CHAPTER 10

Metal Cutting Technology

10. 1 Cutting Action and Chip Formation

A thin sharp knife is used to cut very soft materials like bread , vegetables and fruits where parts of the material are separated without chip formation , fig. 10.1

Fig. 10.1: knife Cutting

For cutting materials with a much higher strength like metals and alloys, a much higher cutting force will be needed. Therefore, the cutting tool will be massive.The intensive pressure between the tool face and the work piece and the sharp cutting edge will shear and tear the work piece and separate small pieces from its surface (chip), see fig. 10.2

Fig. 10.2 Cutting Action

The tearing of the chip from the work naturally leaves the work surface in a torn and rough condition . it is at this point that the extreme tip of the tool does its job by trimming of the irregularities and leaving the surface in a fairly smooth condition . A small worn area may be observed at point (A) of a tool which has been cutting for a long time without having reground .

10. 2 Types of chip

Three types of chip can be identified like those shown in fig 10.3. The type of chip resulting from the cutting process is related mainly to the following variables:

- Work piece material - Cutting tool angles, especially the rake angle .

- Cutting speed and cross sectional area of chip .The main three types of chip are:

A) continuous chip which results when cutting soft materials like copper, aluminum and mild steel or when the rake angle is relatively big.

B) sheared chip which results when the hardness of work piece material increases or when the rake angle decreases.

C) discontinuous chip which results when the brittleness of work piece material increases and the rake angle decreases to become zero or even negative.

Fig. 10.3 Types of Chip

10.3 Cutting tool angles Fig 10.4 shows some of the important angles in a single point tool. The clearance or relief angle avoid rubbing of the tool on the work surface. The lip angle provide for the tool strength. For hard materials, this angle is larger than what it should be for softer materials. The rake angle helps the tool peeling the chip instead of pushing it off. The cutting angle controls the cutting operation. If it is less than 90 (negative rake angle) we say that the tool cuts, and if its larger than 90 we say that the tool scraps the work piece.

Fig 10.4 Tool Angles

α Clearance angle: Is the angle included by the tool flank and a plane containing the main motion. Its purpose is to decrease the friction between the tool and the work piece

β Rake angle: Is the angle included by the tool face and a plane perpendicular to the main motion. It helps the cutting and the chip formation.

γ Wedge angle: Is the angle included by the tool face and flank. Its size depends mainly on the work piece material fig. 10.5. If it is too big than required , the tool will need high force to penetrate the work piece. At the same time , if it is too small , it will weaken the tool and cause its rapid failure.

Fig 10.5 Main Cutting Angles

10.4 Cutting conditions for basic machining processes

Cutting speed: The cutting speed (v) is the speed at which the cutting edge travels relative to the machined surface of the work piece.

Feed rate: The feed rate (f) is the distance advanced by the cutting tool relative to the machined surface in a direction which is usually normal to the cutting speed. Its units can be mm/cycle, mm/min, mm/rev, mm/stroke or mm/tooth depending on the type of the machining operation and the tool used.Depth of cut:

The depth of cut (t) is the normal distance from the original surface before machining to the machined surface. As depicted in fig. 10.6

Chip cross section area = depth of cut x feed = t . f mm2

material removal rate( Vm) = depth of cut x feed x speed = t . f . v mm3 / minThe material removal rate (Vm) is the volume of metal removed per unit time , its units should be mm3 / min. Figure 10.6 shows the basic machining processes and the tool / work piece motions.The material removal rate (Vm) can be calculated as follows:

a) shaping Vm = 1 . f . t . N mm3/ min

Soft materials Medium hard materials Hard materials

where N is the number of strokes per minute and all other dimensions are in mm. b) drilling Vm =(πd2 / 4) . f mm3/min where d is the drill diameter in mm and f is the feed rate in mm/min.c) turning

Vm = πD . N . f . t mm3/min Where; D is the diameter of the work piece in mm, N is the number of revolutions per minute, f is the feed rate in mm/revolution and t is the depth of cut in mm.

d) milling Vm = f . n . N . 1 . t mm3/min

Where; f is the feed rate in mm/tooth , n is the number of teeth of the milling cutter, N is the number of revolutions per minute of the milling cutter , 1 is the width of cut and t is the depth of cut in mm.

Shaping Drilling Turning Milling

Fig 10.6 Tool /workpiece motions for the basic machining processes

10.5 Tool materialsTool materials should possess the following properties:

1 .retain its hardness at elevated temperatures. 2 .high wear resistance .

3.high resistance to withstand vibrations and shocks due to intermittent cutting.4 .low coefficient of friction in order to avoid excessive temperature rise due to

friction at the tool-chip interface.

5 .high thermal conductivity in order to absorb the heat generated during cutting

10.5.1 Carbon steel :Carbon content from 0.9 to 1.1%.Tools are forged or machined to the desired shape then hardened and tempered to reduce internal stresses which enables its use up too 200oC.It has good wear and shock resistance. Carbon steel tools are cheap and they are used for low production runs. The most common use of carbon steel is for reamers, taps, dies and simple tools.

10.5.2 High speed steel (HSS):The most common high speed steel available consists of 18% chromium, 4% nickel and 1% vanadium . HSS is superior to carbon tool steel in that the tools retain a cutting edge at temperatures up to 530 oC and at high speed. It is used for drills, reamers, punches and dies, also for lathe and milling cutting tools ...etc .

10.5.3 Cast alloys:Cast alloys are a combination of chromium, carbon, tungsten and cobalt .It is very hard and difficult to machine, therefore, the mixture is cast into shape. It can be polished to a very smooth surface thus reducing friction. It had high resistance to abrasion and corrosion. cast alloy tools will soften at temperatures above 650oC but will regain their hardness when the tools cool. Higher cutting speeds are possible with cast alloys than with HSS for machining most materials.

10.5.4 Cemented carbides:Composed of carbides, a binder and some alloy which is usually composed of tungsten, tantalum or titanium. The tool is made through the powder metallurgy process. Cobalt is the most common binder used in cemented carbide tools. Increasing the cobalt increases the toughness of the tool but reduces the hardness. It is very hard and retains its hardness up to 800 oC.It is used for cutting tools, machinery parts, dies and gauging tools which require high wear resistance at elevated temperatures .

10.5.5 Coated carbides:Cemented tungsten carbide insert with coating of titanium carbide and titanium nitride have higher wear resistance than uncoated ones. Thus both the tool life and metal removal rate are increased .The thickness of the coating layer ranges between 5 - 7 5 microns (1 micron = 0.001 mm).The effect of coating is to reduce friction at the tool-chip interface thus reducing both the cutting force and the cutting temperature.

10.5.6 Ceramics:Ceramic materials composed of aluminum, silicon or magnesium oxides are manufactured by powder metallurgy in the form of tool tips.it is non corrosive non magnetic, non conductive and have high strength at elevated temperatures up to 1000 oC. Ceramic tools are very brittle and will break if subjected to vibrations or intermittent cutting. That is why they are mainly used in finish machining where high speeds and small cutting depths are employed .

10.5.7 Diamonds:This is the hardest known material. As it is very expensive, artificial diamonds are usually crushed and diamond chips are cutting and grinding wheels.

10.6 Cutting Fluids

The primary purpose of lubrication in metal cutting is to reduce the friction between the tool and the work piece which results in increasing the tool life, improving the surface finish, reducing the power required for machining and enabling using higher cutting speed. The required properties of cutting fluids:

a) high flash point . b) low viscosity.

c) non-toxic and harmless to skin . d) should prevent rust and corrosion of the work piece and machine.

e) will not smoke or fog during application. f) Have high absorption and good thermal conductivity.

cutting fluids can be applied by one of the following methods: a) By flooding .

b) As a pressure jet. c)Mist lubrication systems.

in mist lubrication systems, compressed air and the cutting fluid are pumped together, the fluid is evaporated because of the extreme pressure from the compressed air.

finite particles of the vapor are allowed to penetrate into the cutting area for lubrication while the mist mixture carries away the heat from the tool and the work piece.

10.6.1 Type of cutting fluids:10.6.1.1 Straight cutting oils:

a) vegetable oil such as straight mineral oils have good cooling properties and is used in lubricating machine parts.

b) animal oil such as lard oil provide ample lubrication for heavy machining operations.

It is very expensive. It provides a good medium for bacterial growth which causes bad odour and

may cause skin inflammation. c) A mixture of mineral oil and lard has a wide application.

It has a very good lubricating ability and has low cost. 10.6.1.2 Soluble oil:

Combination of oil and water using an emulsifier such as soap. It is inexpensive and is used in grinding, milling and turning operations.

A mixture of 10:1 (10 parts of water to one part oil ) is suitable for milling while a mixture of 100:1 is used for grinding.

Chemicals such as chlorine and sulphur are added to enhance the properties of the fluid.

A chemical reaction occurs because of the pressure and heat which causes the chlorine or sulphur to from a chloride or sulfide film over the work piece and tool . the film reduces friction , thus , reducing the temperature in the cutting zone.

10.6.1.3 Synthetic cutting fluids:It consists of a chemical mixed with water . The chemical additives aid in reducing friction, prevention of built-up edge and rust while water provides the good cooling properties .

10.6.1.4 Solid lubricant:wax and soap are the most common .the operator rubs the lubricant over the cutting tool .Another solid lubricant is Teflon which is coated over the cutting tool then cured, thus, provides a means of self lubrication and reducing friction .It is used in cutting tools for wood or other non metallic materials .10.6.1.5 Kerosene: kerosene or a mixture with mineral oil, lard or soluble oil provide a means of lubrication and cooling the cutting area .It is used for cutting aluminum, aluminum alloys and brass .10.6.1.6 Dry or compressed air: As air passes the tool and work piece it lower their temperature and removes chips .Compressed air may be used in machining cast iron, brass, bronze, wood and plastics .

Chapter 11 Turning and drilling process

Metal cutting is a technological process during which the work piece is cut to the required shape, dimensions and accuracy by gradual removing of material .The material is removed in the form of chips .

To cut the material, a tool must be forced against the work piece material to a certain depth “ depth of cut ‘t’ “ and a relative motion between the tool and the work piece “ cutting motion “ must be performed . The cutting motion is composed of the main motion and the feed fig. 11.1

Fig 11.1 Elements of cutting motion

The main motion is performed either rotary or rectilinear. The velocity ‘v’ of the main motion is given in meters per minute except in the case of grinding, where the velocity of grinding wheel is given in meters per second. The velocity of the rotary main motion can be calculated from the formula:

V =π Dn/1000 m/min (11.1)

Where D is the diameter of the work piece or tool which performs the main motion (mm.)

Vr cutting motionV main motion (cutting speed)f feed

V

f

V

f

Vr

N is the number of revolutions per minute of the work piece or tool which performs the main action (rev./min.)

The feed f Is the motion of the tool relative to the work piece or vice versa in the direction of feed.Its velocity is given in mm/rev as in turning, mm/min as in milling, mm/tooth as in broaching or milling too and in mm/double stroke as in shaping. The velocity of the main motion ‘v’ is much greater than that of the feed ‘f’ thus the former is considered without a considerable error as the cutting speed v , equation 11.1.The depth of cut t: Is the depth of material removed from the work piece surface by one pass of the tool. It is measured to be machined in mm.It is determined in case of longitudinal turning according to the formula : t = (Do-df)/2 mm. (11.2)

Where Do……. is the diameter of surface to be machined in mm . df……. is the diameter of machined surface in mm .

in case of machining flat surfaces, it is determined according to the formula :

t = H – h mm . (11.3)

Where H……. is the height, width or thickness of the surface to be machined in mm . h……. is the height, width or thickness of the machined surface in mm .

After removing the layer of the thickness t , an in feed motion is performed to remove the next layer. The cutting speed ‘v’ , feed ‘f’ and depth of cut ‘t’ are called the cutting conditions.11.1 Turning Cutting toolsThe cutting tool is composed of two parts : the tool head and the tool body .

fig.11.2 shows the simple and most common used single point tool .

Fig 11.2 Single point tool

H ead sto ck

S p in d le

F eed ro d

B ed

C o m p ou n d restan d slid e (sw iv e ls)

C arriage

A p ro n

se lec to r

F eed chan g eg earb ox

S p in d lesp eed

L ead sc rew

C ro ss s lideD ead cen te r

T oo l p o stG u ide w ay s

T ails tock q u illT a ils tock

H an d le

11.2 Different types of lathes :

1. center lathe . 2. turret lathe .3. capacitance lathe . 4. copping lathe .5. facing lathe . 6. vertical lathe .7. N.C. machine lathe . 8.automatic lathe .

The most common one is the center lathe fig 11.3 which contains of :- A bed – ahead stock – a tailstock a tool post mounted on the cross slide

.

Fig 11.3 Modern center lathe

11.2.2 Turret lathe

It is a production lathe used to manufacture any number of identical pieces in the minimum time. It is a development of center lathe. The main differences between center and turret lathe are :1. the tool post mounted on the cross slide of a turret lathe is a four way tool in

addition there is a rear tool post mounted on the carriage which holds another tool, whereas in the case of center lathe the usual practice is to have only one tool post .

2. The tailstock of the center lathe is replaced by a turret. This is a hexagonal block, each side may carry one or more tools . Thus in place of tailstock in a centre lathe which can accommodate only one tool of limited size, the six faces of the turret hold six or more tools. This is a decisive advantage in serial production. These tools may be indexed one after another to perform different operations in a regular order.

3. The feed movement of each toolset on the turret may be regulated by stops and feed strips. They enable the same tool to perform its operation on each work piece to a predetermined length, hence duplication work without re- measurement.

4. In a turret lathe, two or more tools may be mounted on the same face of the turret, making it possible to machine more than one surface at a time, thus reducing the total operation time. This type of arrangement is uncommon in a centre lathe.

5. The labor cost required to operate a turret lathe is less than that required in a centre lathe. Once the machine has been set up by a skilled machinist, a semi skilled operator can operate the lathe.

11.2.3 Automatic and half automatic lathe Where all motions needed for machining a component are executed by the machine it self using one or more camshafts, cams, and stops. It is used for machining a small component in mass production .it has the highest productivity compared to other lathes. The use of automatic lathe highest production of small number of pieces is uneconomical due to the high cost of machine, tools, tool holders and the long time of machine setting and adjusting.

11.2.4 Facing lathe It has no tailstock. It is used for machining shorter components of great diameters as flywheels, pulleys and the like

11.2.5 Vertical lathe

Its clamping plate is horizontal and rotates around the vertical axis it is used for machining big parts of complicated shapes as boxes, frames, large gears and the

like . 11.2.6 Numerical control lathe

Fig 11.4 illustrates the main components of numerical control (NC) lathe.

The above survey of lathes shows that they are used for single piece serial and mass production.

Fig 11.4Turret lathe

11.3 lathe operations

Although lathes were primarily designed to turn rotary surface, it can perform a number of other operations. Fig 11.5 illustrates the common operations performed on lathe.

Fig. 11. 5 Common lathe operations

11.4 Turning toolsThe tool used in turning is single point cutting tool and classified as follows-:11.4.1According to the method of manufacturing :

- forged tools: - ex. High carbon steel and high speed steel at the end of solid shank .

- Tipped tools brazed to the carbon steel: - ex. sintered carbide tool materials &high speed steel.

- Tipped tools fastened mechanically to the carbon steel shank :- ex . Ceramic tips. &diamond tips. As shown in fig.11.6

Fig. 11.6 Turning tool tips

11.4.2 According to the tool functions (types): Fig (11.13)

Fig. 11.7 Different functions of turning tools

1 .Recessing tool 8. Side round-nosed tool2 .Cutting off tool 9. Bent roughing tool

3 .Straight roughing tool 10. Inside roughing tool 4 .Corner roughing tool 11. Inside corner tool

5 .Straight finishing tool 12. Inside recessing tool 6 .Broad-nosed finishing tool 13. Threading tool

7. Corner round-nosed tool 14. Internal threading tool11.4.3According to the method of applying feed-:

As shown in fig. 11.8, turning tools can be classified into left hand side tools and right hand side tools .

a) right hand b) left hand

Fig. 11.8 Right and left turning tools11.5Tool clamping-:

In turning M/cs . atool post is used to clamp the tool . the common types of tool posts fig (11.9) . are :- 1. Open side tool post .

2 .Four way tool post.

Workpiece

Headstock center(Live Centre)

Tailstock center(Dead Centre)

Workpiece

Tool

Chip

Tool post

S peripheralspeed (m/min)

N (rev/min)

D

Fig. 11.9 Tool post11.6 Work piece clamping:-

May be carried out by one of the following methods-:

1) Between centers: fig (11.10)

Fig. 11.10 Workpiece between two centers 2 (Universal chucks: as pictured in fig.11.11

Wor

kpie

ce

Workpiece Mandrel

Fig. 11.11 Three and four jaws chucks

3) Face plate : used for clamping of non-round parts. Fig (11.12)

Fig. 11.12 Face plate5) Mandrels : is advice for holding and rotating a hollowed work piece and mounted between centers. Fig (11.13)

Fig. 11.13 Work piece mandrel

Jaws

HingeWork Work Jaws

Lathe bed guideways

Carriage

6) Rests : for a long cylinder work piece . Fig (11.14)

Fig. 11.14 Type of tests11.7 Drilling processes Drilling is the process of producing a hole in the workpiece by cutting the volume to be removed into .chips.. The main motions are provided through the cutting tool (twist drill) are shown in fig 11.15 while fig.11.16 illustrates twist drill during cutting

Motion 1 (cutting motion ) About the axis of the twist drill

Motion 2 (feed motion)

Along the axis of the twist drill

Fig.11.15 main motions in drilling

Fig. 11.16Twist drill during cutting11.7.1 Drilling MachinesMain parts of a Drilling MachineFigure11.17 shows the main parts of a drilling machine .It consists of :

- The spindle is used for drill fixation and providing the cutting motion 1- The hand feed lever is used for providing the feed motion 2- The table is used for work piece fixation and height adjustment.

Figure11.17 Bench drill

Types of Drilling Machines

Fig. 11.25 Drilling machines

1) Bench drilling machine Fig.11.24 shows a small drill mounted on a bench. It is used for light jobs

2) Vertical Drilling machine It is similar to a bench type drilling machine. However, it is used for heavier works Fig.11.25.a

3) Radial Drilling Machines The arm movement around the axis of the column covers a circular area which enables the drilling of holes in very heavy work piece put directly on the workshop floor, Fig.11.25b

11.7.2Tools for producing circular holes and holes operations 1) Twist drills Figure 11.27a shows different parts of twist drill while fig.11 27b shows . some types of twist drill for making circular holes.

2) Other tools

Figure 11.28 shows some types of tools for

Core Drilling - Drill a larger hole on a smaller hole.

Step drilling - Double sized drill

Counterboring - stepper hole. Useful to seat bolt heads in the holes.

Countersinking -Hole is cone shaped for flat head screws.

Reaming - Enlarge the hole, provide better tolerance/finish.

Center Drilling - To begin the center for a hole.

Gun Drilling- deep holes with aspect ratios > 300

Figure 11.27a Twist drills

Figure 11.27b Different types of twist drills

Fig 11.30 Operations performed on the drilling machine