BASICS OF DESIGN

BY K. PALANIYAPPAN

TECHNICAL CONSULTANT

CONTENTS

1. Design 1.1 Introduction 1.2 Basics Principles of Design 1.3 Failure Criteria in Machine parts

2. Engineering Materials 2.1 Introduction 2.2 Technical Properties 2.3 Material Selection 2.4 Heat Treatment 3. Design for X 3.1 Introduction 3.2 Design for Manufacture (DFM) 3.3 DFM Approach 3.4 Design for Assembly 3.5 DFA Approach 3.6 Design for Quality 3.7 Design for Cost 3.8 Design for Safety 3.9 Design for Reliability 4. Drawing Practice 4.1 Introduction 4.2 Drawing Standards 4.3 Drawing Numbering System 4.4 Bill of Materials 4.5 Alterations and Modifications 4.6 Filing of Drawings 4.7 Manual Preparation 4.8 Documentation 4.9 Archives 5. Reverse Engineering 5.1 Introduction 5.2 Construction of 3D Model 5.3 Applications i. Further Reading

DESIGN Introduction:

Design is the art of developing new ideas for the construction of machines/ products and expressing those ideas in the form of plan and drawings. The idea may be almost entirely new, as in the case of an invention or an improvement upon existing machinery/ product; or it may be only partially new, as when a product or a machine part is to differ in size, load or materials from those already existing.

For a machine/ product to be well – deigned the parts must be strong enough for the duty required by them and must be adequate for the functions they must perform, but they must not involve unnecessary expenditure of material or prohibitive cost of construction.

To design well any machine or part, the designer 1. Must have working knowledge of the elements of machine construction. 2. Must know how to analyze the applied loads and their reactions.

3. Must know how to determine the resulting stresses.

4. Must possess sufficient information about materials.

5. Must understand the influence of shape, method of assembling, and

working conditions of parts upon the operation and maintenance of the machine.

Thus modern design involves the application of the principles of

fundamental engineering subjects like Mathematics, Mechanisms, Mechanics, Strength of Materials, Engineering Materials, Advanced Technology in Production and Manufacturing and finally Drawing Standards and its Practice. In addition, possession of or access to experimental data on the performance of similar machines already existing is of great value. “The more a man gives of himself to his work, the more he will get out of it, both in wages and satisfaction”. “Co operation is not a sentiment. It is an economic necessity”.

Basic Principles of Design: The following considerations should be made in designing an element or a system.

1. Functional Requirement: The function that the element or the system has to perform is the most important one and correspondingly. Links. Gears, belt drive, etc., can be chosen to perform a particular function in combination with prime-movers.

2. The Strength Requirements should never be sacrificed. Depending upon

the forces, the dimensions of the parts should be so chosen that the strength of the materials of parts is taken care of. The strength also includes the static stiffness and dynamic rigidity of the parts.

3. Alignment of Parts: Depending upon the mating parts or the rotating parts, the dimensions of parts or distances between parts should aim at the proper alignment of bearings or sliding parts, etc.,

4. Use of Indigenous Materials and their proper choice. As far as possible,

indigenous materials should be selected to avoid undue delay in procurement and in production and in maintenance.

5. Use of Standard Sizes of components, Standard Speeds and Preferred

Numbers. The use of standard size of components e.g. bolts nuts, etc., standard speeds of prime-mover and preferred numbers of various sizes, speeds, etc., result in reduced cost.

6. Existing Facilities for Manufacture: In the already existing works, the

existing facilities for manufacture should be utilized for a new product design to start with.

7. Facilities for Wear Adjustment and Protection from Environment conditions. In the case of mating parts with relative motion, the wear adjustment facility results in longer life. Wherever necessary, environmental conditions (eg. in mines, chemical plants, etc.,) should be taken care of in design.

8. Ease of Assembly, Inspection, and Maintenance: Assembly

requirements should be considered in design to reduce wastage. The inspection of various parts subject to wear; Fracture or failure should be convenient and so should be the de-assembly

9. Lubrication and cooling: Wherever necessary the Lubrication and

cooling of parts with relative motion be provided for in the design.

10. Location of Controls Conforming to Standards and should be within the reach of operator. This requires the proper selection of direction, force, and distance.

11. Built in Safety Devices: The design must include built-in safety devices for overload protection and for the safety of the operator. Safety should not be by accident.

12. Overall Weight and Available Space: As for as possible, efforts be made

to reduce the weight of the element or the system be made to the size for a given available space.

13. Manufacturing Tolerances: For all parts, depending upon the specific

use, proper tolerances need to be determined.

14. Aesthetic Appeal: An effort should be made to give the product modern appearances which are best described by words such as ‘Smooth.” ‘Rounded’, ‘Un cluttered’, or ‘stream lined’, etc.,

15. Transportation and Erecting facilities: For proper safe transportation and erection of the machine or subassemblies provision needs to be made in the design for handling purpose like eye bolts, etc.,

16. Cost: The optimized design by compromising with conflicting

requirements, combined with reasonable profits, results in the least cost.

17. Reliability: The reliability of the performance of the machine depends on a number of factors and should be the optimum.

Failure Criteria in Machine parts:

Almost every type of machines consists of a combination of machine elements. Design requires that an element functions without failure for its required life. It is not always necessary, economically feasible, or possible to design for infinite life.

Under certain conditions it is economically possible to design a part to have infinite life, whereas some parts are designed with a finite life. A shaft can be designed to operate without breaking. Gears and bearings however can only be designed with a limited working life, since there is no way of avoiding wear.

Aircraft transmissions are

designed to meet economic and weight criteria different from transmissions on buses. Aircraft are limited by severe weight restrictions, whereas maintenance is critical for buses. Aircraft parts are designed with a much shorter useful life. It is more economical to pay increased maintenance costs and carry larger pay loads.

Failure of machine parts is of two types: functional failure and fracture.

Machine parts fail in their function in many ways. Failure is not always overt or permanent in nature; it may be reversible or irreversible. A shaft which fails to function owing to excessive deflection may perform its function at reduced loads. A spring, which deforms permanently under overload has failed permanently if, as result, it fails to meet the preload of free-length requirements.

Failure may appear as change of the kinematic relationships, noise above a tolerable limit, decrease in mechanical efficiency, increase in heat generation, fire, explosion, or inability of the machine to respond to its controls by either stopping or starting.

Failure of a machine part is due to one or more primary causes like deformation, wear, or corrosion. It is possible, by proper design, to prevent excessive deformation at normal temperatures and, in many cases, to eliminate corrosion; the effect of wear and deformation that are due to high –temperature creep, however, can only be mitigated and taken into account in design, but never avoided. Since a machine always includes moving parts, wear always exists as the life-limiting factor.

“There is nothing more disappointing than failing to accomplish a thing unless it is to see somebody else accomplish it”. “The out standing leaders of every age are those who set up their own quotas and constantly exceed them”.

ENGINEERING MATERIALS Introduction:

The selection of material is one of the decisions that a designer is supposed to make and the selection of the proper material has always been a difficult problem and is one that require an increasing amount of experience as the variety of materials available is constantly increasing.

The proper material to be used is one that meets the engineering requirements of the designers, the production and heat treating requirements of the production department and the cost requirement of the purchase agents. It is important that materials be used in such a way to take full advantage of their characteristics. Thus it is usually desirable to think of design change when selecting materials.

Technical properties: The properties of materials used in machinery may be classified as:

I Mechanical Properties

Tensile Strength Compressive Strength Bending Strength Bearing Strength Shear Strength Torsion Strength Impact Strength Damping Properties, etc.,

II Physical Properties

Wear Resistance Coefficient of Friction Hardenability Thermal Conductivity Specific Gravity Coefficient of Thermal Expansion, etc.,

III Technological Properties

Machinability Formability Weldability Castability Malleability, etc.,

Strength: The strength of a part depends on the type and nature of loading. The static strength of a material is expressed by the corresponding elastic limit stress Se. The impact strength is measured by the corresponding modulus of resilience u. The endurance strength is expressed by the corresponding endurance limit Sen. Stiffness or rigidity: Stiffness or rigidity is measured by the modulus of elasticity, E Being used for tension or compression and G being used for shear. Ductility: A material is ductile if it is capable of undergoing a large permanent deformation without rupturing. There is no absolute measure of ductility. The percentage of elongation or the percentage of reduction of area during a tensile test carried to rupture is used as a relative measure. Ductility helps to relieve localized stress concentration through local yielding. It is necessary characteristic of a material used to sustain live loads, especially where concentrated stress may occur. Brittleness: Brittleness is a characteristic opposite to ductility and toughness. A material may be considered brittle if it is elongation at rupture through tension is less than 5 percent in a specimen 50 mm long. Usually brittleness and hardness are closely associated, and very hard materials are brittle. Salient features of most widely used engineering materials and their applications are discussed below: CAST IRON Cast Iron is primarily an alloy of iron, carbon and silicon. Carbon content is between 1.7 to 4.5%. The physical properties of the casting mainly depends upon the relative amount of graphite or cementite that is present in the cast iron. Also manganese, chromium, nickel, phosphorus, sulphur, etc., are alloyed with cast iron to give special properties. The general term Cast Iron CI includes Grey Cast Iron, Meehanite, Malleable Iron, spheroidal Graphite Iron (SG Iron), etc., Grey Cast Iron is the least expensive of all the metals that could be used for casting. The major structural components of machine tools, gear boxes, brackets, housings, covers, pulleys, etc., are made of grey cast iron in preference to steel due to its excellent castability and better damping properties against vibration. The components made of grey cast iron are normally hardened either by flame or induction hardening to 450 – 500 HBN. “I don’t think much of a man who is not wiser today than he was yesterday”.

- Abraham Lincon

STEELS Steel is very widely used for components as it can be manufactured and processed into number of different specifications each of which has a definite use. Steel is an alloy of iron and carbon. Depending upon the carbon content it is classified as:

Low carbon steel (Carbon Content less than 0.3%) Medium carbon steel (Carbon Content 0.3% to 0.6%) High carbon steel ((Carbon Content more than 0.6%)

Depending upon the application steel can be classified as:

Free Cutting Steel Carbon Steel Alloy Steel

FREE CUTTING STEEL These steels have good machinability and a good surface finish can be achieved on the components. 14Mn1S14 and 40Mn2S12 are preferred steels of this category and are used for handles, levers, spacers, hydraulic fittings, screws, etc., Higher Sulphur content in the composition imparts this property of good machinabilty. MILD STEEL (MS) MS is a low carbon steel with no precise control over the composition or mechanical properties. This type of steels are used for end plates, covers, sheet metal works, tanks, fabricated items, etc., STRUCTURAL STEEL This is typical steel where the main criterion in the selection and inspection of the steel is the tensile strength which is used as the basis of design. Generally load carrying welded structures such as frames, trolleys, racks, machinery structures etc., St 42W is a weldable quality structural steel with high tensile strength, this is a frequently used structural steel in many applications. CARBON STEEL Steels like C40, C45, C55, C75, C80, C85 fall under this group. C45 is the preferred steel of medium carbon steel category and is suitable for application such as shafts, gears, keys, pins, clutchs, threaded fasteners requiring high strength. C75 is the preferred steel of high carbon steel category and can be heat treated to a high hardness in the range of 60 – 64 HRC. “One man with courage makes majority”. - Andrew Jackson

ALLOY STEELS Apart from iron and carbon certain alloying elements such as nickel, manganese, chromium, tungsten, molybdenum etc., are added to enhance certain desirable properties of steel. Alloy steels could be mainly grouped as direct hardening steels and case hardening steels. The table below gives the different types of Carbon Steels, Alloys Steels and their applications. Only important Alloy Steels frequently used in Engineering Design are discussed below. Alloy Steel Application Remarks 10Cr1 (EN31)

Bearing Races, Guide Strips, Tools, Dies, etc.,

High Carbon Low Chromium Alloy Steel Hardness Max. 225 HBN

40Ni2Cr1Mo28 (EN24)

Shafts, Gears, Etc., Components with high wear Resistance

Medium Carbon Alloy Steel Hardness 45 – 50 HRC

15Ni2Cr1Mo15 (EN354)

Spindles, Cams, Lead Screws, and critical components where the case should be hard and wear resistant, leaving the core tough.

Case Hardening Steel (Carburising) Hardness 55 – 60 HRC

40Cr2Al1Mo18 Components requiring high resistance to abrasion, higher surface hardness.

Case Hardening Steel (Nitriding) Hardness Max. 250 HBN

EN1A SAE 1113

Low Duty Bolts Nuts, studs etc.,

Free Cutting Carbon Steel

EN3 SAE 1017

Air Craft Sockets, Highly stressed levers, bolts, and nuts.

Free Cutting Carbon Steel

EN8 SAE 1040

Crank Shafts, Connecting Rods, and parts requiring high strength and wear resistance.

Free Cutting Carbon Steel

EN12

Axle Shafts, Crank Shafts, Connecting Rods, and Stearing Levers

Nickel Steel

“I think and think, for months, for years, ninety nine times the conclusion is false, the hundredth time I am right”. - Albert Einstein.

Alloy Steel Application Remarks EN20A, B Thermal Applications

Chromium Molybdenum Steel

EN30A Spindles, Aero Engines, Connecting Rods, and high duty bolts.

Nickel Chromium Steel

EN32A SAE 1010

Can Shafts, Lightly Stressed Gears, Gudgeon Pins, etc.,

Case Carburizing Steel

EN42B, C SAE 1064

Springs Spring Steel

EN56A SAE 51410

Turbine Blades Chromium Rust resistance Steel.

ALLOYS: The elements most often alloyed with steel, singly or two or more together, are besides carbon, nickel, chromium, silicon, manganese, molybdenum, vanadium, tungsten, and aluminum. Carbon increases the hardness and strength of steel but decreases its ductility. Nickel increases the hardness, toughIless, corrosion resistance, and (up to a 12 per cent content) also the elastic limit of steel, but decreases its ductility slightly. The ratio of elastic limit to ultimate strength increases gradually with increasing nickel content up to about 4 percent, and with a further increase it begins to decrease. Nickel steels are the most important of the commercial alloy steels. Their nickel content varies from 0.50 to 5.25 per cent, while the usual carbon content ranges from 0.30 to 0.60 per cent. The chief uses of nickel steels are for structural shapes, rails, steel castings, engine forgings, and automotive parts. Chromium increases the elastic limit and hardness of steel. It is added either alone or in conjunction with nickel or vanadium. It also increases the resistance to corrosion. Chrome steels are used for ball and roller bearings, gears, and other machine and automotive parts where hardness is essential. Chrome steels are always heat-treated. Steels with chromium content of 11 percent or more-in some cases. Over 20 per cent-are known as stainless steels because of their resistance to corrosion.

“If you want to have success, display enough courage to welcome failure.”

Chrome-nickel steels combine high strength with great hardness. They are produced with a chromium content from 0.6 to 1.2 percent, a nickel content from 1.5 to 3.5 per cent, and various carbon contents. Steels with a carbon content up to 0.2 per cent are used only when case hardened; those having a content of 0.25 to 0.6 per cent are used for structural parts of automobiles; and those having a content of 0.5 per cent and more are used for gears and automotive parts in place of plain chrome steel. Silchrome steel contains 0.5 per cent C, 0.30 per cent Mn, 3.50 per cent Si, 8.0 per cent Cr, not more than 0.02 per cent P, and not more than 0.02 per cent S. This steel is used for exhaust valves in internal combustion engines. It is tough, is hard (even at high temperatures), and has a high resistance to scaling and corrosion. Its drawback is the difficulty of machining it. Silicon-manganese steel contains 0.45 to 0.65 per cent C, 0.60 to 0.90 percent Mn, and 1.8 to 2.0 per cent -Si. It has a high elastic limit and is used extensively for springs and gears. This steel must be given a suitable heat treatment, the type depending on the kind of service for which the steel is intended. Vanadium added to carbon steel nickel steel, or chrome steel, even in such a small amount as 0.15 to 0.25 per cent, increases the elastic limit and resilience. Chrome-vanadium steel is used especially for automotive springs. Steel that has an average composition of 0.47 per cent C, 0.84 per cent Mn, 0.032 per cent S, 0.026 per cent P, 0.10 per cent Si, 1.06 per cent Cr, and 0.15 per cent V. Tungsten and molybdenum are sometimes added to machine steels for strength and toughness, but their chief use is in the production of high-speed cutting tools. Aluminum in small amounts, about 0.10 per cent, increases the fluidity of steel. Aluminum is also added to a ferrous alloy used as stock for nitriding. This alloy, known as nitralloy, has an approximate composition of 0.2 to 0.4 per cent C 0.5 -per cent Mn, O.Z to 0.5 per cent Si, 0.5 to 0.6 per cent Ni, 1.5 to 1.7 per cent Cr, 0.2 per cent Mo, 0.9 to 1.3 per cent Al, and about 95.5 to 96 per cent Fe. NON FERROUS METALS Following are the main non ferrous metals frequently used, owing to their unique characteristics such as low specific gravity, high wear resistance, low coefficient of friction, anticorrosive properties, etc., “We need to teach a man that it is not a disgrace to fail and that he must analyze every failure to find its cause. He must learn how to fail intelligently, for failing is one of the greatest arts in the world.”

- Charles F Kettering.

ALUMINUM Aluminum and its alloys have been used in machine tools owing to their light weight, corrosion resistance, castability into intricate shapes and thin wall thickness, forgeability and ability to be extruded and rolled. Some of these alloys respond to solution treatment and precipitation hardening by means of which the mechanical properties are improved. Aluminum and its alloy castings are standardized as per IS 617. The grades of Aluminum which are frequently used are 4223M & WP (A4), 4600M (A6) and 2280W & WP (A 11) for general castings and 5230M (A5) where decorative anodizing is necessary. COPPER AND ITS ALLOYS Copper: Copper in its pure form is used in electrical industry since it has the best electrical conductivity of any commercially priced metal. It also possesses good resistance to corrosion and can be formed into any shape easily. Owing to their good corrosion resistance, good thermal conductivity, ductility and ease with which they can be brazed, copper tubes find an application for oil circulation in oil hydraulics. Brass: It is an alloy of copper and zinc. It is resistant to corrosion by water and atmosphere. Brass tubes are used for hydraulic cylinders. Brass strips are used for guide way wipers. Brass also finds application where free cutting steels are not permitted as in the case of fittings and other threaded fasteners in corrosive atmospheres. Bronze: It is used as bearing material for worm wheels and lead screw nuts. It is an alloy of copper and tin. It is heavier, stronger and more resistant to corrosion than brass. Phosphor bronze: A small percentage of phosphorous is added to bronze to act as a cleanser to the metal so that sound castings can be produced. Cast phosphor bronze to grade 2 of IS 28 is used for bearing bushes, lead screw nuts and worm wheels. Aluminum bronze: It is harder, stronger and more wear resistant than phosphor bronze. It is also costlier than phosphor bronze. This is used for bearing bushes, air craft engine valve seats and for lead screw nuts where strength and wear are important. Manganese bronze: This is in reality a brass, consisting of copper and zinc. Manganese is used only as a deoxidizer, and the finished product often contains only traces of it. The term bronze is applied generally because of the alloy's red color. Manganese bronze has high strength, is ductile, and resists corrosion in salt water. Cast manganese bronze is used for such parts as ship propel1ers, heavy gears, and heavy-duty bearings. Manganese bronze can be wrought like Tobin bronze.

COMPOSITE MATERIALS: Properties absent in a particular material can be achieved by intimate combination of materials of different properties. Composite materials have the following advantages: 1. Save costly or rare materials by plating the costly material on to the cheaper one such as steel or cast iron plated with copper, bronze or carbide. 2. Impart desirable properties to a material, especially at its surface such as higher tensile strength-reinforced concrete, wire reinforced glass, fiber reinforced plastics, wear resistance as composite rail with hard head. Chemical stability-composite pipes with rust proof surface; electrical or thermal conductivity-composite contact material; low weight with high strength-aluminium carbide composite, poor conductivity, good sliding properties, reflective properties, facility for joining, etc. 3. Achieve new properties such as bimetal as heat indicator and sintered carbides. The combination is brought about by casting, welding, soldering or glueing, by sintering, diffusion, spraying, rolling or by galvanic means. MATERIAL SELECTION: In selecting the material, the requirements concerning the functions, stressing and life of the component are to be considered first; then those concerning the shape and manufacture. And last but not the least, the prime cost. Often the procurement question is also important. Ordinary carbon steels are used for simple axles and shafts keys and pins. High quality steel or special cast iron is used for crankshafts. Gray cast iron for stands, base plates, and housings. If the stresses are high than special cast iron, cast steel or welded steel are used. Hardened steel for parts subjected to high rolling contact pressure (ball bearings, cams, heavily loaded gear teeth). Cast iron, cast steel, steel with carbon 0.2% to 0.6%, heat treated steel, in special cases, plywood ,plastics and non ferrous metals are used for gear wheels. Plastics, soft cast iron, bronze, white metal, Zinc and aluminum alloys are paired for sliding surfaces. Spring steel, rubber are useful for elastic spring. Free cutting or die cast alloys are used for small mass produced components.

Heat resisting or non-scaling steel or ceramics are used for components subjected to heat or fire. Material selection becomes a problem when the available experience is not sufficient i.e. if new considerations, viz. new requirements, new materials, etc., arise. Then close scrunitiny is needed with regard to:

1. The requirements on the component (function, Loading, Life). 2. The conditions for production (number of pieces, shape, Method of

production and cost) 3. The material properties, followed if necessary by tests with materials in

question. In these cases it is advisable for the designer to discuss with material and production experts and with user. The decision is simple if only certain few material properties are of consequence. If several requirements are more or less fulfilled by a number of materials, the decision becomes difficult. The problem of selecting the most suitable material for the body of a motor car —wood, plywood, plastics, light metal as aluminum alloy or sheet steel, for example is thus to be solved by evaluating the influencing factors viz reliability, life, sensitive to environmental conditions, case of forming to required shape, weight, maintenance, repair and cost, etc., “Any man’s success hinges about 5% on what others do for him and 95% on what he does, with the emphasis on the does.” “No one ever attains very eminent success by simply doing what is required for him, it is the amount and excellence of what is over and above the required, which determines success.” “Inventing is quite commonly a process of slowly and determinedly eliminating the impossible solutions until the real one is found. Then see an attorney.”

The mechanical properties of some of the commonly used materials are given in the table below:

Coefficient of Friction for various material combinations are given in the table:

“The man who will use his skill and constructive imagination to see how much he can give for a dollar, instead of how little he can give for a dollar is bound to succeed.”

- Hendry Ford. “Movement in a new direction helps you find new cheese.” “When you move beyond your fear, you feel free.”

HEAT TREATMENT Heat treatment is generally applied to steels to impart specific mechanical

properties such as increased strength or toughness or wear resistance. Heat treatment is also resorted to relieve internal stresses and to soften hard metals to improve machinability. Heat treatment is essentially a process of heating the steels to a predetermined temperature followed by a controlled cooling at a predetermined rate to obtain desired end results. The heat treatment process can be classified into four important groups:

i) Re crystallization annealing which is employed to relieve internal stresses reduces the hardness and to increase the ductility of strain hardened metal. At first, upon an increase in the heating temperature the elastic distortions of the crystal lattices are eliminated. At higher temperature new grains form and begin to grow (re crystallization).

ii) Full annealing which involves phase re crystallization and is achieved by heating alloys above the temperature required for phase transformation. This is followed by slow cooling. Full annealing substantially changes the physical and mechanical properties and may refine a coarse grained structure.

iii) Quenching wherein hardening alloys are heated above the phase transformation temperature and are then rapidly cooled (quenched).

iv) Tempering involves the reheating of hardened steel to a temperature below that required for phase transformation so as to bring it nearer to an equilibrium state.

ANNEALING

Annealing is the process necessary to obtain softness, improve machinability, increase or restore ductility and toughness, relieve internal stresses, reduce structural non-homogeneity and to prepare for subsequent heat treatment operations.

The process consists of heating the metal to the required temperature depending upon the carbon content and other alloying elements of the steel and then cooling in the furnace at a slow rate. Most of the cast iron components are annealed at a low temperature before final machining. NORMALIZING

This is the process necessary to eliminate coarse-grained structure obtained in previous working, to increase the strength of medium carbon steels to a certain extent (in comparison with annealed steel), to improve the machinability of low carbon steels, to reduce internal stresses; etc.

More rapid cooling in air used in normalizing causes the, austenite to decompose at lower temperatures. This increases the disparity of the ferrite-cementite mixture (pearlite) and increases the amount of eutectoid constituent.

Therefore normalized steel has a higher strength and is harder than annealed steel. HARDENING AND TEMPERING

In this process steel is heated to a predetermined temperature and then quenched in water, oil or molten salt baths. HARDENABILITY

Hardenability is defined as the capacity to develop a desired degree of hardness usually measured in terms of depth of penetration. The depth of hardness depends upon the critical rate; since this is not the same for the whole cross-section, full hardening may be , achieved if the actual cooling rates, even at the core, exceed the critical values. The higher the carbon content, the harder a steel will be after hardening owing to a martensitic structure.

Hardening followed by tempering is done to improve the mechanical properties of steel. The aim in structural steels is to obtain a good combination of strength, ductility and toughness. Hardened steel is in a stressed condition and is very brittle so that it cannot be employed for practical purposes. After hardening, steel must be tempered to reduce the brittleness, relieve the internal stresses due to hardening and to obtain predetermined mechanical properties. In spite of the high hardness, hardened steel has a low cohesive strength, a lower tensile strength and particularly a low elastic limit, due to the stress conditions after hardening. The impact strength, relative elongation and reduction of area art: also considerably reduced by hardening.

Tempering the steel at a suitable temperature will enable the steel to attain the desired mechanical properties. Tempering consists of reheating the hardened steels to a temperature below lower critical values followed by cooling at a desired rate. At low tempering temperatures, the hardness changes only by a small extent but the true tensile strength and bending strength are attained. As the tempering temperature is gradually increased, the steel regains its true strength and resistance to shock with a gradual decrease in hardness value. However where toughness is the criterion, tempering in the range of 230-400°C is avoided to overcome the condition of temper brittleness. SURFACE HARDENING

This is a selective heat treatment in which the surface layer of metal is hardened to a certain depth whilst a relatively soft core is maintained. The principal purpose of surface hardening is to increase the hardness and wear resistance of the surface. Surface hardening may be accomplished with or without changing the chemical composition of the surface. While carburizing and nitriding correspond to the first type, flame and induction hardening correspond to the second type, i.e. without changing the chemical composition. Steels with carbon content less than 0.25% can be generally carburized while steels with a minimum carbon content of 0.4% only can be flame or induction hardened.

INDUCTION HARDENING Induction hardening has the advantage that it reduces the time required

for heat treatment. Parts may be hardened with practically no scaling, so that allowance for further machining can be reduced. Deformation due to heat treatment is only marginal. In comparison with other processes for a given tensile strength, induction hardened steels have higher hardness, wear resistance, impact strength and fatigue limit. The increase in the fatigue limit after induction hardening is associated with the appearance of residual compressive stresses in the hardened layer. These stresses reduce the effect of tensile stresses arising from the application of external forces. FLAME HARDENING

In this process, the surface of the part to be hardened is heated by an oxy-acetylene flame at temperature of 30000-32000C. The large amount of heat transferred to the surface rapidly heats it to a hardening temperature before the core is appreciably heated. Subsequent quenching hardens the layer. CARBURIZING

This is a process for saturating the surface layer of low carbon steels with carbon. Several methods are employed for this purpose such as pack carburizing, gas carburizing and liquid carburizing. The advantages of gas carburizing over pack carburizing are:

i) Possibility of better regulation of the process and of obtaining more accurate case depth.

ii) Less time is required for the process iii) The operation is clean and simpler and iv) Process can be mechanised

Liquid carburizing has the advantage that it provides a uniform heating

combined with least deformation of the part. After carburizing, regardless of the process employed, the material is heat treated to produce a hard surface resistant to wear. NITRIDING

This is a process of saturating the surface of steel with nitrogen, by holding it for a prolonged period at a temperature from 480° to 650°C in an atmosphere of ammonia. Nitriding increases the hardness of the surface to a high degree. It also increases the wear resistance and fatigue limit. When high hardness and wear resistance are the chief requirements the part is made of steel containing aluminium, chromium and molybdenum in steel impart an exceptionally high hardness and wear resistance to the nitrided case.

“In a positive environment, a marginal performer’s output goes up. In a negative environment, a good performer’s output goes down.”

DESIGN FOR X INTRODUCTION:

Although the cost of design as such is a small fraction of the cost of product development (only about 8%), a major part of the product's cost (about 80% ) is committed at the design stage (The concept or architecture of the product alone determines 60% of the cost!). The decisions arrived at this stage greatly influence the ease or hardship with which subsequent activities of product development and manufacture are carried out. Apart from these activities, the design decisions also influence other activities during the life of the product up to recycling it. Reorienting the design process so as to make these activities during the product development and product life cycles is referred as Design for X (DFX).

Here, X refers to a number of factors such as

Manufacture Assembly Mass Customization Safety Serviceability Environment etc.

Design for X involves the designer with other functional departments, such

as marketing, manufacturing, and engineering services, via Concurrent Engineering (CE) methods. In fact, DFX is often considered as a subset of CE. Since the other departments involve themselves at the early stages of design and help designers detect and correct flaws, such a CE environment is increasingly being referred as Collaborative Engineering.

DFX can be defined as a knowledge-based approach that attempts to

design products that maximize all desirable characteristics -such as high-quality, reliability, serviceability, safety, user friendliness, environmental friendliness, and short-time-to-market in a product design while at the same time minimizing lifetime costs, including manufacturing costs.

Historically, designers have tended to underemphasize or overlook the

preceding factors, and have concentrated their efforts on only three factors, viz., the function (performance), features and appearance of the product that they develop. They have tended to neglect the "downstream" considerations that affect the usability and cost of the product during its lifetime.

Henry Ford introduced many features which gave the Model the

competitive advantage. He designed the product so that it was easy to drive by virtually anyone. He designed the product to be repairable by any farmer with tools readily available. He designed the parts of the product to be

interchangeable and easy to attach to other parts. He designed the interchangeable worker within the production system. He designed the movable production line. The combination of these sub-designs led to the super-design of mass production, for which he is known. The salient point here is that, in order to

capture the competitive advantage, he designed for many different characteristics. In other words, Henry Ford designed for X. The three elements of competitiveness are;

Timeliness: Can the product be delivered to the market just in time to meet the customer's demand?

Quality: Will the product perform well as intended? Affordability: Do customers wish to buy the product?

These three dimensions largely determine the value of a product to the

customer. To gain the competitive advantage one must design for X. During design, one often focuses on the final product, and not its manufacture. DFX philosophy suggests that a design be continually reviewed from the start to the end to find ways to improve production and other non-functional aspects. These rules are nothing new, they are just common sense items written down, but they can be a good guide through the design process. Advantages of these techniques are as follows:

Shorter production times Fewer production steps Smaller parts inventory More standardized parts Simpler designs that are more likely to be robust They can help when expertise is not available, or as a way to reexamine

traditional designs Proven to be very successful over decades of application.

“Imagining myself enjoying new cheese, even before I find it, leads me to it.”

In the subsequent sections, each feature of DFX will be reviewed. DESIGN FOR MANUFACTURE

Designing a machine part means putting down on paper the dimensioned shape that the part must have to properly fulfill its functions. In order to be able to layout and draw this raw materials. Such knowledge requires a thorough understanding of the various manufacturing methods. These methods may be advantageously divided into two groups: (1) preliminary shaping of the machine parts, mostly, although not always, by using heat; and (2) final shaping by means of cold machining.

Under the first grouping come the processes of casting, welding, riveting, and forging. Each of these methods of forming machine parts has different possibilities, but it also has different limitations, which influence the design of a part. The designer must know and keep in mind these possibilities and limitations. The main features of each method from the standpoint of machine design will be discussed in separate chapters in the order indicated above. The first group includes also such processes as rolling, drawing, extruding, and stamping. However, their purpose is not to produce machine parts as such, but to produce stock material used widely in industry.

Among the methods under the second grouping are a great number of different operations which give he piece the exact dimensions required and produce the surface conditions necessary for its functioning. The main methods in this group are turning, boring, milling, planning, shaping, drilling, reaming, spot facing, broaching, grinding, honing, and polishing. There are additional machining processes, such as screw –cutting, tapping, and gear cutting, which are special adaptations of the basic methods. Some of the machining operations fulfill the same object and are often interchangeable. Examples are milling, planning, and shaping. Other operation is very helpful to a machine designer. However, such knowledge can be acquired only by working in a machine shop, and type of machining operation does not have too much effect on the shape of a part during its design. In practice the proper operation often is selected after the designer consults the man in charge of the machine shop. Therefore no attempt will be made here to give any information about the differences between the various types of machining operations, and no suggestions will be given for selecting the best type of operation for a specific case. The information that must be given to a beginning designer is how to make simpler and easier the machining of the parts he is designing is how to determine the degree of accuracy in machining that he should prescribe on his drawings.

Production conditions: The design of a machine part depends on the facilities of the shop where the part will be built. The facilities in a small jobbing machine shop are naturally different from those in a large plant manufacturing some special machinery.

Also, the design will not be the same when the part will be produced in quantities as when only one piece, or at most a few pieces, must be made. For instance, for quantity production it may be proper to make a part as a die casting, whereas a single piece may be machined from a block. As another example, when it is necessary to replace large sand – molded casting that was produced in quantity, the single piece may be produced more quickly and more cheaply by wielding.

Limitations: The designer must know the limitations of the machine shop in which the part that he is designing will be machined. Such limitations are the biggest diameter and the greatest length or height that can go in the lathe and boring mill and the greatest length or width that can be handled by the planner. He must know for what pitches hobs are available in the shop for cutting gear teeth, as very few shops have all standard hobs in stock. He must have similar information concerning other tools, such as taps, reamers, and broaches.

The designer should know which sizes of cold-rolled and hot –rolled steel material are standard and which standard sizes are kept in stock in the shop where his design will be executed. However, if the design of his part really requires a standard size which the shop is not carrying in stock, he should not hesitate to call for such material. He should not try to use available stock sizes that involve extra machining.

Design for manufacture (DFM) is the process of proactively designing products to

i. Optimize all the manufacturing functions such as fabrication, assembly, test, procurement, shipping, delivery, service, and repair.

ii. Assure the best cost, quality, reliability, regulatory compliance, safety,

time-to-market, and customer satisfaction.

DFM is a proven design methodology that works for any size company. Early consideration of manufacturing issues shortens product development time, minimizes development cost, and ensures a smooth transition into production for quick time to market. Quality can be built in with optimal part selection and proper integration of parts, for minimum interaction problems. By considering the cumulative effect of part quality on product quality, designers are encouraged to carefully specify part quality. Many costs are reduced, since products can be quickly assembled from fewer parts. Thus, products are easier to build and assemble, in less time, with better quality. Parts are designed for ease of fabrication, and commonality with other designs. DFM encourages standardization of parts, maximum use of purchased parts, modular design, and standard design features. Designers will save time and money by not having to "reinvent the wheel." The result is a broader product line that is responsive to customer needs.

In order to design for manufacture, everyone in product development team

needs to: In general, understand how products are manufactured through experience in

manufacturing, training, rules/ guidelines, and/ or multi-functional design teams with manufacturing participation.

Specifically, design for the processes to be used to build tile product he/ she

is designing. If products will be built by standard processes, design teams must understand them and design for them. If processes are new, then design teams must concurrently design the new processes as they design the product.

Before DFM, "I designed it; you build it!" syndrome existed.

Design engineers worked alone or only in the company of other design engineers in "The Engineering Department." Designs were then thrown over the wall leaving manufacturing people with the dilemma of either objecting (but its to late to change the design!) or struggling to launch a product that was not designed for manufacture. Often this delayed both the product launch and the time to ramp up to full production, which is the only meaningful measure of time-to-market.

One way to assure manufacturability is by developing products in multi-functional teams with early and active participation from manufacturing, marketing ( and even customers), finance, industrial designers, quality, service, purchasing, vendors, regulation compliance specialists, lawyers, and factory workers. The team works together to not only design for functionality, but also to optimize cost, delivery , quality, reliability, ease of assembly, testability; ease of service, shipping, human factors, styling, safety, customization, expandability, and various regulatory and environmental compliance.

Paradoxically, one of the first decisions the team has to make is the optimal use of off-the-shelf parts. In. many cases, the architecture may have to literally be designed around the off-the-shelf components, but this can provide substantial benefit to the product and the product development process.

Off-the-shelf parts are less expensive to design considering the cost of design, documentation, prototyping, testing, the overhead cost of purchasing all the constituent parts, and the cost of non-core-competency manufacturing. Off-the-shelf parts save time considering the time to design, document, administer, and build, test, and fix prototype parts.

Suppliers of off-the-shelf parts are more efficient at their specialty, because they are more experienced on their products, continuously improve quality, have proven track records on reliability, design parts better for DFM, dedicate production facilities, produce parts at lower cost, offer standardized

parts, and sometimes pick up warrantee/service costs. Finally, off-the-shelf part utilization helps internal resources focus on their real missions: designing products and building products. Some Key DFM Guidelines

1) Understand manufacturing problems/ issues of current/ past products: In order to learn from the past and not repeat old mistakes, it is important to understand all problems and issues with current and past products with respect to manufacturability, introduction into production, quality, repairability, serviceability, regulatory test performance, and so on. This is especially true if previous engineering is being "leveraged" into new designs.

2) Design for easy fabrication, processing, and assembly: Designing for easy parts fabrication, material processing, and product assembly is a primary design consideration. Even if labor cost is reported to be a small percentage of the selling price, problems in fabrication, processing, and assembly can generate enormous costs, cause production delays, and demand the time of precious resources. 3) Adhere to specific process design guidelines: It is very important to use specific design guidelines for parts to be produced by specific processes such as welding, casting, forging, extruding, forming, stamping, turning, milling, grinding, powdered metallurgy (sintering), plastic molding, etc. Some reference books are available that give a summary of design guidelines for many specific processes. Many specialized books are available devoted to single processes. 4) Avoid right/ left hand parts: Avoid designing mirror image (right or left hand) parts. Design the product, so the same part can function in both, right or left hand modes. If identical parts (cannot perform both functions, add features to both right and left hand' parts to make them the same. Another way of saying this is to use "paired" parts instead of right and left hand parts. Purchasing of paired parts

(plus all the internal material supply functions) is for twice the quantity and half the number of types of parts. This can have a significant impact with many paired parts at high volume. At one time or another, everyone has opened a briefcase or suitcase upside down because the top looks like the bottom. The reason for this is that top and bottom are identical parts used in pairs, 5) Design parts with symmetry: Design each part to be symmetrical from every "view" (in a drafting sense) so that the part does not have to be oriented for assembly. In manual assembly, symmetrical parts cannot be installed backwards, a major potential quality problem associated with manual assembly. In automatic assembly, symmetrical parts do not require special sensors or mechanisms to orient them correctly. The extra cost of making the part symmetrical (the extra holes or whatever other feature is necessary) will probably be saved many times over by not having to develop complex orienting mechanisms and by avoiding quality problems. It is a little known fact that in felt-tipped pens, the felt is pointed on both ends so that automatic assembly machines do not have to orient the felt. 6) If part symmetry is not possible. Make parts very asymmetrical: The best part for assembly is one that is symmetrical in all views. The worst part is one that is slightly asymmetrical which may be installed wrong because the worker or robot could not notice the asymmetry .Or worse, the part may be forced in the wrong orientation by a worker (that thinks the tolerance is wrong) or by a robot (that does not know any better). So, if symmetry cannot be achieved, make the parts very asymmetrical. Then workers will less likely install the part backward because it will not fit backward. Automation machinery may be able to orient the part with less expensive sensors and intelligence.

7) Design for fixturing: Understand the manufacturing process well enough to be able to design parts and dimension them for fixturing. Parts designed for automation or mechanization need registration features for fixturing. Machine tools, assembly stations, automatic transfers and automatic assembly equipment need to be able to grip or fixture the part in a known position for subsequent operations. This requires registration locations on which the part will be gripped or fixtured while part is being transferred, machined, processed or assembled. 8) Minimize tooling complexity by concurrently designing tooling: Use Concurrent Engineering of parts and tooling to minimize tooling complexity, cost, delivery lead-time and maximize throughput, quality and flexibility.

9) Specify optimal tolerances for a Robust Design: Design of Experiments can be used to determine the effect of variations in all tolerances on part or system quality. The result is that all tolerances can be optimized to provide a robust design to provide high quality at low cost. Tolerance also depends of the assembly process adopted. As shown in Figure 8-5, automatic assembly, manual assembly and selective assembly respectively are in the increasing order of tolerances. 10) Specify quality parts from reliable sources: The "rule of ten" specifies that it costs 10 times more to find and repair a defect at the next stage of assembly. Thus, it costs 10 times more to find a part defect at a sub-assembly; 10 times more to find a sub- assembly defect at final assembly; 10 times more in the distribution channel; and so on. All parts must have reliable sources that can deliver consistent quality over time in the volumes required. 11) Minimize Setups: For machined parts, ensure accuracy by designing parts and fixturing so all key dimensions are all cut in the same setup (chucking). Removing the part to reposition for subsequent cutting lowers accuracy relative to cuts made in the original position. Single setup machining is less expensive too. 12) Minimize Cutting Tools: For machined parts, minimize cost by designing parts to be machined with the minimum number of cutting tools. For CNC "hog out" material removal, specify radii that match the preferred cutting tools (avoid arbitrary decisions). Keep tool variety within the capability of the tool changer. 13) Understand tolerance step functions and specify tolerances wisely: The type of process depends on the tolerance. Each process has its practical "limit" to how close a tolerance could be held for a given skill level on the production line. If the tolerance is tighter than the limit, the next most precise (and expensive) process must be used. Designers must understand these "step functions" and know the tolerance limit for each process.

He should remember that the smaller the tolerance, the more accurately a machine part will be finished and the smoother will be the operation of the mechanism that he is designing—at least within certain limits. On the other hand, the smaller the tolerance, the more expensive the machining of the part will be. As a first approximation one may assume that the product of the cost of machining and the tolerance is constant. When this statement is expressed graphically, you can see how fast the cost goes up with a decrease of tolerance. There fore the designer should carefully analyze what tolerance is permissible for every dimension and should not specify a tolerance smaller than that really necessary.

In the shop it is much more difficult to obtain a certain tolerance with a large dimension than with small one. DFM APPROACH

i) Simplify the design and reduce the number of parts: Because for each part, there is an opportunity for a defective part and an assembly error, the probability of a perfect product goes down exponentially as the number of parts increases. As the number of parts goes up, the total cost of fabricating and assembling the product goes up. Automation becomes more difficult and more expensive when more parts are handled and processed. Costs related to purchasing, stocking, and servicing also go down as the number of parts are reduced. Inventory and work-in-process levels will go down with fewer parts. As the product structure and required operations are simplified, fewer fabrication and assembly steps are required, manufacturing processes can be integrated and lead-times further reduced. The designer should go through the assembly part by part and evaluate whether the part can be eliminated, combined with another part, or the function can be performed in another way.

To determine the theoretical minimum number of parts, the following need to be asked:

a. Does the part move relative to all other moving parts? b. Must the part absolutely be of a different material from the other

parts? c. Must the part be different to allow possible disassembly

ii) Standardize and use common parts and materials: This is required to facilitate design activities, to minimize the amount of inventory in the system, and to standardize handling and assembly operations. Common parts will result in lower inventories, reduced costs and higher quality. Operator learning is simplified and there is a greater opportunity for automation as the result of higher production volumes and operation standardization. Limit exotic or unique components

because suppliers are less likely to compete on quality or cost for these components. Group technology (GT) and Component Supplier Management (CSM) systems can be utilized by designers to facilitate retrieval of similar designs and material catalogs or approved parts lists can serve as references for common purchased and stocked parts. GT can also be used to guide in the development of manufacturing cells for common part or product families, thereby minimizing inventory and providing improved effectiveness through manufacturing focus. For example, if different types of fasteners are avoided, a single spanner or screwdriver will be adequate.

iii) Design for ease of fabrication: Select processes compatible with the

materials and production volumes. Select materials compatible with production processes and that minimize processing time while meeting functional requirements. Avoid unnecessary part features because they involve extra processing effort and/ or more complex tooling. Consider specific guidelines appropriate for the fabrication process such as the following for machineability:

a. For higher volume parts, consider

castings or stampings to reduce machining.

b. Use near net shapes for molded and

forged parts to minimize machining and processing effort.

c. Design for ease of fixturing by providing

large solid mounting surface and parallel clamping surfaces.

d. Avoid designs requiring sharp corners or

points in cutting tools -they break easier.

e. Avoid thin walls, thin webs, deep pockets or deep holes to withstand clamping and machining without distortion.

f. Avoid tapers and contours as much as

possible in favor of rectangular shapes.

g. Avoid undercuts, which require special operations and tools.

h. Avoid hardened or difficult-to-machine materials unless essential to requirements.

i. Put machined surfaces on same plane or with same diameter to minimize number of operations.

j. Design work-pieces to use standard cutters, drill bit sizes or other tools.

k. Avoid small holes (drill bit

breakage greater).

l. Avoid length to diameter ratio > 3 (chip clearance & straightness deviation).

iv) Design within process capabilities and avoid unneeded surface finish

requirements: Know the production process capabilities of equipment and establish controlled processes. Avoid unnecessarily tight tolerances that are beyond the natural capability of the manufacturing processes. Otherwise, this will require that parts be inspected or screened for acceptability. Determine when new production process capabilities are needed early to allow sufficient time to determine optimal process parameters and establish a controlled process. Also, avoid tight tolerances on multiple, connected parts. Tolerances on connected parts will "stack-up" making maintenance of overall product tolerance difficult. Design in the center of a component’s parameter range to improve reliability and limit the range of variance around the parameter objective. Surface finish requirements likewise may be established based on standard practices and may be applied to interior surfaces resulting in additional costs where these requirements may not be needed.

“Dissatisfaction with existing designs coupled with determination to improve them has produced many inventions.”

“Winners don’t do different things, they do things differently.”

DESIGN FOR ASSEMBLY Experience shows that it is difficult to make large savings in cost by the

introduction of automatic assembly in the manufacture of an existing product. In those cases where large savings are claimed, examination will show that often the savings are really due to changes in the design of the product necessitated by the introduction of the new process. It can probably be stated that, in most of these instances, even greater savings would be made if the new product were to be assembled manually. Undoubtedly, the greatest cost savings are to be made by careful consideration of the design of the product and its individual component parts.

When a product is designed, consideration is generally given to the ease of manufacture of its individual parts and the function and appearance of the final product. Although for obvious reasons it must be possible to assemble the product, little thought is usually given to those aspects of design that will facilitate assembly of the parts and great reliance is often placed on the dexterity of the assembly operators. An operator is able to select, inspect, orient, transfer, place, and assemble the most complicated parts relatively easily, but many of these operations are difficult, if not possible, to duplicate on the machine. Thus, one of the first steps in the introduction of automation in the assembly process is to reconsider the design of the product so that the individual assembly operations become sufficiently simple for a machine to perform. DFA APPROACH

1) Design for ease of assembly: Simple patterns of movement and minimizing the axes of assembly are recommended. Complex orientation and assembly movements in various directions should be avoided. Part features

should be provided adequate chamfers and tapers. The product design should enable assembly to begin with a base component with a large relative mass and a low center of gravity upon which other parts are added. Assembly should proceed vertically with other parts added on top and positioned with the aid of gravity. This will minimize the need to re-orient the assembly and reduce the need for temporary fastening and more complex fixturing. A product

that is easy to assemble manually will be easily assembled with automation. Assembly that is automated will be more uniform, more reliable, and of a higher quality.

2) Fool-proof product design and assembly: The assembly process should be unambiguous. Components should be designed so that they can only be assembled in one way; they cannot be reversed. Notches, asymmetrical holes and stops can be used to fool-proof the assembly process. Design verifiability into the product and its components is required. For mechanical products, verifiability can be achieved with simple got no go tools in the form of notches or natural stopping points. Products should be designed to avoid or simplify adjustments. Electronic products can be designed to contain self-test and/ or diagnostic capabilities. Of course, the additional cost of building in diagnostics must be weighed against the advantages.

3) Design for Parts orientation and handling: This is required to minimize

non-value-added manual effort and ambiguity in orienting and merging parts. Basic principles to facilitate parts handling and orienting are:

Parts must be designed to consistently orient themselves when fed

into a process.

Product design must avoid parts, which can get entangled, wedged or disoriented.

Avoid holes and tabs and designed "closed" parts. This type of

design will allow the use of automation in parts handling and assembly such as vibratory bowls, tubes, magazines, etc.

Part design should incorporate symmetry around both axes of

insertion wherever possible. Where parts cannot be symmetrical, the asymmetry should be emphasized to assure correct insertion or easily identifiable feature should be provided.

With hidden features that require a particular orientation, provide an

external feature or guide surface to correctly orient the part.

Guide surfaces should be provided to facilitate insertion.

Parts should be designed with surfaces so that they can be easily grasped, placed and fixtured. Ideally this means flat, parallel surfaces that would allow a part to picked-up by a person or a gripper with a pick and place robot and then easily fixtured.

Minimize thin, flat parts that are more difficult to pick up. Avoid very small parts that are difficult to pick-up or require a tool such as a tweezers to pick-up. This will increase handling and orientation time.

Avoid parts with sharp edges, burrs or points. These parts can

injure workers or customers, they require more careful handling, they can damage product finishes, and they may be more susceptible to damage themselves if the sharp edge is an intended feature.

Avoid parts that can be easily damaged or broken. o Avoid parts

that are sticky or slippery (thin oily plates, oily parts, adhesive backed parts, small plastic parts with smooth surfaces, etc.). Suitable dispensers can be used for applying these.

Avoid heavy parts that will increase worker fatigue, increase risk of

worker injury, and slow the assembly process.

Design the workstation area to minimize the distance to access and move a part.

When purchasing components, consider acquiring materials

already oriented in magazines, bands, tape, or strips.

4) Minimize flexible parts and interconnections; Avoid flexible and flimsy parts such as belts, gaskets, tubing, cables and, wire harnesses. Their flexibility makes material handling and assembly more difficult and these parts are more susceptible to damage. Use plug-in boards and back-planes to minimize wire harnesses. Where harnesses are used, consider fool-proofing electrical connectors by using unique connectors to avoid connectors being misconnected. Interconnections such as wire harnesses, hydraulic lines, piping, etc. are expensive to fabricate, assemble and service. Partition the product to minimize interconnections between modules and co-locate related modules to minimize routing of interconnections.

5) Design modular products: This is required to facilitate assembly with

building block components and sub-assemblies. This modular or building block design should minimize the number of part or assembly variants early in the manufacturing process while allowing for greater product variation late in the process during final assembly. This approach minimizes the total number of items to be manufactured, thereby reducing inventory and improving quality. Modules can be manufactured and tested before final assembly. The short final assembly lead-time can result in a wide variety of products being made to a customer's order in a short

period of time without having to stock a significant level of inventory .Production of standard modules can be leveled and repetitive schedules established.

6) Design for automated production: Automated production involves less

flexibility than manual production. The product must be designed in such way that it can be easily handled with automated equipment. There are two automation approaches: flexible robotic assembly and high speed automated assembly. Considerations with flexible robotic assembly are: design parts to utilize standard gripper and avoid gripper/ tool change, use self-locating parts, use simple parts presentation devices, and avoid the need to secure or clamp parts. Considerations with high speed automated assembly are: use a minimum of parts or standard parts for minimum of feeding bowls, etc., use closed parts (no projections, holes or slots) to avoid entangling, consider the potential for multi-axis assembly to speed the assembly cycle time, and use pre-oriented parts.

7) Design for efficient joining and fastening: Threaded fasteners (screws,

bolts, nuts and washers) are time-consuming to assemble and difficult to automate. Where they must be used, standardize to minimize variety and use fasteners such as self-threading screws and captured washers. Evaluate other bonding techniques with adhesives. Match fastening techniques to materials, product functional requirements, and disassembly! Servicing requirements.

8) Design printed circuit boards for assembly: With printed circuit boards

(PCB's), guidelines include: minimizing component variety, standardizing component packaging, using auto-insert- able or place able components, using a common component orientation and component placement to minimize soldering "shadows", selecting component and trace width that is within the process capability, using appropriate pad and trace configuration and spacing to assure good solder-joints and avoid bridging, using standard board and panel sizes, using tooling holes, establishing minimum boarders, and avoiding or minimizing adjustments.

DESIGN FOR QUALITY

Quality is the most effective factor a company can use in the battle for customers. Design for Quality (DFQ) emphasizes on the role of quality in the total production cycle, including customer inputs, competitive benchmarking, performance specifications, product and process design, manufacturing variability and product reliability.

To be competitive, we must satisfy the customers. In order to be more competitive, we must delight the customer. Quality is defined here as the measure of customer delight. Note that customer satisfaction is a region on the

scale of customer delightment. To delight the customer, we must design for quality.

The domain of DFQ includes: anticipating and satisfying customer expectations; a fundamental understanding of "variability" and the way it affects production processes; the new product life cycle and how to lower costs through merging design specifications and production; and more advanced topics such as robust design and the optimization of manufacturing processes.

Kaizen, a Japanese concept for continuous improvement, provides the philosophy and driving force for DFQ. Total Quality Control provides the implementation. The concepts are elegant. If quality is made the global driving force, then the customers will obtain the best value possible. DESIGN FOR COST One of the most important things that the designer must keep in mind at all times is the cost of any part and assembly of parts depends on the material used; the amount of labor involved in machining and assembling; and in any multiple reproduction, features of standardization and sales appeal. Materials .As a result of the metallurgical developments of the last years a great numbers of material are available for every machine part. These materials vary in quality and in price. A good designer will select the least expensive material that will be satisfactory for the duty of the part. For example, he will use more –expensive alloy steels only when ordinary low carbon steel cannot give satisfactory service. When forced to use alloy steel he should compare the properties and costs of different kinds and should again follow the same principle. In selecting materials the designer must consider not only their strength but also their rigidity and their resistance to wear. The permissible wear of a part depends on its duty and also on the length of service that must give. If a part is subjected to severe wear when in operation but is operated intermittently and nit very often, a comparatively inexpensive material may be satisfactory; whereas a similar part in continuous operation will require a more wear-resistant material. The expected life of a part or a machine must also be taken into account .The longer the intended life; the better should be the grade of the material used. The same consideration applies to parts purchased from other manufactures, such as bolts, setscrews, bushings, and ball bearings. For instance, if the intended life of a certain machine is 3,000 hr, its cost will be unnecessarily increased by using in it ball bearings with an expected life of 10,000 hr. Whenever possible, standard stock sizes of rolled and extruded materials should be used without changing their cross-sectional dimensions.

Effective product cost management requires a Design for Cost (DFC) philosophy as its basis since a substantial portion of the product's cost is dictated by decisions at the design. Design for cost is a management strategy and supporting methodologies to achieve an affordable product by treating target cost as an independent design parameter that needs to be achieved during the development of a producl. A design for cost approach consists of the following elements:

An understanding of customer affordability or competitive pricing requirements by the key participants in the development process.

Establishment and allocation of target costs down to a level of the

hardware where costs can be effectively managed.

Commitment by development personnel to development budgets and target costs.

Stability and management of requirements to balance requirements with

affordability and to avoid creeping elegance.

An understanding of the product's cost drivers and consideration of cost drivers in establishing product specifications and in focusing attention on cost reduction

Product cost models and life cycle cost models to project costs early in the

development cycle to support decision-making

Active consideration of costs during development as an important design parameter appropriately weighted with other decision parameters

Creative exploration of concept and design alternatives as a basis for

developing lower cost design approaches.

Access to cost data to support this process and empower development team members

Use of value analysis / function analysis and its derivatives (e.g., function

analysis system technique) to understand essential product functions and to identify functions with a high cost to function ratio for further cost reduction

Application of DFMA principles as a key cost reduction tactic

Meaningful cost accounting systems using cost techniques such as

Activity-Based Costing (ABC) to provide improved cost data

Consistency of accounting methods between cost systems and product cost models as well as periodic validation of product cost models

Continuous improvement through Value Engineering to improve product

value over the longer term. Machining:

Any kind of machining should be specified only where it is necessary to permit the part to function properly. Informer years it was considered necessary to machine the outsides of parts that were to be put together in contact, such as the top of a cylinder and the outside of the cylinder head, in order to match them accurately. This is not necessary, and the method is less expensive and Justas good. Finishing cover plates on the edges and from the top is another waste of machining.

All that is necessary is to spot-face around the holes for the nuts.

Where machining is necessary it should be done by the least expensive method that is consistent with the purpose of the machining. If, for example a part must be turned for the sake of balancing, rough turning is satisfactory ,and specifying finish turning would be a mistake .Similarly ,if planning is satisfactory, more expensive milling should not be specified on the drawing, and reaming should not be specified if simple drilling is satisfactory.

Where fitting of parts requires tolerances, the specified tolerances should not be closer than those absolutely, do not specify a smaller rms roughness than that actually necessary.

In order to reduce the cost of assembling a machine, the parts of the machine should as far as possible be so designed and built that they will place and align themselves automatically when brought together. Usually, the more complicated a machine is, the more important it is that the case and cost of assembling be given careful consideration. If bench work and hand fitting cannot be avoided, they should be reduced to a minimum.

The number of machines to be built has an important bearing on the design of a machine and its parts. If only one machine or a few machines of a certain kind and size are to be built, the limitations of the available plant equipment should be kept in mind in the design. The capacity of the equipment of the foundry or machine tools at hand may limit the methods of machining and thus influence the design. In Certain cases it maybe found more economical to weld a frame than to make a large, expensive pattern for it.

On the other hand, mass production or interchangeable manufacture justifies the use of special molding machines in the foundry, special jigs and

fixtures, special production and inspection gages, and special tools, dies, and machines in the welding and machine shops. Standardization:

The purpose of standardization is to establish mandatory of obligatory norms or standards to which the different types, grads, parameters, quality characteristics, test methods, rule of marking, packing and storage of finished items, raw materials and semi finished products to conform. Its aim is to minimize variety so that the number of types, dimensions and size etc., are limited to a definite number of models. Standardization is of great importance in mechanical engineering.

If a part is made in lots, especially if the part is manufactured on a mass –production basis, it is important to follow certain standards. Once a part is designed and developed, it should be considered standardized; and no changes should be made that would make the part not interchangeable with the original design. Standard stock parts should be used without any additional machining. Preferred numbers: When a machine is to be made in several sizes having different powers or capacities, it is necessary to decide what capacities will cover a certain range efficiently with a minimum number of sizes. When a larger similar machine is built, its relation to the original smaller machine is complicated. When preferred numbers are used, fewer stock sizes can cover certain ranges. Such a reduction would mean a great saving in inventory and probably in manufacturing cost too. There is a wide field for the application of preferred numbers in various fields, and machine designers can contribute a good share to our economy by using them for serial designs in proper places.

DESIGN FOR SAFETY

Design for Safety (DFS) of an engineering system is a process of identifying the possible failure events (top events) and the associated consequences, estimating them and finally evaluating them. It provides the designer with a systematic approach to identify high-risk areas and attain explicit levels of safety by identifying and implementing ways to reduce the hazard frequency of occurrence and the extent of respective consequences. In such a process risk identification and risk- assessment may be the most difficult and important steps that always attract a great deal of attention by safety researchers.

“Success doesn’t mean the absence of failures; it means the attainment of ultimate objectives. It means winning the war, not every battle.”

“To a positive thinker, attitude can be a stepping stone to success. To a negative

thinker, it can be a stumbling block.”

DESIGN FOR RELIABILITY Reliability consideration has tended to be more of an after-thought in the

development of many new products. Many companies' reliability activities have been performed primarily to satisfy internal procedures or customer requirements. Where reliability is actively considered in product design, it tends to be done relatively late in the development process. Some companies focus their efforts on developing reliability predictions when this effort instead could be better utilized understanding and mitigating failure modes, thereby developing improved product reliability. Organizations will go through repeated (and planned) design/ build/ test iterations to develop higher reliability products. Overall, this focus is reactive in nature, and the time pressures to bring a product to market limit the reliability improvements that might be made. Specific Design for Reliability guidelines includes the following:

Design based on the expected range of the operating environment. Design to minimize or balance stresses and thermal loads and/or reduce

sensitivity to these stresses or loads. De-rate components for added margin. Provide subsystem redundancy. Use proven component parts & materials with well-characterized reliability. Reduce parts count & interconnections (and their failure opportunities). Improve process capabilities to deliver more reliable components and

assemblies. “How much easier our work would be if we put forth as much effort trying to improve the quality of it as most of us do trying to find excuses for not properly attending to it.”

“Queer thing, but we always think every other man’s job is easier than our own, and the better de does it, the easier it looks.”

“If you are sufficiently disgusted with a present design to do something about it, you

are on the right road, to create an invention.”

DRAWING OFFICE PRACTICE INTRODUCTION

Drawing is the universal language of the engineers. It is a graphic language used to describe the size and shape of individual components or to describe the mutual disposition of a group of components. It can also be a schematic representation, drawn by using clearly defined or standardised symbols, explaining or detailing a function. Engineering drawing is such an important means of communication.

The drawings are the final output of any design activity. This forms

the basic document for the further manufacture of components, procurement of bought out items, inspection, subassembly and assembly of the products. Drawings should contain the complete information in clear unambiguous forms for further processing. ISO (International Organisation for Standardisation) has brought out a series of standards dealing with all aspects, which guide in the preparation of drawings. The same has been adopted by Bureau of Indian Standards (BIS) as Indian Standards. Adoption of these standards during preparation of drawings will ensure aligning the individual company drawings to the international practices, so that they are easily interpreted by all users nationally and internationally and these have minimized misunderstanding and consequent delays among all concerned.

Engineering office practice involves methods adopted for the

systematic preparation of engineering drawings and their documentation and archiving.

They can be listed under the following main groups:

♦ Drawing standards ♦ Drawing numbering system ♦ Documentation ♦ Archiving ♦ Procedure for effecting alterations/modifications

To ensure complete compliance to the accepted drafting standards, to ensure manufacturability and suitable inspection, to effect economy at the design stage itself by ensuring the variety reduction of bought out (standard parts) components and tools & tooling, and to make sure that the drawings are indeed complete in all respects as a means of providing effective and unambiguous communication, it is necessary to check the design drawings for these possible deviations or deficiencies before they are released for manufacture.

DRAWING STANDARDS: Drawing standards comprise of standards for:

Drawing sheets The drawing sheets are standardized and designated as A0, A1, A2, A3 & A4 The following table gives the sheet size of standard drawing sheets available.

Two successive format of a series of sizes are obtained by halving along the length or doubling along the width. The areas of the two sizes are in the ratio 1:2 as shown in the figure

“The three essentials to achieve anything worthwhile are: hard work, courage and

common sense.”

“Progress is impossible without change, and those who cannot change their minds cannot

change anything.”

The above figure shows a typical drawing sheet layout.

Scales, lines and lettering Since manufacturing drawings convey technical information, for correct interpretation, they have to be drawn by lines of standard thickness and types, with standard lettering practice and to recommended standard scales. For engineering drawings, the types of lines recommended are as follows:

A. Continuous thick: Visible outlines (outlines to be bold & dark) B. Continuous thin: Dimension lines, leader lines, extension lines, construction lines, outlines of adjacent parts & hatch lines. C. Continuous thin wavy: Irregular boundary lines, short break lines. D. Short dashes medium: Hidden outline & edges. E. Long chain thin: Center lines, pitch circles, extreme positions of movable parts. F. Long chain thick at ends & thin elsewhere: Cutting plane lines G. Long chain thick: To indicate surfaces which are to receive additional treatment. H. Short zigzag thin : Long break lines.

The fiure above shows the application of different types of lines.

The scales have to be chosen depending upon the complexity of the object and the size of the drawing sheet. In all cases, the selected scale should be just large enough to permit easy and clear interpretation of the information depicted. The choosen scale must be indicated in the title block. The table below gives the preferred Scales that can be used in the drawings.

All engineering drawings consist of some written details to convey the technical information. Writing of these particulars and dimension figures for indicating the sizes of the object is called lettering. The height of the letters for different applications may be selected from the table given below.

♦ Projection, sectioning & conventional representation ♦ Dimensioning and tolerancing of dimensions (to be discussed in

detail) ♦ Tolerancing of form and position (to be discussed in detail) ♦ Surface roughness and surface treatment (to be discussed in detail) ♦ Symbols and symbolic representation (to be discussed in detail) ♦ Folding of prints etc,.

Title Block

On any drawing, the title block is of great significance. It contains all the details pertaining to the component represented on the drawing. The title block is normally placed at the right hand bottom corner of the drawing and includes such details as the title of the component, the drawing number, the scale, drawing number of the immediate assembly into which the component/sub assembly goes, material, surface roughness, number of pieces required for the assembly, etc,. and it provides space to indicate future alterations on the drawing.

ABC ABC INC.

DRAWING NUMBERING SYSTEM Project Number

Each project taken up by the design department should have a unique project number which could be a 3 digit number with the prefix of the year in which it is taken up Eg. 98.205, 99.206 etc,. Project code is also to be given in the title block for eg,. Drilling machine, radial, 25 mm capacity could be indicated as DR25.

A good drawing numbering system provides for a systematic and good scientific coding method that enables the designer to identify drawings based on the mutual relationship, size and technological features of the component and avoid duplication.

Drawing numbering system may differ from industry to industry and from company to company depending on the products/machine manufactured etc,.

Drawing numbering can be broadly be classified into: ♦ Numbering of part drawings ♦ Numbering of assembly drawings Numbering of Part Drawings

Numbering of parts drawings provides an easy identification of a component, which may be based on the technological features of the component such as size or weight, shape, machining features, material etc,. the size of the drawing sheet and the serial number with the required number of digits. In order to give the serial number, particular project will be assigned a particular project number and all the components intended for that project may be numbered serially starting from, say 0001. Numbering of Assembly Drawings

Assembly drawings are essentially numbered, independently of the part drawings. The assembly drawing number could be a three digit number beginning from 001 for overall assembly. Nature of Assembly Number corresponding to the nature of the assembly is selected, as follows: ♦ Overall assembly (Main assembly of machine/ product.) ♦ Sub-assembly (All sub assemblies coming in a group or main assembly other

than those indicated below) ♦ All sub assemblies involving welding, brazing, riveting operations etc,. ♦ Auxiliary drawings (Erection drawings, tubing assembly, etc,.) ♦ Circuits and schematics (Electric circuits, hydraulic schemes, kinematic

schemes, etc,.) ♦ Dimensional sketches of the machines. ♦ Operating and instruction manuals, servicing manuals etc,.

“The quicker you let go of old cheese, the sooner you find new cheese.”

“Give a man a fish, you feed him for a day; teach him how to fish you feed him for a lifetime.”

“Experience is what you get when you don’t get what you want.”

Number Register For each design, a separate number register is prepared containing the

list of assemblies and list of parts, both serially numbered. This enables documentation of drawings prepared for a particular design. The number register also provides for an easy location of the part drawing, clearly indicating the group in which it is used. A specimen format for no. register, which is being used, is attached for information.

NUMBER REGISTER

Assy / Part Dwg No. Name Used in Assy

Project No. Prepared By Project Name Approved By LOGO Date Revision

PART LIST (BILL OF MATERIALS) Part list is a list giving the material number, and size of all sub assemblies,

manufactured parts, procured parts, and standard parts that go into an assembly, arranged in that order. The arrangement in that order makes the process planning work and purchase of raw material and standard parts easier.

Every assembly shall have a corresponding part list or a set of part lists, depending on the size of the assembly and the number of parts. A welded assembly having a few individual parts may have only one part list, while group assembly or the main assembly having several sub-assemblies and a number of individual parts followed by some standard parts, will have a set of part lists, as illustrated in enclosed specimen part list.

In the part list, the individual parts, sub-assemblies and standard parts are incorporated in a methodical way, which makes the process planning work, as well as the purchase of the raw materials and standard parts, more easier. A specimen format of part list is enclosed for information. A schematic for distribution of part lists in a particular design is indicated below

PART LIST / BILL OF MATERIAL Sl No. Name Dwg No. Materia

l Qty Size Std

No. Remarks

Project No. Prepared By Project Name Approved By LOGO Assy/ Sub-assy No.

Date

Revision

ALTERATIONS AND MODIFICATIONS Procedure on effecting alterations/modifications on a drawing depends on

whether the main features of the component are altered or modified.

When alterations/modifications do not alter the main features of a component, the alterations are effected on the drawing and are marked by indices a, b, c, etc,.. These alterations/modifications are entered in the appropriate column in the title block by giving reference to the area in which alterations/modifications are affected and the corresponding index.

If the alterations/modification is such that it alters the main features, then a new drawing under the same drawing number is prepared and the old drawing is superceded. In the title block of superceding drawings, a clear mention of the drawing number and the date of superceded drawing is made at an appropriate place. Likewise, a mention of the drawing number and the date of superceding modifications should be made also on all the assembly drawings in which the part is used, in part list and number register and other related places. FILING OF DRAWINGS

Order of filing of drawing sheets, part lists and no. register for a particular design is as given below. This method of filing helps in easy identification of drawings and makes it possible to file all drawings, assemblies, sub-assemblies and part lists belonging to a group at one place. Order for Filing for Project ♦ Number register. ♦ Overall Assembly drawings. ♦ All parts lists, in a serial order ♦ All part drawings filed in the same serial order as listed in the no. register. MANUAL PREPARATION

Each and every machine is provided with an instruction manual with a view to enable the customer to get accustomed to the machine regarding its installation, use and proper upkeep. The instruction manual should contain all information necessary so that installation, operation and maintenance of the machine is possible without further guidance from the manufacturer. Sometimes separate manuals are prepared for maintenance, installation, spare parts, safety etc,. The Instruction manual, should contain the following: ♦ A perspective view of the machine. ♦ Detailed technical specifications, with the list of extra accessories and special

accessories.

♦ General description of the machine indicating the operational features. If necessary the operational features of the sub-units, accessories and additional equipment, to be given in detail.

♦ Kinematic diagram for the machine tool as a whole and if necessary for each and every sub-units and accessory.

♦ Clear description of all the auxiliary systems such as hydraulic, pneumatic or electric circuits, lubrication, etc,. Schematic diagrams, with universally accepted or standardised symbols, are to be given.

♦ A brief note, giving the details of the possible faults, their detection and rectification.

♦ A description of all such items, which require frequent attention for maintenance, such as belt transmission, brakes, hydraulic seals, etc,.

♦ Spare parts with their identification numbers. This should also clearly indicate the method of specifying spare parts, while ordering.

♦ Clear instructions for handling, transportation and lifting of machine. If possible detailed instruction with diagrams should be given for do’s and don’ts during handling and transportation.

♦ Foundation plan, method of installation and precautions while installing the machine.

♦ Clear instructions for initial starting and running of the machines. ♦ Lubrication chart using standardised symbols, indicating the type of lubricant

to be used, frequency, and points, which are to be lubricated. The following points are to be borne in mind while preparing the instruction manual: ♦ The information given should be complete in all respects and should not

make the customer feel lacking in information. The information should be brief and clear and should not contain ambiguous statements.

♦ All drawings, diagrams and symbolic representations should confirm to standard conventions and methods.

♦ The instruction manuals, for the machine for export, should be prepared in the language of that country.

♦ Information given in the manual should be consistent with the actual machine tool that is being supplied. If there is any difference, due to developmental and technological advances, it should be clearly indicated.

♦ The instruction manuals should preferably be on A4 size and printed on quality paper and bound so that any removal of intermediate sheets can be easily detected. Blue printing or cyclostyling is also permissible.

“Unless you try to do something beyond what you have already mastered, you will never grow.”

DOCUMENTATION As far as drawing office practice is concerned, documentation relates to

collection of all drawings and related documents and supplying copies of the same for further processing, whenever required. The following are the list of documents that have to collected: • Dimensional sketch of the machine with the main specifications. • Number register. • All group assembly, sub-assembly and welding assembly, drawings with

corresponding part lists. • All circuit diagrams and schematics. • All part drawings. • Instruction and operating manuals. • All auxiliary drawings, such as special fixtures, which are exclusively prepared

for components called for in the project, or tubing assembly drawings etc,. • Test certificate format. • Test reports. • Typified lists of standards (Company standards referred to in the part lists). ARCHIVES

Originals of all the documents are stored in the archives. The following points are to be noted while storing: • The originals are so arranged that they can be easily located when required,

and can be produced with unreasonable delay. • The originals are protected against burglary, heat, fire hazards, insects, dust

and dampness.

The drawing filing containers used for storing the originals must be capable of storing an A0 size unrolled. If required, it must be capable of being suitably partitioned to enable storing of different sizes of sheets from A4 to A1. The equipment used should not unnecessarily require undue physical effort, for locating, inserting, and extracting the originals.

If required, suitable arrangements should be made for safe storing and identification of microfilms and all necessary precautions against damage or loss of these microfilms must be taken. If however the drawings are prepared on CAD systems, they could be stored in hard discs, floppies, CDs or optical discs.

“If you do not change, you can become extinct.”

“There is always something about your success that displeases even your best friends.”

REVERSE ENGINEERING INTRODUCTION:

Reverse engineering (RE) is the process of creating a mathematical representation or CAD model of an object from its physical form. RE is necessary when

• A part is first modeled in clay, wood or foam and needs to be transferred into CAD

• Only 2D drawings or master models of physical tooling exist • A Competitor’s product needs to be analyzed • A change in a physical part or tool must be captured in CAD • Final parts have to be verified against the original CAD design.

The process consists of first scanning the object to create clouds of points

(Cops) that represent the skin of the object. These points on the boundary of the object are used to create the surface model of the object using software such as surfacer. If enough topological details are available, the surface model can be converted into solid model. Otherwise, user can provide the necessary information to convert it into a solid model. The solid model thus created can be then used for analysis, documentation and NC cutter path generation or for prototyping. RE essentially consists of two steps

I. Acquiring point data II. Constructing 3D model.

Acquiring point data is carried out by an appropriate hardware and the

construction of 3D model from the acquired data is carries out using suitable software. ACQUIRING POINT DATA

The models used to produce CoPs or point data as output can be divided into two major types:

I. Contact type II. Non –contact type.

Touch probe based systems such as Coordinate Measuring Machines (CMMs) are contact type, whereas laser scanners, optical fringing photoprammetry systems come under non-contact type. While the contact types are more accurate, they are slow, labor intensive and they cannot reach interior locations. On the other hand, the non contact type acquisition systems can produce millions

of points within a short time but here the skill of the person doing the scanning is a crucial factor. Contact type Methods

The various contact type measurement methods are discussed in this section. Manual Measurement

Manual measurement using simple instruments such as scale, measuring tape, vernier calipers, micrometer, bore guage, height gauge, templates, slip gauge, standard pins etc., will be enough for objects with simple geometry. This is the most labor-intensive method. Furthermore, this method does not create 3 D data directly; one needs to use the measured values to create 3D model. Touch Probe Measurement

The surface of the object is measured by using contact sensitive sensor or touch probe mounted on Coordinate Measuring Machine (CMM) Shown in Figure. When the probe touches the surface, a signal is sent back to the machine, which records the x, y, and z coordinates of the contact point. The CMM is generally a portal frame type and fixed in a particular location with an environment controlled for temperature humidity and dust. Depending on the investment, it may have features to automatically scan the object in a predetermined path.

The advantage of this method is that the

object to be measured does not always have to be moved for full 3D scans since the conventional CMM will have multiple probes calibrated at different orientations and the articulated one is inherently dexterous. Additionally, by scanning only corners and edges, it is easier to recover the topology of the 3D object. Internal openings pose less of a problem. However, this method is labor intensive and time consuming. Furthermore, this method can generate only smaller data sets, typically

around 120 points per minute only.

There are also portable CMMs, which look like an articulated mechanical, arm .Its articulated are generally consists of 3 parts, Viz, the base, elbow and wrist. The wrist that holds the calibrated probe can be moved manually. Data are collected at the probe tip, most often through the use of a manual switch or button .A portable CMM requires either an integrated or separate controller unit that calculates the probe position based on the angles of the encodes at each joint and the lengths of each link. The x, y and z values are then transferred via a serial line to an application software executing on a computer.

Non –contact type Methods Laser Scanning

In a laser scanner pulsed laser is directed at the object and the reflections are measured .The time lag between emitting pulse and receiving it by the detector after reflection from the surface point is the measure of its distance. The resulting CoP has an accuracy of around 0.5mm.This method produces large data sets (in the order of 106 points) within a very short time. The disadvantages are that the object or the measuring device has to be moved to produce a full 3D scan and interior openings might be in the shadow of the laser beam.

A common method for acquiring range data is active optical triangulation. Measuring an object’s surface depth on a regular sampling lattice produces range data. The by connecting triangular elements with the nearest neighbors, a range image is created.

Generally, a 1D or 2D sensor is swept linearly across the object or circularly around it. This is usually not enough information to reconstruct the entire object being scanned. Therefore, multiple passes must be made from different orientations. Algorithms are required to merge multiple range images into single description of the surface.

Although this technique has been in use for more than 20years,its speed and accuracy have increased dramatically in recent years with the development of stable imaging sensors such as CCDs and lateral effect photodiodes.

There are several different types of scanners that accomplish this, their primary difference being in the structure of illuminant (typically point, stripe, multipoint, or multi stripe), dimensionality of the sensor (linear array or CCD grid), and the scanning method (move the object or move the scanner hardware). One of the most obvious benefits to laser scanning is the tremendous increase in speed with which a prototype can be reproduced.

Traditional methods call for the object to be measured manually and converted into a CAD model. Not only is this extremely time consuming. But organic shapes are almost impossible to create using this method. Objects such as an ergonomically designed handle or new toy designs can be easily sculpted and then scanned to ensure the intended result. Laser scanning is at its best when dealing with ergonomic shapes .The entire scanning and post –editing process can happen in hours. This time saving means faster response to requirements. Because laser –scanning technology is relatively quick, it is generally much cheaper than other types of scanning.

Operator experience is a critical factor with optical laser scanning the operator must follow certain guidelines and be able to predict how the laser will react. Discretion must be shown when viewing the individual scans before

merging so that any unacceptable data will be discarded .It is necessary to have a clear understanding of how lasers work so as to know how to deal with them .The lighting in the location, the object’s distance from scanner, and the object’s color can all potentially affect the laser scanning process .The operator needs to be able to clearly distinguish acceptable from unacceptable data. The operator needs to be able to read and recognize clearly certain things in the point cloud, the native product of laser scanners.

Product verification is another example of the benefits of scanning. After a product has been produced, it can be scanned and the resultant data compared to the CAD/CAM design .The deviation of the part can then be accurately determined. Scanning is routinely used for periodic inspection of multiple parts to analyze how closely the product adheres to the original. This allows for greatly improved quality control and the ability to identify errors in the manufacturing process. CONSTRUCTING 3 D MODEL

At the end of measurement, the point data that lie on the boundary of the object only is available. These may be random cloud of points or in some order. If it is available in some order, say as in the case of CT scans, this order must be exploited. One has to obtain the solid model of the object from the point data using appropriate software. The details of constructing the 3D model are discussed in this section. Procedure:

Raw 3D digitized, or point cloud data, is memory hungry, static, and awkward. While it is possible to export raw data directly into CAD software .It can be very painful. Software such as surfacer, STRIM, Pro/Scan Tools, and Alias are specially developed to handle huge data sets. The following is a description of how such software is used for point cloud manipulation: Data Orientation:

Orienting data is the first step. For Injection –molded parts; the data is often oriented relative to the parting plane of the tool. If the part is symmetrical, the point cloud is oriented such that the mirror plane is defined. Once orientation is complete, all data exported to CD software will be located correctly for easier model creation. Manipulation of Data:

Often the final product is a variation of the part that was digitized. Therefore, data manipulation is required to reflect the desired changes. . Scaling data is the most common manipulation. Where critical assembly is involved, one may have to obtain the required fits, tolerances, and surface finish requirements based on other engineering considerations.

“The ladder of life is full of splinters, but they always peril the hardest when we are sliding down.”

Extraction Of Reference Curves and base Geometry: After data orientation and manipulation information is extracted, one piece

at a time, to aid in the CAD model creation. Curves:

Cross –sectional and 3D curves can easily be extracted from the point cloud data .The point could data can be sectioned through any plane, and a 3Dcurve can be created through any ridge or feature. In some cases such as CT data, the information is already available in slice form. These planar cross –sectional curve are created quickly. No time is spent smoothing the curves. The curves, the Curves are only going to be used as templates during the model creation. Geometric Features:

Points that make up a flat surface, cylinder, or sphere are isolated, and a best-fit surface is created. Verification of final surfaces:

Software is used for verification of the CAD model. Surfaces are exported several times during CAD model creation and compared, via a color variance plot, to the point cloud. Reconstruction of model

There are several CAD softwares that convert the available CoP into a 3D object. The data of cloud of points is passed to the CAD software in formats such as IGES, DXF or 3D StudioMAX. Various approaches have been used to take the problem of solid reconstruction form a given CoP. APPLICATIONS

Some important applications of Reverse engineering are presented in this section. Film Industry

The film industry quite frequently needs models of things or creatures, which do not exist. Therefore, plasticine models are created by hand, which are subsequently scanned. Once the fitted surfaces are available in the computer, the creatures can be brought to life and animated. Examples for this are images created for the films Godzilla and Jurassic Park. Similar applications can be found across the entertainment industry and the production of commercials. Security

It is common to restrict access to security sensitive areas by various means such as keys, cards, and fingerprint or eye recognition. A Novel approach is to use face recognition to perform the same task. The face of the visitor is scanned, a surface is fitted over the CoP and this surface is then compared with one, which has been stored. Only when the surfaces agree, access is granted.

Face recognition might also play an important role in the future of education with the rise of distance learning over the Internet. When examining students, it will be necessary to verify that the correct student is sitting in front of the computer answering the test questions .Another security application is the safeguarding of museum pieces .By scanning the exhibits and keeping an electronic copy, it is easier to detect false from original artifacts. Accident Assessment

After road accidents, the police typically measure out the accident spot and the position of the involved partners relative to each other. When using the laser cameras, the police simply take a few snap shots of the accident. These pictures are then fed into a computer, which recreates the accident scene. Additionally, it is possible to reconstruct the accident. Product Design /Rapid Prototyping

Engineering components can sometimes be styled rather then engineered. Typical examples include the shapes of cars, kettles, or telephones. These parts may be asclay or plasticine models. Rather then recreating them in the computer, it is possible to scan them and generate geometric CAD models of them. This styled part can be manufactured using any of the Reprocesses early in the design cycle. Quality Control

One part of quality is to compare the manufactured shape to the required shape. For simple –shaped products, this can be done by means of simple measurement. However, this is feasible for complex, large parts. Using scanning and surface fitting techniques, it is possible to capture and recreate the manufactured shape and compare it with the required shape. Fashion

The fashion industry has started to make extensive use if these technologies. They perform full body scans of people to generate geometric models. This allows designers to create their new clothes directly on the models in the computer .In this way; the designers are able to create a very good image of the appearance of their clothes. Restoration

When carrying out restoration work, there are usually no drawings of the components available. In such cases, the components can be scanned; a geometric model can be manufactured using modern CNC machines. Similarly, when engineering drawings are lost the CAD models can be reconstructed from an existing part.

“If you and I were to exchange dollar bills, neither would be any richer. But if we were to exchange new ideas, each would increase our knowledge.”

Medicine In medicine, Cavities in teeth are scanned, which then enable a filling to

be exactly milled from solid ceramics, and glued into the tooth for a longer lasting filling. Another area is the assessment of tumor grown. Sometimes, it can be difficult to judge whether a tumor has grown between two consecutive body scans. When extracting the CoP determining the tumor and fitting a surface to this CoP, it is possible to easily assess tumor growth accurately.

Traditionally, to manufacture orthopedic shoes it is necessary to wrap the patients foot into plaster, let the plaster harden then remove it. This is not only time consuming but it is untidy when scanning the foot, it is easily possible to generate a computer –based image of patient’s foot. From this, the orthopedic shoe can be manufactured using CNC machining.

A precise reverse engineered model facilitates the pre –operative planning on an optimal surgical approach and enables selection of correct or appropriate implants .The reliability and the accuracy of a reverse engineered model in surgical application allow surgeons to rehearse the re-alignment of bones or fitting of implants on the reverse engineered models prior to operating the patient, to evaluate and gain confidence in the planned approach. Surgical procedures continue to be more effective day by day with reduced risk and expense to both the patient and the hospital.

Biomechanical design work is closely related to sculptural work .The human body does not have sharp corners or edges, thus it was necessary to select CAD software that is versatile enough to give the model irregular shape. This software accepts data in neutral formats such as STL and gives us the opportunity to interface CTM to CAD. Segmented data can be translated into STL file format and imported into the CAD environment .The CAD environment allows for both the surgeon and the designer to determine critical dimension and mass properties from the CAD model to aid Surgeons in their assessment. Simulation

Simulation of parts using numerical methods is only possible when a computational model of the object exist. This is usually not the case for natural objects. These objects can be scanned and a computational object can be created for them. This makes subsequent numerical analyses possible.

“The successful person must learn to take with grace the jealousy of the less ambitious person.”

“I never did any thing worth doing by accident, nor did any of my inventions come

by accidents, they came by work.” - Thomas A Edison

“Smell the cheese often, so you know when it is getting old.”

FURTHER READING

Machine Tool Design Hand Book, CMTI, Tata McGraw-Hill. Machinery’s Hand Book, The Machinery Publishing Co. Ltd. Machine Design, Maleev & Hartman. Machine Design, Sharma & Agarwal. Mechanical Engineering Design, Red Ford Machine Drawing, K R Gopalakrishna. Production Technology, HMT, Tata McGraw-Hill. Production Technology, R K Jain Engineering Mechanics, Bavikatti. Strength of Materials, Ramamurtham. Theory of Machines, Ballaney.

When I Asked God for Strength

He Gave Me Difficult Situations to Face

When I Asked God for Brain & Brown

He Gave Me Puzzles in Life to Solve

When I Asked God for Happiness

He Showed Me Some Unhappy People

When I Asked God for Wealth

He Showed Me How to Work Hard

When I Asked God for Favors

He Showed Me Opportunities to Work Hard

When I Asked God for Peace

He Showed Me How to Help Others

God Gave Me Nothing I Wanted

He Gave Me Everything I Needed

- Swami Vivekananda