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Design For Manufacturing and AssemblyEffect of Materials and Manufacturing processes on Design

1.EFFECT OF MATERIAL ANDMANUFACTURING PROCESSES IN DESIGN

1. Introduction:Design is the process of translating a new idea or a market need into thedetailed information from which a product can be manufactured. Each ofits stages requires decisions about the materials from which the product isto be made and the process for making it. The number of materialsavailable to the engineer is vast: between 40000 and 80000. At thebeginning the design is fluid and the options are wide; all materials mustbe considered. As the design becomes more focused and takes shape, theselection criteria sharpen and the shortlist of materials, which can satisfy

them, narrows.

Then more accurate data are required and a different way of analyzing thechoice must be used. In final stages of design, precise data are neededand the search finally comes to only one. The procedure must recognizethe initial choice, the narrow this to a small subset, and provide theprecision and detail on which final design calculations can be based.

The choice of material cannot be made independently of the choice ofprocess by which the material to be formed, joined, finished, and

otherwise treated. Cost enters, both in the choice of material is processed.Good design alone will not sell a product. Industrial design is one that, ifneglected, can also loss the manufacturer his market.

So, Engineering materials are evolving faster, so there are wide options,which pave way for new innovations. It is important in the early part ofdesign to examine the full materials, which fulfill the requirements, andsubsequently deciding upon the manufacturing processes. For this, theknowledge of the Effect of material properties and manufacturingprocesses is required.

1.1. Major Phases of Design:

Introduction:

Engineering design work is usually performed on three different levels:

1. Development of existing products or designs, i.e.,redesign, by introducing minor modifications in size, shapeor materials to improve performance.2. Adaptation of an existing product or design to operatein new environment or to perform a different function.

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3. Creation of totally new design that has no precedent.This work is more demanding in experience and creativity of thedesigner.

1.1.1. Major Phases of Design:

Engineering design is usually an iterative process, which involves a seriesof decision-making steps where each decision establishes the frameworkfor the next one. There is no single, universally recognized sequence ofsteps that leads to a workable design as these depends on nature of theproblem being solved as well as the size and structure of the organization.

However, a design usually passes through most of the phases, which areshown in the Fig 1.

1. Identification of the problem and evaluating the need in order to

define the objective of the design represent the first phase of thedesign in most cases.

2. Functional requirements and operational limitations are directlyrelated to the required characteristics of the product and arespecified as a result of the active phase I.

3. System definition, concept formulation, and preliminary layout areusually completed, in this order, before evaluating the operatingloads and determining the form of the different components orstructural members.

4. Consulting design codes and collecting information on materialproperties will allow the designer to perform preliminary material

selection, preliminary design calculations, and rough estimation ofmanufacturing requirements.5. The evaluation phase involves a comparison of the expected

performance of the design with the performance requirementsestablished in phase 2.Evaluation of the different solution andselection of the optimum alternative can be performed usingdecision-making techniques, modeling techniques, experimentalwork and /or prototypes.

6. In some cases, it is not possible to arrive at a design that fulfills allthe requirements and compiles with all the limitations established inphase2. This means that these requirements and compiles with allthe limitations established in phase 2.

7. Having arrived at final design, the project then enters the detaileddesign stage where it is converted in to a detailed and finished formfor suitable for use in manufacturing. The preliminary design layout,any available detail drawings, models and prototypes, and access tothe developer of the preliminary design usually form the basis ofthe detailed design.

8. The next step in the detailed design phase is detailing, whichinvolves the creation of detail drawings for every part .All theinformation that is necessary to unambiguously define the partshould be recorded in detailed drawing. The material of the partshould also be selected and specified by reference to standardcodes.

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Major phases of designConstraintsSafety, LOP Fig 1

3

1. Identification of the problem

Unavailabl

e

informatio

n

2. Functional requirements

3. Concept formulationand preliminary layout.

4. Preliminary material and processselection.

Information

sufficient toreach feasible

solution?

5. Evaluate solution with

functional requirements.

Acceptable

Design?

Detail Design

Detailing

Materials and processesspecified.

Design

Changes

necessary

4. Bill of Materials

Manufacturing

Customer

Files

R&D

Patents

Material

properties, DesignCodes

Modeling and simulation

PrototypeExpt.Work.

Sales

MarketingProspective customers

Revise Functionalrequirements.

Specifications for standarditems.

MarketingPurchase andAccounting.

No

Yes

No

No

Yes

Yes

Yes

No


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9. An important part of the detailed design phase is the preparation ofthe bill of materials, sometimes called parts list .The bill of materialsis a hierarchical listing of everything that goes into the final productincluding fasteners and purchased parts. Close interaction betweendesign, manufacturing, and materials engineers is important at thisstage.

10.The relationship between the designer and the product does notusually end at the manufacturing or even delivery stages. Themanufacturing engineer may ask the detailed designer for a changein some parts to make fabrication easier or cheaper. Finally whenthe product gets in to use, the reaction of the consumer and theperformance of the product in service are of concern to thedesigner as the feedback represents an important source ofinformation for the future design modifications.

1.2. Effect of Material Properties onDesign:Introduction:

Materials are the food of design. A successful product is one that performswell, is good value for money and gives pleasure to the user. A successfuldesign should take in to account the function, material properties andmanufacturing processes, as shown in the following fig., in the context ofselection of material, there are many classes of materials metals,

polymers, and ceramics but in the end, what we seek is a profile ofproperties.

Fig 2 Factors that should be considered in component design.

4

Function

And

Consumer

Requirement

Component

Design

Material

Properties

Manufacturing

Process


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This figure shows that there are other secondary relationships betweenmaterial properties and manufacturing processes, and between functionand material properties.

The relationship between design and material properties is complexbecause the behavior of the material in the finished product is quitedifferent from that of stock material used in making it. This point isillustrated in the following Fig.3

Fig 3, Factors that should be considered in anticipating the behavior ofmaterial in the component.

This figure shows the direct influence of the stock material propertiesproduction method, and component geometry and external forces on thebehavior of materials in the finished component. It also shows thesecondary relationships exist between geometry and production method,and between stock materials and component geometry.

1.2.1 Effect of Component Geometry:

In most cases, engineering components and machine elements have toincorporate design features, which introduce changes in cross-section.

These changes cause localized stress concentrations, which are higherthan those, based upon the nominal cross-section of the part.

1.2.2 Stress Concentration Factor:

A geometrical or theoretical stress concentration factor Kt, is usually used

to relate the maximum stress, Smax, at the discontinuity to nominalstress, Sav, according to the relationship:

5

Properties of

Stock

materials.

Behavior of

material in

the

Component

Effect of

fabrication

method

Component

Geometry and

External forces


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Kt = Smax/ SavIn making a design, Kt is usually determined from the geometry of thepart. Under static loading Kt gives an upper limit to the stressconcentration value and applies only to brittle and notch sensitivematerials. With more ductile materials, local yielding in the very smallarea of maximum stress causes a considerable relief in the stressconcentration. So, for ductile materials under static loading, it is notusually necessary to consider the stress concentration factor.

Guidelines for design:

Stress concentration can be a source of failure in many cases,especially when designing with the high-strength materials and underfatigue loading. In such cases, the following guidelines should be observedif the stress concentrations are to be kept minimum.

1. Abrupt changes in cross-section should be avoided. If they arenecessary, generous fillet radii or stress-relieving grooves should be

provided.2. Slots and grooves should be provided with the generous run-out

radii in all corners.3. Stress-relieving grooves or undercuts should be provided at the

ends of threads and spines.4. Sharp internal corners and external edges should be avoided.5. Oil holes and similar features should be chamfered and the bore

should be smooth.6. Weakening features like the bolt and oil holes, identification marks,

and the part numbers should not be located in highly stressedareas.

7. Weakening features should be staggered to avoid the addition oftheir stress concentration factors.

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Fig 4 Stress concentration factor on Design.

1.2.3 Designing for Static Strength:

Designs bases on static strength usually aims at avoiding yielding of thecomponent in the case of soft, ductile materials and at avoiding fracture inthe case of strong, low-toughness materials.

Designing for Simple Axial Loading:

Components and structures made from ductile materials are usuallydesigned so that no yield will take place under the expected static loadingconditions. When a component is subjected to uniaxial stress, yielding willtake place when the local stress reaches the yield strength of the material.

The critical cross-sectional area, A,Of such a component can be estimated as :

A= KtnL/YSWhere Kt = Stress concentration factor,

L = applied Load,N = factor of safety,YS= yield strength of the material

Designing for Torsional Loading:

The critical cross-sectional area of a circular shaft subjected to torsionalloading can be determined from the relationship:

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2Ip/d = Kt nT/where d = shaft diameter at the critical cross-section,

= Maximum shear strength of the materialT = transmitted Torque,Ip = polar moment of inertia of the cross-section

= d

4

/ 32 for a solid circular shaft= (d4o d4i)/ 32 for a hollow shaft of inner dia di and outer dia do

Design for Bending:

When a relatively long beam is subjected to ending, the bending moment,the maximum allowable stress, and the dimensions of the cross-sectionare related by the equation:

Z = (nM)/YS

where M = bending moment.

Z = section modulus = I/c,I = moment of inertia of the cross-section with respect to the

neutral axis normal to the direction of the load.c = distance from the center of gravity of the cross-section to theoutermost fiber.

1.2.4 Designing for Stiffness:

In addition to being strong enough to resist the expected service loads,there may also be the added requirement of stiffness to ensure thatdeflections do not exceed certain limits.

When an initially straight beam is loaded, it becomes curved as a result ofits deflection. As the deflection at a given point increases, the radius ofcurvature at this point decreases. The radius of curvature, r, at any pointon the curve is given by the relationship:

r= EI /M

The equation shows us that the stiffness of a beam under bending isproportional to the elastic constant of the material, E, and the moment ofinertia of the cross-section, I. Therefore, selecting materials with higherelastic constant and efficient disposition of material in the cross-section

are essential in designing beams for stiffness.

Torsional Rigidity of Shafts:

The torsional rigidity of a component is usually measured by the angle oftwist, , per unit length, where

= T/ G IpWhere G = modulus of elasticity in shear

= E/2(1+v)Where v = Poissons ratio.

The usual practice is to limit the angular deflection in shafts to about 1

degree in a length of 20 times the diameter.

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1.2.5 Designing With High-Strength, Low ToughnessMaterials:

High-strength is being increasingly used in designing critical componentsto save weight or to meet difficult service conditions. These materials tendto be less tolerant of defects than the traditional lower-strength, toughermaterials. While a crack-like defect can safely exist in a part of lower-strength ductile material, it can cause a catastrophic failure if the samepart is made of a high-strength, low toughness material.


In designing with the high-strength, low toughness materials, theinteraction between fracture toughness of the material, the allowable

crack size, and the design stress should be considered. In the case of high-strength, low-toughness material, as the design stress increases (or as thesize of the flaw increases) the stress concentration at the edge of thecrack, the stress intensity KI increases until it reaches KIC and fractureoccurs.

KI = KIC = YFs(a)1/2

where Fs = fracture stress (controlled by the applied load and shape of thepart)

a = quality control parameter (controlled by the manufacturingmethod)

Y = dimensionless shape factor. (Estimated experimentally,analytically or numerically)

1.2.6 Designing against Fatigue:

In majority of cases the reported fatigue strengths or endurance limits ofmaterials are based on tests of carefully prepared small samples underlaboratory conditions. Such values cannot be directly used for designpurposes because the behavior of the component or structure underfatigue loading does depend not only on the fatigue or endurance limit ofthe material used in making it, but also on several other factors including:

Size and shape of the component or structure Type of loading and state of stress.

Stress concentration Surface finish Operating temperature Service environment

Method of Fabrication.

The influence of the above factors on the fatigue behavior of thecomponent can be accounted for by modifying the endurance limit of thematerial using a number of factors. Each of these factors is less than unity

and each one is intended to account for a single effect.

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Se = ka kb kc kd ke kfkg kh S eWhere, Se =endurancelimit of the material in the component.

S e = endurance limit of the material as determined by laboratoryfatigue test.

ka = surface finish factor.

Surface finish factor varies between unity and 0.2 depending uponsurface finish and strength of the material.kb = size factor.Size factor is 1.0 for component diameter less than 10mm; 0.9 forcomponent diameter in the range of 10 to 50 mm.

kc = reliability factor.Reliability factor is 0.900 for 90% reliability

0.814 For 99% reliability0.752 For 99.9% reliability

kd = operating temperature factor.

Operating temperature value is 1.0 in the range of -45 to 450CIts value is 1- 5800(T-450) for T between 450 - 550CIts value is 1- 3200(T- 840) for T between 840- 1020C

ke = loading factor.Loading factor is equal to 1 for applications involving bending.It is equal to 0.9 for axial loading.It is equal to 0.58 for torsional loading.kf=stress concentration factor.kg = service environment factor.Service Environment factor varies from 0.72 to 0.19kh = manufacturing process factor.

Manufacturing factor is generally taken as 0.3-0.5.

The above equation can be used to predict the behavior of the componentor a structure under fatigue conditions provided that the values of thedifferent modifying factors are known.Cumulative Fatigue Damage:

Engineering components and structures are often subjected to differentfatigue stresses in service. Estimation of the fatigue life under variableloading conditions is normally based on the concept of cumulative fatiguedamage, which assumes that successive stress cycles cause a progressive

deterioration in the component.

The Palmgren -Miner rule, also called Miner's rule proposes that if a cyclicstressing occurs at a series of stress levels S1, S2, S3..Si each of whichwould correspond to a failure life of N1, N2, N3,.Ni if applied singly, thenthe fraction of total life used a each stress level is the actual number ofcycles applied at this level n1, n2, n3, .ni divided by the corresponding life.

The part is expected to fail when the cumulative damage satisfies therelationship:

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CNi

ni.........

3N

3n

2N

2n

1N

1n=++++


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The constant C can be determined experimentally and is usually found tobe in the range of 0.7-2.2. The Palmgren - Miner rule does not take in toaccount the sequence of loading nor the effect of mean stress and itshould be taken as rough guide to design.

1.2.7 Designing under High-TemperatureConditions:

Service temperature has a considerable influence on the strength ofmaterials and consequently, on the working stress used in design.Depending on the temperature range, the design can be based on:

1. Short-time properties of the material, i.e., ultimate tensilestrength, yield strength for moderate temperatures.2. Both the short time and creep properties for intermediatetemperature range.3. Creep properties of the materials for high temperatures.

In addition to creep, the other factors, which must be taken in toconsideration when designing for elevated temperatures, include:

1. Metallurgical and micro structural changes, which occur inthe material owing to long-time exposure to elevatedtemperature.2. Influence of method of fabrication, especially welding, oncreep behavior.3. Oxidation and hot corrosion, which may take, place duringservice and shutdown periods.

Design guidelines:For design purposes, creep properties are usually presented on plots,which yield reasonable straight lines. Common methods of presentationinclude log-log plots of stress vs. steady state creep rates and stress vs.time to produce different amounts of total strain as shown in the Fig.5. Achange in the microstructure of the material is usually accompanied by achange in creep properties, and consequently a change in the slope of theline.

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Increasingtemperature

Stress(

logscale

)


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Creep rate (%/1000h) (log scale)

Fig5, Variation of stress with steady-state creep rate at varioustemperatures.

Fig.6, Variation of stress with time to produced different amounts of totalstrain at a given temperature.

Larsen- Miller Parameter:

In many cases, creep data are incomplete and have to supplemented orextended by interpolation or, more hazardously, extrapolation. This isparticularly true of long-time creep and stress-rupture data where the100,000 hour (11.4 years) creep resistance of newly developed materials

is required. Reliable extrapolation of creep and stress-rupture curves tolonger times can be made only when no structural changes occur in theregion of extrapolation. Such changes can affect the creep resistance,which would result in considerable errors in the extrapolated values.

The basic idea of these parameters is that they permit the prediction oflong-time creep behavior from the results of shorter time tests at highertemperatures at the same stress. A widely used parameter for correlatingthe stress rupture data is the Larson-Miller parameter (LMP), where LMP isdescribed as,

LMP = T(C + log tr)

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Time (h) (Log scale)

Rupture

Strength

Stress(logscale)

Increasing totalstrain


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Where T= the test temperature in kelvin (C+273) or degrees Rankine (F+460)

tr= time to rupture in hours (the log is to the base 10)C= the Larson- Miller constant which generally falls between 17

and 23, but is often taken to be 20.

Fig.7 Larsen-Miller Plots.

Life under Variable Loading

The stress-rupture life of a part or a structure, which is subjected to avariable loading, can be roughly estimated if the expected life at eachstress level is known. Under such conditions, the life fraction rule assumesthat rupture occurs when:

Where t1, t2, t3, are the times spent by the part under stress levels1, 2, 3 respectively.

tr1, tr2, tr3. are the rupture lives of the part under stress levels 1,2, 3...

respectively.

Life under Combined Fatigue And Creep Loading:

Similar reasoning can also be applied to predict the life of a part or astructure when subjected to combined creep and fatigue loading.Cumulative fatigue damage laws,e.g. Palmgren-Miner Law, can be

13

10

100

403020

S

tress(Mpa

)

T(C+logt)

.1....3tr3t

2tr2t

1tr1t =++++


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combined with the life fraction rule, given in the equation, to give a roughestimate of expected life under combined creep-fatigue loading. Thus:

Where n1, n2, n3... are the number of cycles at stress levels 1, 2, 3respectively.N1, N2, N3 are the fatigue lives at stress levels 1, 2, 3 respectively.

1.3 Effect of Manufacturing Process onDesign

Introduction

It is now widely recognized that design, materials selection, andmanufacturing are intimately related activities, which cannot beperformed in isolation of each other. Creative designs may never developinto marketable products unless they can be manufactured economicallyat the required level of performance. In many cases, design modificationsare made to achieve production economy or to suit existing productionfacilities and environment. Modifications of design may also be made inorder to improve quality and performance, in which case the cost ofproduction may increase.

1.3.1 Design Considerations for Cast Components

Casting covers a wide range of processes which can be used to shapealmost any metallic and some plastics in a variety of shapes, sizes,accuracy, and surface finish. In some cases, casting represents theobvious and only way of manufacturing, as in the case of componentsmade of the different types of cast iron or cast alloys. In many otherapplications, however a decision has to be made whether it asadvantageous to cast a product or to use another method of manufacture.In such cases, the following factors should be considered:

1. Casting is particularly suited for parts which contain internal cavitiesthat are inaccessible, too complex, or too large to be easily producedby machining.

2. It is advantageous to cast complex parts when required in largenumbers, especially if they are to be made of aluminum or zinc alloys.

3. Casting techniques can be used to produce a part, which is one of akind in a variety of materials, especially when it is not feasible to makeit by machining.

4. Precious metals are usually shaped by casting, since there is little or noloss of materials.

5. Parts produced by casting have isotropic properties, which could beimportant requirements in some applications.

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1.....3N

3n

2N

2n

1N

1n...

3tr

3t

2tr

2t

1tr

1t=+++++++


15/35Stagger section Use a core or

internal chill

Use External

ChillsUse a riser


6. Casting is not competitive when the parts can be produced bypunching from sheet or by deep drawing.

7. Extrusion can be preferable to casting in some cases, especially in thecase of lower- melting nonferrous alloys.

8. Castings are not usually a viable solution when the material is noteasily melted, as in the case of metals with very high melting pointssuch as tungsten.


A general rule of solidification is that the shape of the casting should allowthe solidification front to move uniformly from one end toward the feedingend, i.e. directional solidification. This can most easily be achieved whenthe casting has virtually uniform thickness in all sections. In most casesthis is not possible. However, when section thickness must change, suchchange should be gradual, in order to give rise to stress concentration andpossible hot tears in the casting. Figure 8.gives some guidelines to avoid

these defects.

Another problem, which arises in solidification, is caused by sharp corners;these also give rise to stress concentration and should be replaced bylarger radii. When two sections cross or join, the solidification process isinterrupted and a hot spot results. Hot spots retard solidification andusually cause porosity and shrinkage cavities.

Effect of material properties

The type and composition of the material play an important part indetermining the shape, minimum section thickness, and strength of thecasting. Materials, which have large solidification shrinkage and containlow melting phases are susceptible to hot tears. Another materialvariable is cast ability, which can be related to the minimum sectionthickness, which can be achieved. It should be noted that the shape andsize of the casting as well as the casting process and foundry practicecould affect the minimum section thickness.

15

Incorrect designsCorrect Designs

Solidifications of intersecting sections results in hot

spots and shrinkage activities


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Fig 8


1.3.2 Design Considerations for Molded PlasticComponents

Compression, transfer, and injection molding processes are the commonlyused methods of molding plastic components. These processes involve theintroduction of fluid or a semi fluid material into a mould cavity andpermitting it to solidify into the desired shape.

Guidelines for design

Experience shows that the mechanical, electrical, and chemical propertiesof molded components are influenced by the flow of the molten plastic asit fills the mold cavity. Streamlined flow will avoid gas pockets in heavy sectioned areas.

An important common feature in molding processes is draft, which isrequired for easy ejection of molded parts from the mold cavity. A taper of1 to 4 degree is usually used for polymers, but tapers of less than 1

degree can be used for deep articles. Another common feature is theuniform thickness. Non-uniformity of thickness in a molded piece tends toproduce non-uniform cooling and unbalanced shrinkage leading to internalstresses and warpage.

If thickness variations are necessary, generous fillets should be used toallow a gradual change in thickness. The effect of junctions and cornerscan also be reduced by using a radius instead, as shown in Fig 9.Thenominal wall thickness must obviously such that the part is sufficientstrong to carry the expected service loads. However, it is better to adjustthe shape of the part to cope with the applied load than to increase thewall thickness. This is because thick sections retard the molding cycle and

require more materials.

The presence of holes disturbs the flow of the material during molding anda weld line occurs in the side of the hole away from the direction of flow.

This results in a potentially weak point and some from of strengthening,such as bosses may be necessary as in Fig 10.Through holes are preferredto blind holes from a manufacturing standpoint. This is because core printscan often be supported in both halves of the mold in the case of throughholes, but can only be supported from one end in the case of blind holes.

Accuracy of molded parts.

Dimensional tolerances in molded plastic parts are affected by the typeand constitution of the material, shrinkage of the material, heat and

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pressure variables in the molding process, and the toolmakers toleranceson the mold manufacture. Shrinkage has two components:Mold shrinkage, which occurs upon solidification; andAfter shrinkage, this occurs in some materials after 24 hours.For example, a thermosetting plastic like melamine has mold shrinkage ofabout 0.7 to 0.9 %, and an after shrinkage of 0.6 to 0.8%. Thus a totalshrinkage of about 1.3 to 1.7 % should be considered. On the other hand,a thermoplastic like polyethylene may shrink as much as 5% and nylon asmuch as 4%. In addition, the value of tolerance depends on the size of thepart. Larger dimensions are normally accompanied by larger tolerances.For example, dimensions less than 25mm (1 in) can be held within 50 m. Larger dimensions are usually given tolerances of 10 to 20 m/cm.

The value of tolerances also depends on the direction in relation to theparting plane.

Fig 9 some design features of plastic parts. (a) Using radii instead of sharpcorners.

Fig 10 some design features of plastic parts (b) Use of bosses tostrengthen areas round holes and slots.

1.3.3 Design Considerations for Forged Components

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Poor Design Better Design

(a)

(b)


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Forging processes represent an important means of producing relativelycomplex parts for high-performance applications. In many cases forgingrepresents a serious competitor to casting especially for solid parts thathave no internal cavities. Forged parts have wrought structures, which areusually stronger, more ductile, contain less segregation, and are likely tohave less internal defects than cast parts. This is because the extensivehot working, which is usually involved in forging, closes existing porosity,refines the grains, and homogenizes the structure.

On the other hand, cast parts are more isotropic than forged parts, whichusually have directional properties. This directionality is due to the fibrestructure, which results from grain flow and elongation of second phasesin the direction of deformation. Forged components are generally strongerand more ductile in the direction of fibres than across the fibres.

Guidelines for Design

Rapid changes in thickness should be avoided because these could resultin laps and cracks in the forged metal as it flows in the die cavity. Toprevent these defects, generous radii must be provided at the locations oflarge changes in thickness. Another similarity with casting is that verticalsurfaces of a forging must be tapered to permit removal from the diecavity.

A draft of 5 to 10 degrees is usually provided. It is better to locate theparting line near the middle of the part in order to avoid deep impressionin either of the two halves of the die and allows easier filling of the diecavity. A design would be more economically produced by forging ifdimensions across the parting line are given appropriate mismatchallowance, and parallel dimension are given a reasonable die closureallowance. Specifying close tolerances to these dimensions could requireextensive machining which would be expensive.

Fig 11 Schematic comparison of the grain flow in forged and machined

components.

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Machined Forged


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1.3.4 Design Considerations For Powder MetallurgyParts

Powder metallurgy (P/M) techniques can be used to produce a large

number of small parts to the final shape in few steps, with little or nomachining, and at high rates. Many metallic alloys, ceramic materials, andparticulate reinforced composites can be processed by P/M techniques.Generally, parts produced by the traditional P/M techniques contain 4 to10 vol % porosity. The amount of porosity depends on part shape, typeand size of powder, lubrication used, pressing pressure, sinteringtemperature and time, and finishing treatments.

The distribution and volume fraction of porosity greatly affect themechanical, chemical, and physical properties of parts prepared by P/Mtechniques. An added advantage of P/M is versatility. Materials that can becombined in no other way can be produced by P/M. Aluminum - graphitebearings, copper - graphite electrical brushes, cobalt - tungsten carbidecutting tools (cermets), and porous bearings and filters are such.


The Powder Metallurgy Parts Association and Metal Powder IndustriesFederation have made certain rules. They are:1. The shape of the part must permit ejection from the die, Fig 122. Parts with straight walls are preferred. No draft is required for ejectionfrom lubricated dies.

3. Parts with undercuts or holes at right angles to the direction of pressingcannot be made, Fig 13.4. Straight serrations can be made easily, but diamond knurls cannot, Fig

14.5. Since pressure is not transmitted uniformly through a deep bed ofpowder, the length/diameter ratio of a mechanical pressed part should notexceed about 2.5: 1.

Fig 12 Reverse taper should be avoided, use parallel sides and machinetheRequired taper after sintering.

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Fig 13 undercuts and holes at right angles to pressing direction should beavoided; if necessary such features are introduced by machining aftersintering.

1.3.5 Design of Sheet - Metal Parts

Parts made from sheet metal cover a wide variety of shapes, sizes, andmaterials. Many examples are found in the automotive, aircraft, andconsumer industries. Generally, sheet-metal parts are produced byshearing, bending, and/or drawing. The grain size of the sheet material isimportant and should be closely controlled. Steel of 0.035 - 0.040 mm(0.001 - 0.0016) grain size is generally acceptable for deep- drawingapplications. When formability is the main requirement in a sheetmaterial, drawing - quality low carbon steels represents the mosteconomic alternative.


The most important factor, which should be considered when designing

parts that are to be made by bending, is bend ability. This is related to theductility of the material and is expressed in terms of the smallest bendradius that does not crack the material. Bend ability of a sheet is usuallyexpressed as 2T, 3T, 4T, etc. A 2T material has greater bend ability than a3T material.

Another factor which should be considered when designing for bending isspring back, which is caused by the elastic recovery of the material whenthe bending forces are removed. One way of compensating for spring backis to over bend the sheet. Another method is bottoming which eliminatesthe elastic recovery by subjecting the bend area to high-localized stresses.

20

Fig 14 Diamond knurls should be replaced by straight serrations.


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1.3.6 Designs Involving Joining Processes

The major function of a joint is to transmit stress from one part to anotherand in such case the strength of the joint should be sufficient to carry theexpected service loads. In some applications, tightness of the joint is also

necessary to prevent leakage. Because joints represent areas ofdiscontinuities in the assembly, they should be located in low-stressregions especially in dynamically loads structures.

Welding

Welding has replaced riveting in many applications including steelstructures, boilers, tanks, and motorcar chassis. This is because riveting isless versatile and always requires lap joint. Also, the holes and rivetssubtract from strength, and a riveted joint can only be about 85%asstrong, whereas a welded joint can be as strong as the parent metal.Welded joints are easier to inspect and can be made gas and liquid-tight

without the caulking which has to be done in riveted joints. On thenegative side, however, structures produced by welding are monolithicand behave as one piece. This could adversely affect the fracture behaviorof the structure. For example, a crack in one piece of a multipiece rivetedstructure may not be serious, as it will seldom progress beyond the piecewithout detection. However, in the case of a welded structure, a crack thatstarts in a single plate or weld may progress for a large distance andcause complete failure.

Another factor, which should be considered when designing a weldedstructure, is the effect of size on the energy-absorption ability to steel. A

Charpy impact specimen could show a much lower brittle-ductile transitiontemperature than a large welded structure made of the same material.

Guidelines for design of weldments

1.Welded structures and joints should be designed to have sufficientflexibility. Structures that are too rigid do not allow shrinkage of the weldmetal, have restricted ability to redistribute stress, and are subjected to

distortions and failure.

2. Accessibility of the joint for welding, welding position and componentmatch-up are important elements of the design.

3. Thin sections are easier to weld than thick ones.

4. Welded section should be about the same thickness to avoid excessiveheat distortion.

5. It is better to locate welded joints symmetrically around the axis of anassembly in order to reduce distortion.

6. Whenever possible the meet of several welds should be avoided.

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7. Use weld fixtures and clamps to avoid distortion.

Adhesive BondingAdhesives represent an attractive method of joining and their use isincreasing in many applications. Some of main advantages in usingAdhesives are as follows:

1. Thin sheets and parts of dissimilar thickness can be easily bonded.2. Adhesive bonding is the most logical method of joining polymer-

Matrix composites.3. Adhesives are electrical insulators and can prevent galvanic

Action in joints between dissimilar metals.4. Flexible adhesives spread bonding stresses over wide areas and

Accommodate differential thermal operation.

5. Flexible adhesives can absorb shocks and vibrations, whichIncreases fatigue life.

6. The preparation of bonded joins requires no fastener holds, whichGives better structural integrity and allows thinner gage materials tobe used.

The main limitations of adhesives are as follows:

1. Bonded joints are weaker under cleavage and peel loading thanunder tension or shear.

2. Most adhesives cannot be used at service temperatures above 300degree C(600 degree F).

3. Solvents can attack adhesive-bonded joints.4. Some adhesives are attacked by ultraviolet light, water, and ozone.5. The designer should also be aware of the adhesive's impact resistance

and creep, or cold flow, strength.

Design of adhesive joints

The strength of the adhesive joint depends on the geometry, the directionof loading in relation to the adhesive material, surface preparation, andapplication and curing technique. As the bonded area limits the strengthof an adhesive joint, lab and double-strap joints are generally prepared to

butt joints. If the geometry constrains do not allow for such joints, a scarfor double -scarf joint should be made.

When a lab joint is used to bond thin sections, tensile shear causesdeflection, and this results in stress concentration at the end of the lab.

Tapering the ends of the joints, gives more uniform loading throughout thejoint. Since adhesive joints are weaker under peeling forces, joint designshould avoid this type of loading.

1.3.7 Designs Involving Heat Treatment:

Heat treatment represents an important step in the sequence ofprocesses that are usually performed in the manufacture of metallic parts.Almost all ferrous and many nonferrous alloys can be heat treated to

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achieve certain desired properties. Heat treatment can be used to makethe material hard and brittle, as in the case of annealing.

Generally, hardening of steels involves heating to the austenitictemperature range, usually 750 to 900 C (1400 to 1650 F), and then

quenching to form the hard martensitic phase. The nonuniformtemperature distribution that occurs during quenching and the volumechange that accompanies the martenstic transformation can combine tocause distortions, internal stresses, and even cracks in the heat treatedpart. Internal stresses can warp or dimensional changes when thequenched part is subsequently machined or can combine with externallyapplied stresses to cause failure. Corrosion problems can also beaggravated owing to the presence of internal stresses. These difficultiescan be reduced or eliminated by selecting steels with hardenability asthey require a less cooling rate to achieve a given hardness value.Manganese, chromium, molybdenum are commonly added to steels toincrease their hardenability.

1.3.8 Designs Involving Machining Processes

Guidelines for designThe following discussion illustrates some component shapes and featureswhich can cause difficulties in machining, take an undue length of time tomachine, call for precision and skill that may not be available, or whichmay even be impossible to machine by standard machine tools andcutting tools.1. The workpiece must have a reference surface, which is suitable for

holding it on the machine tool or in a fixture. This could be a flat base

or a cylindrical surface.2. Whenever possible, the design should allow all the machiningoperations to be completed without resetting or reclamping.

3. Whenever possible, the radii between the different machined surfacesshould be equal to the nose radius of the cutting tool.

4. If the part is to be machined by traditional cutting methods, deflectionunder cutting forces should be taken into account. For the same cuttingforce, the deflection is higher for thinner parts and for lower elasticmoduli. Under these conditions, some means of support is necessary toensure the accuracy of the machined part.

5. Features at an angle to the main machining direction should beavoided as they may require special attachments or tooling. Fig 15

6. To reduce the cost of machining, machined areas should be minimumas shown Fig 16

7. Cutting tools often require run-out space, as they cannot be retractedimmediately. This is particularly important in the case of grindingwhere the edges of the grinding wheel wear out faster than the center.Fig 17 gives some examples to illustrate this point.

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(a) (b) (c)

Reliefs toreduce

machinedareas


Fig 15 (a) Poor design as drill enters and exists at an angle to the surface.(b) Better design, but drilling the holes need a special attachment.

(c) Best design.

Fig 16 Some design details which can be introduced to reduce machining.

Fig 17 Some design details which can be introduced to give run-out forgrinding wheels.

1.4.The Materials Selection Process:

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Poor design Better Design

Added materials toreduce machine

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One of the most important requisites for the development of a satisfactoryproduct at a competitive cost is making sound economic choices ofengineering designs, materials, and manufacturing processes. The largenumber of materials and the many manufacturing process available to theengineer, coupled with the complex relationships between the differentselection parameters, often make the selection process a difficult task. Arigorous and through approach to materials selection is, however, oftennot followed in industry and much selection is based on past experience.

It is often said, When in doubt make it stout out of the stuff you knowabout. While it is unwise to totally ignore past experience, the frequentintroduction of new materials and manufacturing process, in addition tothe increasing pressure to produce more economic and competitiveproducts, make it necessary for the engineer to be always on the lookoutfor possible improvement. The reasons for reviewing the types of materialand processes used in making an existing product are:

1. Taking advantage of new materials or processes.2. Improving service performance, including longer life andhigher reliability.3. Meeting new legal requirements.4. Accounting for changed operating conditions.5. Reducing cost and making the product more competitive.

Selecting the optimum combination of material and process can beperformed at one certain stage in the history of a project; it shouldgradually evolve during the different stages of product development.

These are:

1. Analysis of the performance requirements.2. Development of alternative solutions to the problem.3. Evaluation of the different solutions.4. Decision on the optimum solution

1.4.1 Analysis of the Material PerformanceRequirements:

Functional Requirements:

Functional requirements are directly related to the required characteristicsof the part or the product. For example, if the part carries a uniaxialtensile load, the yield strength of the material can be directly related tothe load-carrying capacity of the product. For the evaluation process ofthe characteristics of material properties like thermal shock resistance,wear resistance, reliability etc., and simulation service tests are employed.

Processability Requirements:

The processability of the material is a measure of its ability to be worked

and shaped in to a finished part. With the reference to a specificmanufacturing method, processability can be defined as a castability,

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weldability, machinability etc.,Ductility and hardenability can be relevantto processability if the material is to be deformed or hardened by heattreatment respectively. The closeness of the stock form to the requiredproduct form can be taken as a measure of processability in some cases.

The material properties are closely related to functional requirements.

Cost:

Cost is usually the controlling factor in evaluating materials, because inmany applications there is a cost limit for a material intended to meet theapplication requirements. When the cost limit is exceeded, the design mayhave to be changed to allow the use of a less expensive material. The costof the processing often exceeds the cost of the stock material.

Reliability Requirements:

The reliability of the material can be defined as the probability that it will

perform the intended function for the expected life without failure.Material reliability is difficult to measure, because it is not only dependentupon the materials inherent properties, but also greatly affected by itsproduction and processing history.

Though there are difficulties in evaluating reliability, it is often animportant selection factor that must be taken in to account. Failureanalysis techniques are usually used to predict the different ways in whicha product can fail, and can be considered as a systematic approach toreliability evaluation.

Resistance to Service Conditions:

The environment in which the product or part will operate plays animportant role in determining the material performance requirements.Corrosive environments, as well as high or low temperatures, canadversely affect the performance of most materials in service. Wheneverthere is more than one material involved in an application, compatibilitybecomes a selection consideration. For example, In thermal environment,the coefficient of thermal expansion of all the materials involved may haveto be similar in order to avoid thermal stresses. In applications whererelative movements exist between different parts, wear resistance of thematerials involved should be considered.

1.4.2 Cost per Unit Property Method:

In simplest cases of optimizing the selection of materials, one propertystands out as the most critical service requirement. In such simple casesthe cost per unit property can use as a criterion for selecting the optimummaterial. Consider the case of a bar of given length (L) to support a tensileforce (F). The cross-sectional area (A) of the bar is given by:

A=F/S ()

Where S is the working stress of the material, which is related to its yieldstrength by an appropriate factor of safety.

The cost of the bar is given by:

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C = C AL = (C FL)/SWhere C = cost of the material per unit mass, and = Density of the material.In comparing different candidate materials, only the quantity (C )/S,

which is the cost of unit strength, needs to be compared, as F and L areconstant for all material. The material with the lowest cost per unitstrength is the optimum material.When one material is considered as a substitute for an existing material,the two materials a and b can be compared on the basis of relative costper unit strength (RC ):

RC = (C )a

(C )b

which is equal to Ca aSbCb bSa

RC less than unity indicates that the material a is preferable to materialb.

Equations similar to () an () can be used to compare the materials on costbasis.

1.4.3 Weighted Properties Method:

The weighted properties method can be used in optimizing materialsselection when several properties should be taken into consideration. Inthis method each material requirement, or property, is assigned a certainweight, depending on its importance. A weighted property value is

obtained by multiplying the numerical value of the property by theweighting factor ( ). The individual weighted property values of eachmaterial are then summed to give a comparative materials performanceindex ( ). The material with the highest performance index ( ) isconsidered as the optimum for the application.

When evaluating a list of candidate materials, one property is consideredat a time. The best value in this list is rated as 100 and the others arescaled proportionally.

B= scaled property = Numerical value of property x 100Maximum value in the list

For properties like cost, corrosion or wear loss, weight gain inoxidation, etc., a lower value is more desirable. In such cases, the lowestvalue is rated as 100 and B is calculated as:

B= scaled property = Minimum value in the list x 100Numerical value of property

For material properties that can be represented by numerical values,applying the above procedure is simple. However, with properties likecorrosion and wear resistance, machinability and weldability, etc., arerarely given and materials are usually rated as very good, good, fair, poor

etc. In such cases, the rating can be converted to numerical values usingan arbitrary scale. For example, a corrosion resistance rating of excellent,

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very good, good, fair and poor can be given numerical values of 5,4,3,2and 1 respectively. Then,

n

Material performance index, = Bi i

i=1Where iis summed over all the n relevant properties.

In the cases where numerous material properties are specified, the digitallogic approach is used as a systematic tool to determine . In thisprocedure evaluations are arranged such that only two properties areconsidered at a time. Every possible combination of the properties orperformance goals is compared and no shades of choice are required, onlya yes or no decision for each evaluation. To determine the relativeimportance of each properties or goal a table is constructed, theproperties or goals are listed in the left hand column, and comparisons aremade in the columns to the right, as shown in the table.

Table 5.1 Determination of the relative importance of performance goalsusing the digital logic method

Goals Number of possible decisions[N=n(n-1)/2]

Positivedecisions

Relative EmphasisCoefficient ( )

1 2 3 4 5 6 7 8 9 10

1 1 1 0 1 3 1=0.32 0 1 0 1 2 2=0.23 0 0 1 0 1

3=0.14 1 1 0 0 2 4=0.25 0 0 1 1 2 5=0.2

Total number of positivedecisions

=10 =1.0

In comparing two properties or performance goals, the more important isgiven numerical one (1) and the less important is given zero(0).The totalnumber of possible decisions N=n(n-1)/2 , where n is the number of theproperties or goals under consideration. A relative emphasis coefficient orweighting factor, for each goal is obtained by dividing the number ofpositive divisions for each goal (m) into the total number of possibledecisions (N). In this case =1.

However, if there are large numbers of properties to consider theimportance of cost may be emphasized by considering it separately as amodifier to the material performance index ( ). In the cases where thematerial is used for space filling, cost can be introduced on a per unitvolume basis. A figure of merit (M) for the material can then be defined as:

M= /(C )

Where C= total cost of the material per unit weight (stock, processing,finishing, etc.)

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= Density of the material

The weighted properties method can be used when a material isconsidered as a substitute for an existing one. This is done by computingthe relative figure of merit (RM), which is defined as,

RM = Mn/McWhere Mn and Mc are the figures of merit of the new and existingmaterials respectively. If the RM is greater than unity, the new material ismore suitable than the existing material.

The steps involved in the weighted properties method can be written inthe form of a simple computer program to select materials from the databank. An interactive program can also include the digital logic method tohelp in determining weighting factors.

1.4.4 Limits On Properties Method:

In the limits on properties method, the performance requirements aredivided into three categories:

1. Lower limit properties2. Upper limit properties3. Target value properties

The limits on properties method are usually suitable for optimizingmaterial and process selection when the number of possible alternatives isrelatively large. This is because the limits, which are specified for thedifferent properties, can be used for eliminating unsuitable materials from

data bank. The remaining materials are those whose properties are abovethe lower limits, below the upper, and within the limits of target values ofthe respective specified requirements. After the screening stage, the limitson properties method can be used to optimize the selection from amongthe remaining materials.As in the case of the weighted properties method, each of therequirements or properties is assigned a weighted factor, , which can bedetermined using the digital logic method, as discussed earlier. A meritparameter, m,is then calculated for each material according to therelationship:

+

+

= 1

1 k

k

j

j

j

i

i

i

Y

X

Y

X

X

Ym k

where l,u, and t stand for lower limit, upper limit, and target valueproperties respectively.

nl,nu,and nt are the numbers of the lower limit, upper limit, andtarget value properties respectively. i, j, k are the weighting factors of the lower limit, upper limit,and target value properties respectively.Xi,Xj and Xk are the candidate material lower limit, upper limit, and

target value properties respectively.

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Yi,Yj,and Yk are the specified lower limit, upper limit, and targetvalue properties respectively.

According to the equation the lower the value of the merit parameter m,the better the material.

As in the weighted properties method, the cost can be considered in twoways:

1. Cost is treated as an upper limit property and given theappropriate weight.

2.Cost is included as a modifier to the merit parameter as follows:

m = (CX/CY)mWhere CY and CX are the specified cost upper limit and candidate materialcost,In this case the material with the lowest cost-modified merit parameter, m, is the optimum.

1.5. Case Study for Material Selection:

1.5.1 Materials for springs:

Springs come in many shapes as shown in the Fig 18, and havemany purposes: one thinks of axial springs, leaf springs, helical springs,spiral springs, torsion bars. Regardless of their shape or use, the bestmaterial for a spring of minimum volume is that with the greatest value of

Ef

/2

, and for minimum weight it is that with the greatest value Ef /2 .

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Fig 18 springs store energy. The best material for any spring, regardless ofits shape or the way in which it is loaded, is that with the highest value of

Ef

/2

Or if weight is important, Ef /2

.

The primary function of the spring is that of storing elastic energy andwhen required releasing it again.The elastic energy stored per unit volumeof material stressed uniformly to a stress is

EWv

2

2

1 =

Where E is youngs modulus. It is Wv that to be maximize. The spring willbe damaged if the stress exceeds the yield stress or the failure stress f; the constraint is


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Fig 19 Materials for small springs. High strength (spring) steel is good.Glass, CFRP and GFRP all under right circumstances, make good springs.Elastomers are excellent. Ceramics are eliminated by their low tensilestrength.

1.5.2 The Selection

The choice of materials for springs of minimum volume is shown in

the Fig 19 family lines of slope link materials with equal values of

EM

f2

1

=

Those with the highest values of M1 lie towards the bottom right.The heavy line is one of the families; it is positioned so that a subset ofmaterials is left exposed. The best choices are a high0strength steel

(spring steel) lying near the top end of the line, and at the other end,

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rubber. But certain other materials are suggested too: GFRP (trucksprings), titanium alloys, glass andNylon.

1.6.Problem :1. Suggest a suitable operation sequence for the stub carrier shown inFig.20 and redraw the component incorporating features to facilitatemanufacture. The carrier is to be produced from a steel casting and thesymbol indicates a ground surface for the 30 mm diameter f8 limits.

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2. The proposed machining procedure for the plate Fig 21(1) Bore and face, reverse, face other side - turret.(2) Drill and ream four 25 mm H8holes - drill, drill jig.

Suggest a design modification which will permit of an alternativeprocedure to achieve a substantial reduction in machining time. State theprocedure for producing the modified design.

3. A Cast iron bearing bracket is shown in Fig 22. Indicate the preferredparting line and any necessary sand cores. Offer a design modificationthat will reduce or eliminate the need for sand cores.

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