemsin many situations, so hard-and-fast categorization often can’tbe done. We’ll discuss each of these systems, but you maynever run into a “pure” example of any of these methods.

Manual Assembly System

FlexibleSystem

Dedicated System

Single Station Transfer System

IntermittentContinuous

Conveyor CarouselSynchronous/Indexing

(in-line, rotary)Nonsynchronous

Single or Multistation Synchronous Transfer System Nonsynchronous

Synchronous

FIGURE 58—Assembly systems can be classified by the amount of automation incorporated into theworkstations, although these systems tend to overlap in many cases.


Early manufactured products were all assembled by hand, asmachine tools weren’t widely available and most products werecustom made. As factory systems became more prevalent,people were used to assemble a collection of previously man-ufactured parts into completed products. As mentioned before,most people recall Henry Ford’s contribution to the develop-ment of the production line and the ability of products to bemass produced. Ford used one type of manual assembly, withconveyor lines transferring product between workstations.

Manual assembly takes a variety of forms: the single work-station,a workstation with in-line transfer, and in-line transfer withmechanized assistance (Figure 59). Manual assembly is usedto assemble products such as automobiles, toys, appliances,machine tools, furniture, footwear, and clothing. Productsdesigned for manual assembly should use design-for-assembly(DFA) techniques to optimize the design by minimizing assem-bly operations and parts count. Assembly sequence diagramsshould be developed to aid the operators and managers, andfixtures and component features should be designed to preventimproper assembly and other errors.

Part Holder

Part Storage Bins

Single-Station In-Line Transfer System Transfer System with Assistance

Part FeederParts

Conveyor

FIGURE 59—Manual assembly systems are the most labor intensive, but often use mechanization,such as power tools and conveyor lines.


Manual assembly methods are very flexible, as people can adaptto many different situations and can be trained to accommo-date many different variations of a particular assembly. Theycan also immediately recognize and correct errors, a skill thatonly the most advanced robots offer today. Flexible and dedi-cated systems incorporate much more automation and oftenaren’t as adaptable as manual assembly.

Flexible assembly methods integrate a substantial amount of robotic or automated equipment to assemble parts or subassemblies at workstations. Materials can be organizedand presented in bins, and picked up by robots and accu-rately placed on the assembly; parts can be held by vacuum, pneumatic, or hydraulic actuators; robots can weld, paint, orassemble parts and subassemblies; and completed products canbe transferred to subsequent stations by robots or conveyorlines. Human involvement may be minimal for these types ofassembly systems. Figure 60 shows examples of two types offlexible assembly stations. About half of all flexible assemblyproblems are caused by inadequate incoming part quality.

As you can imagine, such a system is expensive and it cantake a long time to set up such a factory. All of the assemblyproblems we mentioned before become important to addressas these systems, once configured, despite their name, aresomewhat inflexible in adapting to product modifications.

Pallets for Parts

Robot

Holder

Workstation

Parts

Single Station Synchronous Transfer System

Conveyor

FIGURE 60—FlexibleAssembly Systems


Tooling costs are high, production rates are moderate, andthe installation process can be lengthy. These types of sys-tems require a highly skilled workforce for programming andmaintaining the equipment, although one technician canservice many machines and much manual, low-skill labor is eliminated or minimized.

Despite some of the disadvantages listed, flexible assemblysystems can be desirable for many applications. They’re particularly useful for automated painting and welding operations, abrasive jet machining and cutting, and variousgrinding procedures. They’re a necessity for hazardous envi-ronments in places such as nuclear reactors, hazardousmaterials manufacturing and chemical plants, and biologicalhazard rooms.

Dedicated assembly systems are fully automated, special-purpose assembly workstations that completely assemble oneproduct or subassembly (Figure 61). Often, a robotic transfersystem will move one completed subassembly to anotherworkstation, to be used as a raw material in the next assembly.Various types of bowl feeders, magazines, and vibratory platesorient parts. Robotic arms or different types of escapementmechanisms handle parts and place them in the proper position,and assembly is done with procedures that lend themselvesto automation such as welding or adhesives.

Synchronous Transfer System

Carrier

Part Feeder

StationaryWork Head

Conveyor Line

In-Line Transfer System

Part Feeder

FIGURE 61—Dedicated Assembly Systems


Dedicated assembly systems are used advantageously in electronics assemblies where components are picked andplaced on a circuit board and then moved to an automaticwave soldering machine for final assembly. They’re also usedfor assembling subassemblies such as relays, motors, smallappliances, and toys. Production rates for such methods arevery high and direct labor costs are very low. But equipmentcosts are high, and setup and installation of dedicated sys-tems takes time, so they’re suitable only for high productionvolumes. Dedicated systems are almost totally inflexible.Changes to the product are hard to incorporate, unlessthey’re simply an absence of components or parts from anoriginal design. Again, these systems excel where issues ofoperator safety in hazardous environments are present.


Self-Check 5


1. Four general classifications of parts-joining methods are _______, _______, _______, and_______.

2. A metallurgically compatible material used to aid the fusion of the base materials is _______.

3. Manual welding using a flux-coated electrode is called _______ welding.

4. Two welding processes that don’t leave slag over the welded area are _______ and _______welding.

5. A process that’s commonly used to join metals to ceramics is _______.

6. _______ is similar to brazing except that it uses alloys that melt below _______ ºC.

7. The _______ of adhesives will determine how fast manufacturing processes can take place.

8. A permanent mechanical joining method that upsets one or both ends of the fastener is _______.

9. One of the principal reasons for using heat treatment during manufacturing is to _______.

10. _______ assembly is more adaptable than flexible assembly, but is not able to provide as highproduction rates.

11. _______ is a surface treatment that’s useful for food preparation products because bacteriadon’t grow well on the surface.

12. Applying a zinc coating to protect a steel surface from corrosion is a process called _______.

13. One of the most popular coatings for aluminum, which can protect and color the surface, is _______.



NOTES

111

An

sw

er

sA

ns

we

rs

Self-Check 11. primary, secondary

2. primary

3. continuous, discrete

4. durable, nondurable

5. petroleum or petrochemical

Self-Check 21. material removal, material forming, casting/molding

2. contraction

3. pattern

4. shell

5. die, gravity die casting

6. centrifugal

7. injection molding

8. reaction injection

9. Vacuum

Self-Check 31. hot working; cold working

2. oxide scale

3. recrystalization

4. 1020 carbon steel

5. Cold heading/upset forging

6. extrusion

7. drawn

8. sintering

9. oil/lubricant/plastic

10. coining

Self-Check Answers112

Self-Check 41. Machine tools

2. shearing, abrasion

3. Indexable

4. mechanical, thermal, chemical, abrasive

5. Cutting fluids

6. 416 stainless steel

7. lathe

8. Grinding

9. electrical-discharge machining (EDM)

10. Electrochemical machining (ECM)

11. abrasive jet machining

Self-Check 51. welding, brazing and soldering, adhesive, mechanical

2. filler metal

3. stick, or manual metal arc

4. metal inert gas welding (MIG) and tungsten inert gas welding (TIG)

5. brazing

6. Soldering, 500

7. curing rate

8. riveting

9. relieve internal stresses

10. Manual

11. Electropolishing

12. galvanizing

13. anodizing

113

1. Drilling a long, 0.001-inch-diameter radial hole in the curvedsurface of a round rod would require

A. the use of special drilling tools such as an electron beamor laser drill.

B. a special drill with a locating tip. C. a rigid drill press, and starting the hole with a thin drill. D. careful perpendicular drilling.

2. Two technicians are discussing the advantages of rolledthreads on fasteners over machined threads. Technician Asays machined threads can’t be put on long parts. TechnicianB says that rolling is better because it can produce threads inharder materials. Which of the following statements is correct?

A. Only Technician A is correct.B. Only Technician B is correct. C. Both technicians are correct. D. Neither technician is correct.

Ex

am

ina

tion

Ex

am

ina

tion


When you feel confident that you have mastered the material

in this study unit, go to http://www.takeexamsonline.com and

submit your answers online. If you don’t have access to the

Internet, you can phone in or mail in your exam. Submit your

answers for this examination as soon as you complete it. Do not

wait until another examination is ready.

Questions 1–20: Select the one best answer to each question.

EXAMINATION NUMBER

18607700Whichever method you use in submitting your exam

answers to the school, you must use the number above.

For the quickest test results, go to

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Examination114

3. A roll of uncoated wire in the assembly room of a factory is to be used in a joiningprocess. It has a diameter of 0.010” and is labeled with its melting-point temperatureof 780ºC. This wire is most likely suitable for use

A. in forging threaded fasteners. C. as solder wire.B. as raw material to make forging dies. D. as brazing wire.

4. A side effect of forging as a manufacturing process is that

A. it leaves the parts with a superior surface finish.B. the parts are harder and stronger when finished.C. no additional finishing is required.D. it’s easy to machine compared to cast parts.

5. Among the main advantages of investment casting is the

A. ability to create intricately detailed parts.B. ability to cast very large parts with precision dimensions. C. ability to avoid part breakage when removing parts from the mold.D. relative cost compared to sand casting the same part.

6. For high-volume production runs, machining parts from solid material might not be thebest choice of manufacturing operations because

A. manufacturing lead times are too long, and the material choices are limited.B. tolerance capability is too low and piece price is very high. C. tooling costs are high but surface finish capability is low. D. tooling and piece-part costs are high.

7. A foreman ordering drill bits for a high-speed drilling machine would

A. first consider moving the drilling operation to the EDM department because this process is much faster.

B. order chromium plating for increased life.C. use HS steel bits for the longest tool life possible.D. specify TiN coated steel drills.

8. Due to their low peak-power needs, pointing lasers are most likely _______ type.

A. CO2 C. helium neon B. ruby D. argon

9. Two technicians are discussing extrusion processes used in manufacturing. Technician Asays extrusion is used for prototyping new parts because of the ease of producing extru-sion dies. Technician B says that it’s primarily used for manufacturing wire. Which ofthe following statements is correct?

A. Only Technician A is correct. C. Both technicians are correct. B. Only Technician B is correct. D. Neither technician is correct.

Examination 115

10. A screw acts on molten plastic, pressurizing it and forcing it into a mold during

A. compression molding. C. injection molding. B. blow molding. D. thermoset molding.

11. A vacuum tight seal is to be made on a flat plate welded to a cylinder. The edges to bewelded are 0.014� thick and are to be joined without a filler metal. The best processfor this weld joint would be

A. MMAW. C. MIG.B. SMAW. D. TIG.

12. The purpose of the flux coating on a welding electrode for manual metal arc welding(MMA) is to

A. protect the electrode from damage until it’s melted. B. alloy with the melted material to increase strength of the weld. C. generate inert gases to prevent oxidation of the weld. D. form a protective slag coating of the weld area.

13. Spinning would be a preferred process to produce parts such as

A. metal funnels. C. automotive pistons. B. cap screws. D. spark plug heads.

14. An impeller for a pump on a submarine must be made from a single piece of tungsten.The impeller resembles a large screw about 3” in diameter and is 12" long. What manufacturing process would best produce this part?

A. chemical machining C. sand casting B. electrochemical machining D. closed die forging

15. Two technicians are discussing ways of creating different parts. Technician A says thatforming is the process of creating a part by pouring or placing liquid metal or plasticinto a mold. Technician B says that the described process is also called shaping. Whichof the following statements is correct?

A. Only Technician A is correct. C. Both technicians are correct. B. Only Technician B is correct. D. Neither technician is correct.

16. Using mechanically mounted inserts as cutting tools

A. allows rapid changeover to new cutting edges. B. makes the part dimensions more precise. C. avoids the necessity of using cutting fluids. D. allows more complex cuts with turned parts.

Examination116

17. The brown trim color found on aluminum window frames is made by

A. anodizing. C. ECM.B. EDM. D. chromating.

18. A primary manufacturing process usually

A. uses subassemblies to build a final product.B. uses naturally occurring raw materials. C. doesn’t use much manual labor. D. doesn’t require as much capital equipment as secondary manufacturing processes.

19. A carpenter constructed a tool shed that showed streaks of rust from the exposed nailheads after three weeks of rainy weather. This could have been avoided by

A. using chrome-plated nails. B. painting the nails before use.C. using galvanized nails. D. painting the heads after installation.

20. The best way to produce small threaded plastic bottles is probably

A. shell casting. C. vacuum forming. B. sand casting. D. blow molding.

study unit manufacturing processes, part 3f01.justanswer.com/i3hfsufm/186077.pdf4 manufacturing...

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