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Objectives: Upon completion of this chapter the reader will be able to: 1. Establish the factors that affect the design of a product. 2. Establish the general aspects of manufacturing factors that affect the design of the product. 3. Establish the manufacturing factors that affect Design with particular reference to manufacturing processes. 4. Design products considering the above factors. 3.1 INTRODUCTION: The great multiplicity of design possibilities that may be drawn up in a specific situation-either by such methods which were described in the previous chapter or simply by intuition may frighten the unprepared. The question quickly arises of how to reject the unsuitable solutions. It was seen earlier how the products synthesis might be characterized by a continuous alternation between searching for and selecting ideas, and it was mentioned that one ought to select only if suitable criteria for selection are present. The designer on the basis of requirements from the outside world formulates the criteria used. As all the stages in the life of the product can give rise to demands and requirements in respect of the product we can get a general idea of where these originate. The situation may be described as a force field where a number of forces try to pull the product in different directions. The final product will then represent equilibrium- a compromise where the forces balance each other. Figure 3.1 illustrates the various factors that affect the Design of a Product. On the outside are number of requirements, product factors, stemming from the life of the product, which influence it through the criteria that are formulated. In the middle are the five basic properties-structure, form, material, dimension and surface-which specify the product. These properties are specified in the product synthesis in such a way that the criteria are fulfilled as far as Prepared by Prof.R.Panneer, Assistant Professor.

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possible. From the basic properties, all other properties of the product are derived, particularly the function, which is the central property in the process of use. If we consider the basic property of design in particular, we find that the influences arise, on the one hand, from the product factors mentioned above, and, on the other hand, from the other basic properties, as these are not independent variables. The dependence of design on other factors will be discussed in the following section. The remainder of the chapter deals with the dependence of design on the manufacturing factors.

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Figure 3.1 Factors affecting the design of a product The fact that the basic properties are not independent can be seen in practice by the impossibility of deciding on them separately. The structure cannot be finally chosen until the consequences to the design have been estimated, and the design cannot be determined until material, dimensions and surface have been considered. It is therefore very important to recognize the interplay between the basic properties and the design. The influence of the structure on the design is direct, as already mentioned in the previous chapter and as mentioned in Fig 3.2. The dependence of the design on the material, dimension and surface is a little more difficult to identify, because it takes place indirectly. The interaction of the material and design occurs in two ways. Firstly, the design depends on the production/manufacturing processes by which the material can be shaped, and so the design depends indirectly on the material. Secondly, the properties of the material play a major role in determining the form e.g. the strength, elasticity and weight of the material (Fig.3.3).

Figure 3.2 Two Drawing Equipments with Different Structures (Dependence of design on structure)

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Figure 3.3 Springs made of two different materials- rubber and steel. The different properties of materials result in very different form designs. The influence of the dimensions on the design is illustrated in Figure 3.4. What counts here is the fact that, with the change of size of the object, the practicable production processes alter. The criteria, e.g. the material and the weight, also change. The influence of the surface on the form takes place indirectly through the choice of production process.

Figure 3.4 Cog-wheels showing the influence of the dimensions (Size) on the design 3.2 GENERAL ASPECTS OF DESIGN FOR MANUFACTURE: One of the most important questions to be faced when designing a product is how the production will be carried out. Production can be divided into the manufacture of the parts, the assembly, the testing and quality control. The manufacturing and the assembly processes are very closely linked to the design/form of the parts. In the earlier chapter on form variation, the different examples did not end with detailed design suggestions, but with a series of form concepts. There is a natural reason for this, namely that a product or a part cannot be designed in detail until one has chosen the material, manufacturing process and assembly process. The stages in the design of an element are shown in Figure 3.5 starting from considerations of functional surfaces over to form concept levels to the choice of processes and a final design of Prepared by Prof.R.Panneer, Assistant Professor.

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details, the loop round the choice of processes means that, at any point, it is possible to split the element into several part elements which are later assembled. Correspondingly, it is sometimes possible to integrate several elements into one. The stage of form division/form integration must thus be thought of as a last level of ideas, which may be used if one cannot easily produce and assemble one's elements. Figure 3.6 shows different detailed designs for a pulley. From these diagrams it can be seen that there is a close relationship between elements, the manufacturing and the assembly processes.

Figure 3.5 The form design stages in a design project

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Figure 3.6 Different detailed designs for a pulley. In Figure 3.7 we have taken as a starting point the earlier mentioned fork joint. The form concept on which the detailed design proposals are built is shown top left, and it is assumed that the joint is approx. 100mm long and that the material is steel. We first examine by which processes or combinations of processes the fork joint can be manufactured as one element. Next, we suggest four new form concepts by form division, and finally each of these is examined for possibilities for manufacturing the elements and for the assembly process. Altogether the example gives fourteen practical ways in which the joint can be produced. Obviously not all are equally suitable in a specific situation. The choice depends on such factors as the number to be produced, the tolerance required, surface requirements and many other factors. These requirements will be dealt with more thoroughly in the following section.

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Figure 3.7 Examples of interplay between form concept, form division and choice of manufacturing and assembly processes A condition for being able to choose an optimum manufacturing process is that the best possible agreement can be achieved between the design and the process requirements. This means that the order shown in Figure 3.5 form concept, choice of manufacturing and assembly processes, design of details-must be understood, bearing in mind that first the form concepts are drawn up, then the process possibilities are examined, and finally the form concept and the processes are chosen as far as possible simultaneously. It is thus usually not enough to adjust the detailed form to the process, if an optimum product is to emerge. The problem of choosing the manufacturing process before the design of the details has been taken too far often occurs in discussions between the designer and the process technician. The former often tends to forget the manufacturing process so that the latter has no possibility of optimizing his contribution. The ideal would be if the process technician could come into the picture early so that he could take part in assessing the form concepts at the first stage. In a possible discussion of proposals for alterations based on the manufacturing process, the idea of functional surfaces is valuable. A functional surface can only be altered if other alterations are made simultaneously elsewhere in the system, while an alteration of the areas between the functional surfaces can be made with much greater freedom. As a rule the designer must have an intimate knowledge of the manufacturing processes available. The fact that (in the bigger firms) there may be process technicians who can assist in the detailed design does not excuse the designer from knowing intimately the existing processes. The designer must know about the form geometries that can be created with a given process, including the tools and fixings. He must also know what materials can be used in the process and the tolerances, which can be achieved, and the surface finish. Using this information as a background the designer must be able to design his object so that it is cheap to manufacture. How in practice is it possible to choose the best possible form concept and manufacturing process? Obviously this can only be done from a number of criteria which may be divided into the Prepared by Prof.R.Panneer, Assistant Professor.

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operator

3.2.1 The Manufacturing Process: How Feasibility affects? The factors concerning feasibility in connection with the choice of process are as follows: • Form geometry

• • • • •

Material Size/Dimensions Surface requirements Tolerance requirements Form (availability) of input materials.

The first three factors decide whether a given process is possible. Each process has its own characteristics and limitations, as shown in Figure 3.8. When initially considering various processes one should not choose those at the extremes, so that the size is theoretically possible but in practice difficult to achieve.

Figure 3.8 Possibilities concerning form geometry and dimensions that can be realized by turning

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Figure 3.9 Bearings bracket where the forces acting on it during manufacture have been taken into consideration, so that the tolerances can be maintained. The factors of surface and tolerance requirements must also be included when choosing the process. It is not enough that these requirements can theoretically be met, but they must also apply to the specific object. The example in figure 3.9 shows a bearing bracket, the design of which is suited to the forces that act on it during production. The only purpose the stiffening rib serves, in this case, is to restrict the flexibility during the production sufficiently for the desired tolerances to be achieved. The last factor mentioned in the list is the form of the materials used. It is necessary that the required materials exist or can be obtained in the desired form. 3.2.2 The Manufacturing Process; How the Economics involved in the choice of process affects? Economic factors in connection with the choice of process are:

• • • • • •

The number of processes required Materials: supply, price, quantity or own manufacture Quantities to be produced Machinery Investment in new machinery Special tools

An object may be produced directly in one process, or in several successive processes. The economics of the manufacture depend on which and how many processes must be gone through before the object is finished. One must also consider the necessary transport, handling and 'fixings' between the separate processes. The availability of the desired materials must be examined. It should be decided whether they can be bought in the required form, and under what conditions, or whether the company itself must produce them. The quantity in which the object is to be produced is decisive when deciding which manufacturing processes will be economic. Processes that require big investments in tools and machinery (e.g. turning, milling and welding) are well suited to the Prepared by Prof.R.Panneer, Assistant Professor.

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production of multiple objects or series of objects. Figure 4.15 shows an example of this. As already mentioned, the last three factors machinery, investment in new machinery and special tools are closely connected with the quantities to be produced. 3.2.3 The Manufacturing Process: How the operator situation affects? At the same time as an object is designed and a process decided on, a job for an operator is laid down. This must be done as a conscious effort, where the operator's situation is used to influence the form design and the choice of process. One must ensure that the operator can carry out the process appropriately without unnecessary workload and risk and, for instance, without unnecessary demands for precision or speed. But even if in principle the operator's conditions may be allowed for, there is still a decisive factor remaining. Does the company has the necessary know-how, can others be trained, or must new workers be employed? 3.2.4 The Manufacturing Process: How Economics of the detailed design affects? After the form concept and the manufacturing process are chosen according to the criteria of feasibility, economics and operator situation, there is still the detailed design to be decided (see Figure 3.5). The last task is to design the details in such a way that the object can be manufactured in the most suitable way by the chosen process, and that the desired function may be sufficiently well realized. Form design guidelines for all the usual processes can be found in the specialist literature, and therefore the characteristics of the various processes will not be discussed here. A few general guidelines can, however, be laid down for making an economic form design. They are:

• • • • •

Number and nature of fixings Number and nature of tools Number and extent of processes Accessibility for tools Consumption of materials Prepared by Prof.R.Panneer, Assistant Professor.

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These guidelines are illustrated in Figure 3.10 to 3.12. Figure 3.13 shows a complete example in which many of the economically important factors are mentioned.

Number and Nature of Fixing

Consumption of Material

Accessibility for Tools

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Figure 3.10 Form designs that take into account the economics of production. Number and extent of Process

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Figure 3.11 Form designs that take into account the economics of production

Number and Nature of Tools

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Figure 3.12 Form designs that take into account the economics of production

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Figure 3.13 Three versions of a threaded spindle corresponding to different product quantities. The most important economic factors are listed. 3.2.5 The Manufacturing Process: How Assembly affects? The close connection between the assembly process and the manufacturing processes is illustrated in the following examples. The assessments that must be made before choosing an assembly process are (as in the case of the manufacturing process) feasibility, economics and situation of the operator, and the factors are completely parallel to the factors in choosing the manufacturing process. Figure 3.14 shows examples from a photocopier where the prototype and the final machine are compared. The illustration shown in the top, explains the way in which the number of operations in fitting a mirror can be reduced if one goes to the expense of a die cast tool. The lower illustration shows an example of how a traditional way of fitting a pin can be simplified if the assembly process is carefully thought out. After the assembly process has been chosen (and the manufacturing process as well) the product details must be designed in such a way that an optimum assembly can be achieved. As a check the following list of general sub-operations in assembly may be used: Recognize Grasp Move to contact area Orientate

Line up Fit in Move along contact area Secure

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Figure 3.14 Comparison between form design details in a prototype and the final version. Above, fixing a mirror to a frame; below, fixing a cog wheel on an axle

Figure 3.15 Form design details that illustrate how the assembly is taken into account. Figure 3.15 shows examples of the way in which some suboperations can be made easier by the form designs. These considerations apply whether the assembly is manual or automatic.

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Figure 3.16 Form design details that ensure a stop button is fitted in the correct position Figure 3.16 shows the assembly of a stop button in an electric switch. On the button is printed the word STOP, and in order to ensure that it is put on the right way up, the bottom is designed with a groove that corresponds to a knob on the edge of the hole. 3.3 DESIGN FOR CASTINGS: The material of paramount importance for general engineering purpose is cast iron. It is also one of the shortest routes from raw material to finished product.

Figure 3.17 Conventional Sand Casting Process Basically the molding and casting operation consists of packing sand around a pattern (which resembles the unmachined casting), removing the pattern from the mold and assembling the sections of the mold, pouring metal and removing & cleaning the casting. The mold is usually in two parts and the pattern or part-pattern is seated on board or is mounted on a plate when each part of the mold is produced. (Fig 3.17). Heat is extracted through mold (in this case sand mold), and the molten metal solidifies into the final solid shape. This seemingly simple process can be quite complex metallurgically, since the metal undergoes a complete excursion from the superheated molten state to the solid state. The problems associated with sand-casting can be identified as (a) Manufacture of the mold (b) The behaviour of the metal during pouring, solidification and cooling in the solid state. (c) The fettling of casting

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In addition to these problems, the cost of producing and machining the casting must be considered. Features that increase the cost of molding and fettling and those demand additional machining should be avoided. Hollow castings can be produced by including a sand core to restrict the shape into which the metal can flow(Fig 3.17), but if the core is a separate piece the cost of molding will increase. Therefore although a characteristic of sand casting is its ability to produce hollow, box like shapes, a less complicated shape should used if possible. Design considerations in Casting: 3.3.1 Design to simplify molding: (i) Location and Shape of Joint Surface:

Fig 3.18 Casting that requires cranked joint line

Fig 3.19 Casting that requires a straight joint line

The joint surface between sections of the mold must be such that the pattern can be removed after the moulding has been done. As this form the surface upon which the pattern, or part-pattern will sit at the start of the molding it should, if possible be a plane surface. Fig 3.18 shows a casting that can only be produced by splitting the mould in the direction shown but which, because of the position of the circular flange, require a cranked joint line. When the flange is repositioned, as shown in Fig 3.19, the joint line is straight and the pattern can be seated on a plane surface simplifying the molding operation and enabling higher degree of accuracy to be obtained. (ii) Draw Angles & Reentrant Shapes:

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Fig 3.20 Function of the draw angle

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Fig 3.21 Casting that requires a three part mold

Surfaces that would lie exactly in the direction of pattern withdrawal are inclined slightly by a small angle known as the draw angle as shown in Fig. 3.20 to prevent the impression being scuffled by the pattern as it is withdrawn from the mold. The designer should specify both the angle and the location. Re-entrant shapes should, if possible be avoided because they require special arrangements to enable the pattern to be removed. This increases the cost of molding and the accuracy of casting. For example, if the casting under consideration included the flange as shown in Fig 3.21, the pattern and the mould would need to be split in two places, requiring three part mould, so that the pattern could be removed. (iii) Location of Bosses:

Fig 3.22 Casting with large number of bosses

Fig 3.23 Casting with single facing

Bosses are often included to cut down the amount of machining that is required, by reducing the machined surface area, as an alternative to localizing by spot facing. Every boss that lies on an upper, horizontal face will require a riser to allow air to escape and so ensure that a complete casting is produced. These additional risers will increase the molding time and also the fettling time because the metal in them will have to be removed. Therefore introduction of more bosses will become uneconomical. Fig.3.23 illustrates a casting that includes a single facing instead such a large number of bosses. Prepared by Prof.R.Panneer, Assistant Professor.

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3.3.2 Design to control the behavior of the metal during the sand casting process: In order to appreciate the relationship between casting design and the behaviour of the metal, the pouring and solidification of metal will be briefly discussed here. To ensure continuity and completeness the molten metal should reach all parts of the mold before the solidification starts. The molten metal in the runners and risers will act as feeders for this purpose. Upon cooling metal, non-metal consists of grains and grain boundaries as shown in Fig 3.24. Figure illustrates three main types of grain (in thick casting). The structure that consists of small grains (fine structure) is stronger than one that consists of large grains (coarse structure). A fine structure is produced by rapid solidification and a coarse structure by slow solidification. The solidification is controlled by; (a) the mould material, (b) the mould Fig.3.24 Types of grain in thickness, (c) the mould temperature and the (d) thickness of the casting a thick casting being produced (i) Section thickness:

Fig 3.26 Casting design that Fig 3.25 Casting design that prevents shrinkage defects produces shrinkage defects It has already been stated that the contraction of molten metal during solidification will be allowed for supply from runners/risers, and it will be appreciated that a thicker section requires more molten metal to allow for contraction than does a thin section. This causes problem when it is necessary to feed a thick section through a thin section. The metal in the thin section will solidify before that in the thick section, cutting of supply of molten metal and producing shrinkage defect as shown in Fig.3.25. Fig. 3.26

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shows how the casting can be redesigned to prevent the defect from occurring.

Fig 3.28 Casting supporting Fig 3.27 Casting with Heavy Riband rigidity, section Instead of making a casting thicker to obtain strength it may often be made of larger ‘outside’ shape and hollow to reduce the section thickness. Similarly the thick section shown in Fig. 3.27 can be replaced by rib as in Fig. 3.28 to obtain local support.

Fig 3.29 Tall Bosses Causing Heavy Section

Fig 3.30 Modified Design to avoid Heavy Section

The tall bosses on the flange in Fig. 3.29 can be modified as shown in Fig.3.30 to combine height with uniform section. (ii) The junction of wall sections and of ribs with wall sections:

Fig 3.31 Design causing plane Fig 3.32 Design to eliminate of weakness and hot spot plane of weakness and hot spot s The effect of section thickness, and also the variation of grain shape shown in Fig. 3.24, must be considered when the casting includes junctions. Fig. 3.31 illustrates how plane of weakness is produced when two sections join at right angles, and Fig 3.32 Prepared by Prof.R.Panneer, Assistant Professor.

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illustrates how this can be eliminated by making the surface curved at the intersection.

Fig 3.34 Design to blend Fig 3.33 Design to further different sections reduce and hot spot In addition to the weakness associated with the thickness grain shape, the sharp angle at the inside of the casting (Fig. 3.31) causes the part of the casting to solidify more slowly because the heat cannot escape so easily in to the core producing a hot spot. This is reduced considerably in the design shown in Fig 3.32, but is further reduced by making the section locally thinner, as shown in Fig. 3.33. When it is necessary to design a casting having different wall thickness, there should be gradual changes as shown in Fig. 3.34.

Fig 3.36 Design to minimize hotspot s The hotspot effect is made worse when sections and ribs join at acute angle as in Fig. 3.35. The acute angle junction can be retained if the shape is opened out as shown in Fig. 3.36. Fig 3.35 Design that causes hot spot

Fig 3.37 The inscribed circle system

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Fig 3.38 Cored Boss

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Fig 3.39 Junction with local depression

The extent of the variation of metal mass produced at junctions can be indicated by the inscribed circle system. (Fig 3.37) in which the mass at the junction (represented by ‘D’) is compare with that of the sections that join (represented by ‘d’). It will be seen that the variation increases as Fig 3.40 Local depressions and the number of sections that meet increases and as the angle of the staggered joints junction becomes more acute. The mass at the junction can be reduced by using cored boss, as shown in Fig. 3.38. But introduction of core increases the cost of moulding operation. An alternative is to reduce the thickness by a local depression as shown in Fig. 3.39, and to combine this with staggering when sever ribs are involved as shown in Fig 3.40. (iii) Bosses and section thickness:

Fig 3.41 Boss causing hot spot

Fig 3.42 Modified Boss

The section thickness will be increased where boss is present. The effect is unnecessarily great if the boss is considered as a short cylinder projecting from the main body of the casting, to which it is blended by a fillet (Fig. 3.41). The hotspot effect and Prepared by Prof.R.Panneer, Assistant Professor.

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unnecessary local thickness can be eliminated by reducing the height of the boss and making the fillet form such that a more gradual change in thickness is produced (Fig. 3.42). (iv) Separate Cores:

Fig 3.43 Badly designed core Fig 3.44 Correctly designed core When separate cores have to be used, gas from the molten metal should be allowed to escape through them in the same way as it is allowed to escape through the mould. This can only occur if the cores extend into or beyond the mold. Fig. 3.43 shows a design in which the core is badly vented and the core sand awkward to remove. Fig. 3.44 shows a variation in the design eliminating these problems. (iv) Prevention of stresses and possible fracture during cooling in the solid state:

Fig 3.45 Ribs that may crack during cooling

Fig 3.46 Rib arrangement that will absorb contraction

Contraction of solid metal as it cools to room temperature cannot be avoided and so the casting must be deigned with this in mind. For example, when a casting is ribbed to combine stiffness with minimum weight there is a tendency for the ribs to crack as a result of contraction when they are arranged as in Fig. 3.45. This cracking can be avoided by arranging them as shown in Fig. 3.46.

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Fig 3.48 Spoke arrangement that will absorb contraction

Fig 3.47 Spokes that may crack during cooling

The above design consideration may be extended to include the spokes of a ring (for example a cast spoked flywheel). When there is even number of spokes, as shown in Fig. 3.47, distortion during the cooling will cause cracking; but when there is an odd number of curved spokes as in Fig. 3.48, each one can take up a greater or smaller curvature, depending upon the distortion, without itself cracking or causing crack elsewhere. 3.3.3 Design considerations in Die-Casting: (i) Tapers: Surfaces that would lie exactly in the direction in which the dies move to release the casting should be inclined at an angle known as taper (Similar to draw angle in sand casting) so that the casting can Fig 3.49 Location of Taper be rapidly removed from the die without scuffing. (Fig 3.49) (ii) Re-entrant Shapes:

Fig 3.50 Design that requires a collapsible core

Fig 3.51 Design that does not requires a collapsible core

Castings should be designed without reentrant shapes. For example, the casting shown in Fig 3.50 has an internal flange which requires a collapsible core. The version shown in Fig 3.51 can be easily produced. iii) Bosses and Reentrant Effects:

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Fig 3.53 External Bosses Fig 3.52 Internal Bosses Fig 3.52 shows a box-like casting with bosses that are placed on the inside to produce clean external shape. This shape requires reentrant shapes which demand collapsible core. By placing bosses on outside as shown in Fig 3.53 there will be no core problems. iv) Design to avoid retracting cores:

Fig 3.54 Incorrectly designed elbow piece

Fig 3.55 Correctly designed elbow piece

Fig 3.54 shows an elbow piece that could be produced by a sand core but could not be produced by the metal cores that are necessary when pressure die casting because of the shape at ‘A’. Fig 3.55 shows how the inside shape can be modified to enable the cores to be withdrawn from the finished casting. v) Core Support:

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Fig 3.57 Design that allows Fig 3.56 Design that core to be supported requires unsupported core When a long slender core is used to produce holes (as shown in Fig 3.56) it should, if possible, be supported by the mating die so that it does not deflect during the injection of the metal, causing in accuracy or making it difficult to remove the finished casting. This can be only done by making the hole through hole as shown in Fig 3.57 which will enable the core to be supported at both ends. 3.4 DESIGN FOR HOT METAL WORKING - FORGING: Forging involves hitting material using hammer (manually or by machine) or squeezing it using a press. Simple tools are used when small quantities are required and dies are used for large quantities. (Closed die forging, drop forging or stamping). Forging is a bulk deformation process in which a solid billet is forced under high pressure to undergo extensive plastic deformation in to a final near to finished shape. Because of extensive plastic deformation that occurs in forging, the metal undergoes metallurgical changes and develops a fibre structure as shown in Fig 3.59. This mechanical fibering introduces directionality to the structure sensitive properties such as ductility, fatigue strength, and fracture toughness. (ST) (LT) (L) Fig 3.58 Schematic of Closed Die forging – Block Die The principal direction of (long axis of bar) working is known as longitudinal direction (L). The short transverse direction (ST) is the minimum dimension of the forging such as thickness of a plate like shape. The long transverse (LT) direction is perpendicular both to longitudinal and short-transverse direction. (Refer Fig 3.58)

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Fig 3.59 Closed Die forging – Formation of Flash Design Considerations in forging: 3.4.1 Grain Fibre Direction: The development of grain fibre direction should coincide with that of the major stress when the product is in service. The direction of grain fibre depends on the shape of the die cavity. In order to obtain the required grain fibre direction, the shape of the forging may differ quite considerably from that of a finished product. Manufacturing involving a forging involves removal of metal to obtain the required shape and accuracy. The change of grain fibre direction should not be abrupt; otherwise regions of weakness and stress concentration will be produced. 3.4.2 Separation of forging from Dies: The location of the parting line between the two sections of the die must be such that the forging will not be trapped. Also there must be no reentrant features and surfaces that would lie exactly in the direction of the die and forging movement must be inclined at an angle called draft angle. (Fig 3.60). The normal draft angle on external surfaces is 5° to 7° and for internal surfaces it is 7° to 10°.

Fig 3.60 Closed Die forging – Schematic of Finishing Die Prepared by Prof.R.Panneer, Assistant Professor.

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3.4.3 Location of Parting Line: The location of parting line produces more problems than casting. This is because of unpredictable flow of metal due to greater volume of metal is placed between the upper and lower die than is required in the finished forging. (Refer Fig 3.58 and 3.59). The excess metal is allowed to flow out through the flash lane and into the gutter to form a flash that is later removed. The forging design should be such that the location of parting line ensures that the product is of a shape to suit its manufacture and has the required directional strength.

Fig 3.61 Example of desirable and undesirable location of parting line – Case 1

Fig 3.62 Example of undesirable and desirable location of parting line – Case 2

Fig 3.63 Example of undesirable and desirable location of parting line – Case 3

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Fig 3.64 Example of undesirable and desirable location of line – Case The depth of recessesparting in the die into which4 the material must flow will affect the orientation of the forging with respect to the parting line as shown in Fig 3.61 (Case 1) the aim being to minimize the depth to which the material must flow. The deep impression also requires high forging pressure for complete filling and might lead to breakage. For optimum economy the parting line should be kept in a single plane, since that will make the forging less costly. Because the forging fiber is unavoidably cut through when the flash is trimmed, the parting line is best placed where minimum stresses arise in the service performance of the forging. Fig 3.62 to Fig 3.64 illustrates variety of simple shapes and the correct and incorrect parting line locations. Case 2 and 3 represent unsymmetrical parts in which the parting line and the forging plane are no longer coincident. The forging plane must be perpendicular to the direction of ram travel. When the parting line is not in one plane as in Case 2 or 3, die construction is more costly. Some time the most economical solution is to produce non-symmetrical part is to build a die with two cavities in mirror imaging positions. A good rule in forging die design is to locate the parting line near the central height of the part. That avoids deep impression in either top or bottom die. When parts are dished or hollow as shown in Fig 3.64 (Case 4) they may not produce the best strength because a centrally located parting line interrupts the grain flow. In this case, the satisfactory location of parting line provides the least expensive design because only top half of the die requires the impression. However the most desirable grain flow pattern is produced when the parting lone is at the top of the dish. 3.4.4 Corner and Fillet Radii:

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Fig 3.65 Example of fillet formation in closed die forging Corner and fillet radii should be as generous as possible as the service of the product permits. As Fig 3.65 (Type A) illustrates fillet can be sharper when the metal flows away from them is more than the metal that flows toward them. In the same figure in Type B situation, the metal is in danger of folding back on itself to produce a defect known as a Lap. In general 6.4 mm fillet radius is the absolute minimum for high strength materials. To produce sharper radius the forging will have to be done in several steps. As a general rule one additional die impression is required to reduce the fillet radius by 50 %. Generally corner radii are not as critical as that of fillets. The minimum corner radius is about one half of the minimum fillet radius. 3.4.5 General Design Considerations:

•

The thickness of flash must be large enough to ensure that the die cavities are filled before metal escapes.

•

The flash thickness is related to the weight of the forging (it increases with forging weight) and to the thickness of webs in forging. (no webs that are thinner than flash can be produced).

•

Whenever possible in the design of forgings, it is desirable to maintain all adjacent sections as uniform as possible. Rapid changes in section thickness should be avoided.

•

Laps and cracks are most likely where metal flow changes because of large differences in the bulk of sections.

•

The machining envelope is the excess metal that must be removed to bring the forging to the finished size.

•

The ultimate in precision forging is the net shape forging in which the machining allowance is zero.

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•

Generally, however allowance must be made for removing surface scales (oxides), and correcting warpage.

•

Allowance must be also given to remove mismatch (where upper and lower dies shit parallel to the parting plane), and for dimensional mistakes due to thermal contraction or die wear.

3.5 DESIGN FOR COLD METAL WORKING - SHEET METAL FORMING: The cold working improves the dimensional and surface accuracy o the material and also causes work-hardening. Components are often produced from cold rolled sheet metal using presswork. Presswork can be broadly classified into (i) Cutting Operations and (ii) Non-cutting operations. Piercing is a cutting operation that produces holes in the work piece and blanking is a cutting operation that cuts the work piece from the sheet – these operations are generally combined. The principal non-cutting operations can be classified into two groups. One group consists of forming and bending; the other group consists of operations in which the sheet metal is pushed to form a Cylinder or Can like part (the operation that produces shallow cylinders are called cupping and the operation that produces cylinders that are deep in depth are called deep drawing). Components produced by presswork are basically of simple shape compared to those produced by operations such as forging, casting and machining. Presswork is ideal for manufacturing of tab-washers, clips, cup-like containers etc. Design Considerations in sheet metal forming: 3.5.1 Stiffening of Sheet Metal Parts:

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Fig 3.67 Ribs formed to stiffen Fig 3.66 Bracket stiffened The lack of stiffness associated with sheet ametal can be overcome flat sheet by forming a rib by the shape of the product. For example, a reasonably stiff shelf bracket can be obtained by forming it to produce a rib as shown in Fig 3.66. A flat sheet can be stiffened by introducing number of ribs as shown in Fig 3.67 or by corrugating it. This increases the stiffness in one direction only. To increase the stiffness in the second direction additional ribs may be introduced. . 3.5.2 The manufacture of Die-set: The cost of manufacturing die-set depends upon the complexity of the product, which should therefore be as simple as possible. Apparently minor changes can considerably reduce the cost of Die-set. For example, the rounded end shown in Fig 3.68 requires

Fig 3.68 Unsuitable Profile

Fig 3.70 Compromise Profile

Fig 3.69 Suitable Profile

Fig 3.71 Compromise Profile with radii

more costly die set than does the square end shown in Fig 3.69; the 45° chamfers shown in Fig 3.70 is a compromise. When rounded ends are essential it must be remembered that the tangential radii shown in Fig 3.70 cost more than the one shown in Fig 3.71.

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Fig 3.72 Pressing with Fig 3.73 Pressing with Thetwo diedirectional cost can be reduced by designing the product so that it forming single directional forming can be produced using the minimum number of forming operations as illustrated in Fig 3.72 and 3.73. 3.5.3 The behaviour of metal during its manipulation:

Fig 3.74 Sheet Metal Component

The cold rolling done to prepare the sheet metal for presswork increases the directional effect by deforming the grains. The directional properties of these grains should be taken into account when designing parts that are produced by bending. Consider the component shown in Fig 3.74.

Fig 3.75 Piercing and Blanking Layout If the material is blanked prior to bending using the layout shown by ‘A’ in Fig 3.75 there will be no trouble. But if blanked using the layout shown by ‘B’ in Fig 3.75 it will probably split when bent because the line of the bend lies in the same direction s the grain fibre. In this example the problem is overcome by using a suitable blanking layout.

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Fig 3.76 Pressing that would be troublesome

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Fig 3.77 Re-designed Pressing

A component such as the one shown in Fig 3.76 causes problems because it has two tabs, one of which must be in an unfavorable position; but by changing the design to that shown in Fig 3.77 the problem is eliminated.

Fig 3.78 Pressing that Fig 3.79 Re-designed Pressing would tear

Fig 3.80 Re-designed Pressing

Abrupt bends should be avoided because they tend to produce stress concentration. Similarly the metal will tear when bent to for the tab shown in Fig 3.78. The tearing will not occur if relief notches shown in Fig 3.79 are introduced or if the form is modified to that shown in Fig 3.80. 3.5.4 The separation of pressed part from the die-set:

Fig 3.81 Pressing that would be difficult to remove from the die

Fig 3.82 Redesigned Pressing

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Fig 3.84 Redesigned Fig 3.83 Awkward Pressing Pressing The pressing should be designed in such a way that it can be easily removed from the die-set. For example the pressing shown in Fig 3.81 is more difficult to remove from the die-set than that shown in Fig 3.82. Similarly the pressing shown in Fig 3.83 can be produced by forming the two flanges with two slide tools, but it must be slid from the punch by the operator. The modified version shown in Fig 3.84 has two external flanges that can be formed by bending before the U-bend is done. (a double acting press is used) and the completed pressing is removed from the punch during its upward movement by a stripper plate. 3.5.5 General Design Considerations:

• •

It may be les expensive to construct a component from several simple parts than to make an intricate blanked part. Blanking with sharp corners are expensive to produce. The layout of the blanks on the sheet should be such as to minimize scrap loss. As Fig 3.85 illustrates, a simple change in design can often greatly improve the material utilization

Component Scrap Area

Fig 3.85 Design to minimize scrap loss

• •

The usual tolerances for blanked parts are ± 0.076 mm. When holes are punched in sheet, only part of the metal thickness is sheared cleanly; a hole with tapered side is created. If a punched hole is to be used as a bearing surface, then subsequent operation will be required to obtain parallel walls.

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Chapter 3 Design for Manufacture

•

Diameter of punched holes should not be less than thickness of the sheet or a minimum of 0.635mm. Smaller holes results in excessive punch breakage and should be produced by drilling.

•

The minimum distance between the holes or between a hole and the edge of the sheet should be at least equal to the thickness of the sheet.

≥t ≥t •

Figare 3.86 Minimum Holes must be at least If holes to be threadedDistance the sheetof thickness one-half of the thread diameter.

•

The greatest formability in bending is obtained when then bend is made across (perpendicular) the metal grain.(Refer Fig 3.75)

•

The largest bend radius should be used and the bend radius should not be less than the sheet thickness‘t’.

•

The total length of the sheet required for bending is the sum of two legs plus bend allowance. The bend allowance depends upon how much the metal stretches on bending, which is function of the angles of bend (A) and bend radius (r).

Bending Die

Bent material

Bent material A

r Leg Length

Bend Allowance

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Fig 3.87 Bend Allowance, Bend Angle and Bend Radius

•

After bending and upon release of the load, the bent materials springs back and both the angle of bend and bend radius increases. Therefore to compensate for the springback, the metal must bend to a smaller angel and sharer radius so that when the metal springs back it is at desired values. Springback becomes more severe with increasing yield Punch load Spring back after strength and section thickness. removed

Fig 3.88 Springback after removal of load 3.6 DESIGN FOR JOINING - WELDING:

Fig 3.89 Classification of Welding Processes

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Welding is the most prominent process for joining complex parts. Fabrication by welding, as an alternative to production by casting or by working and by machining has become important during recent years. The different types of welding processes can be classified as shown in the Fig 3.89. Design Considerations in welding: 3.6.1 Selection of material and electrode: When selecting the parent material of a weld joint the following points should be considered. Material selection for welding involves choosing a material with high weldability. Weldability is a complex technological property that combines many basic properties such as melting point, specific heat and latent heat of fusion, thermal conductivity, thermal expansion etc. The melting point of the material, together with the specific heat and latent heat of fusion will determine the heat input necessary to produce fusion. A high thermal conductivity allows heat to dissipate and therefore required high rate of heat input. Also metals with high thermal conductivity result in more rapid cooling and hence result in problems with weld cracking. Greater distortion result from high thermal expansion, with residual stresses and greater danger of welding. The electrodes that are used in electric arc welding are specified with a code such as E60XX. The last two digits indicate the type of welding application, and the two digits immediately following E indicate the tensile strength of the weld metal in kips per square inch. (ksi). According to the tensile strength required and how the welding is going to be applied an appropriate electrode can be selected. For example, E 7024 electrode has a 70 ksi tensile strength and is intended for ac or dc electric arc welding for steel fillet welds in the horizontal or flat position. 3.6.2 The Design of Weld Joint: The basic types of weld joints and the variations of the basic types are shown below. (Fig. 3.90 to Fig 3.93)

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Corner Joint

Edge Joint Fig 3.90 Basic types of welded joints

Fig 3.91 Variations of Butt Joints

Fig 3.92 Variations of Tee joints

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Fig 3.93 Variations of Corner joints The types of weld joints that can be used for particular type of applications are shown below. (Fig. 3.94 to Fig 3.98)

Fig 3.94 Welds used when plate thickness are unequal

Fig 3.95 Flank Fillet Weld to join plates

Fig 3.96 Ring Fillet Weld to join a shaft and plate

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Fig 3.97 Slot welds and plug welds to join plates (flank fillet weld is preferred over this type. To be avoided unless required)

Fig 3.98 Types of Box Section welding

Fig 3.99 Types of ribs to be provided when welding large walls Vulnerable point a

b

Fig 3.100 Dish End Welding (Weld should not be placed at vulnerable points, ‘a’ – wrong, ‘b’ - right) 3.6.3 General Design Considerations of welding:

•

A square edged butt joint (Fig 3.101) requires minimum edge preparation. However an important parameter in controlling weld cracking is the ratio of the width (w) of the weld bead to the depth (d) of the weld bead. It should be close to unity. Since narrow weld joints with deep weld pools are susceptible to cracking, the most economical solution is to spend machining money to shape the edges of the plate to produce a joint with more acceptable width to depth ratio. w d Prepared by Prof.R.Panneer, Assistant Professor.

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Fig 3.101 Square Edged Butt Joint

•

Ideally a butt weld should be full penetration weld that fills the joints completely throughout its depth. When the gap in a butt weld is wide, a backing strip (Fig 3.102) is used at the bottom of the joint. Backing Strip Fig 3.102 Backing Strip

•

Welds are made with weld metal ‘reinforcement’ (Fig 3.103) that extends above or below the surface of the base metal plate. Some Designers believe that this increases the strength of the joint and compensate for any weld imperfection. However these reinforcements serve as a stress concentration under fatigue loading. Therefore reinforcements on welds should not be used when the welds are subjected to fatigue loading.

Weld Reinforcement Fig 3.103 Full penetration Butt Weld with Reinforcement

•

Fillet welds are the welds most commonly used in structural design. They are inherently weaker than full penetration butt welds. A fillet weld fails in shear at the weld throat, (as shown Fig 3.104) given by 0.707h and American Welding Society code allows a shearing yield strength of 30% of the tensile strength designation of an electrode. Thus for an E60XX electrode the load carrying capacity of a fillet weld is = 0.30 (60) (0.707h) kips/sq.inch.

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Fig 3.104 Fillet Weld in a Lap Joint

•

Since welding involves rapid application of heat in a localized area and rapid removal of heat/cooling, distortion is ever present. One of the best ways to eliminate the welding distortion is to design the welding sequence. An example sequence or a step-by-step welding principle is shown below. (Fig 3.105)

Fig 3.105 Welding Sequence •

If because of the Geometry of the part to be welded, the welding distortion cannot be avoided, then the forces that produce the distortion should be balanced with other forces provided by clamps and fixtures.

3.6.4 General rules of Welding: 1.

Do not attempt copy blindly from cast, riveted and forged designs.

2. Provide a straight line force pattern as far as possible. 3.

Avoid laps, straps, and stiffening angles.

4.

Use Butt Welds as far as possible.

5.

Limit the number of welds used.

6.

Make sure that ends to be welded are equal thickness.

7.

Avoid placing welds in vulnerable sections.

8.

Avoid using welding fixtures as far as possible.

9.

Provide easy access to welds.

10. Carefully consider the sequence with which the oarts to be welded together and include the information in the drawing.

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11. Use horizontal and fillet welding. Try to avoid overhead welding. 12. Avoid subjecting welds to bending loads.

13.Distribute heavy loading over long welds in the longitudinal direction. 14. Do not position the weld at the point of maximum deflection.

15.Make sure that the working drawing contains the relevant and necessary information such as details of weld quality, weld form, weld length etc.

3.7 DESIGN FOR MACHINING: Machining operations represent the most versatile and most common manufacturing processes. Practically every part is subjected to some kind of machining operation in its final finishing stages of manufacture. There is wide variety of machining processes with which the designer should be familiar. The classification of machining processes according to translation and rotation of tool or work piece is shown below in Fig 3.106 & Fig. 3.107

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Fig 3.106 Classification according to work piece rotation

Fig 3.107 Classification according to tool translation/rotation

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The operations and machines that can be used to generate flat surfaces are shown below. Table 3.1

The operations and machines that can be used to generate external cylindrical surfaces are shown below. Table 3.2

The operations and machines that can be used to generate internal cylindrical surfaces are shown below. Table 3.3

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Design considerations for machining: 3.7.1 Design to suit the relative work piece: The shape to be produced by machining must be such that it can be obtained by the work piece and cutting tool movements of the machines that are available. For example the basic movements of a lathe are rotation of the work piece and linear movement of the cutting tool. This movement may be parallel to axis of rotation or right angle to it. Other cutting tool movements can be obtained by setting the machine slides or by using a copying system. Shapes that can be directly produced by the basic movements of the machines are the most convenient and those requiring the copying systems are least convenient. The order of preference regarding shape to suit lathe work is as follows.

Fig 3.108 First Choice of Shape Fig 3.109 Second Choice of lathe The workfirst choice (Fig 3.108) Shapeisfor (i) First for Choice: a lathe shapework that consists of concentric cylindrical surfaces. (D1, D2 and D3) that are right angle to the axis ( R, S and T) with the recesses, chamfers (W, X, Y and Z) etc. that can be produced using form tools and basic movements. Screw Threads are a special case of cylindrical surfaces. (ii) Second Choice: The second choice includes concentric conical surfaces (A and B of Fig 3.109) that are too long to be produced by a form tool because of chatter problems. These require setting of compound slide, the use of taper turning attachment, or the setting over the tail stock, to obtain a cutting tool movement that is inclined to axis.

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Fig 3.110 Third Choice of shape for lathe work

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Fig 3.111 Fourth Choice of shape for lathe work

(iii) Third Choice: Cylindrical Features that are eccentric with respect to the main axis (Fig 3.110) require the setting over of the workpiece so that the axis of the eccentric features coincides with that of the machine (refer dimension E). this requires additional machining operation, or operations, and either individual setting or use of fixtures. The problem of the workpiece being out of balance should also be considered. (iv) Fourth Choice: Surfaces that involve a frequent or continuous change of cutting tool direction usually cannot be obtained by the basic machine movements or by changes in setting. Form tools can be used if the surface is short, or if long contour is broken by recesses to enable short contact form tools to be used without inevitable step between each section of the contour. A long contour shown in Fig 3.111 can only be produced by one cutting tool with its movement controlled by a copying system. Function controls the shape but the problem can be minimized if considered at the initial design stages. 3.7.2 Design to suit location, seating and clamping:

Fig 3.112 Double Cone Component

Fig 3.113 Design for location/clamping

Fig 3.114 Further Improved design

The basic requirements that must be met before a workpiece is machined are • that it is located with respect to the path of the cutting tool (or its own path with respect to the cutting tool)

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that it is seated so that it does not deflect under the cutting forces and that it is clamped with sufficient force to prevent it from moving during machining.

Ideally a location point or points should be established at the first machining operation and be used at all subsequent operations. If a suitable location feature is not part of the basic design of a part it may be necessary to introduce one to assist in machining. Similarly although a part may not have to be seated upon assembly it may be necessary to introduce a suitable feature for seating and clamping at the machining operations. For example Fig 3.112 shows a double cone component with slot S that is to be milled in a later operation. At the milling operation it could be located from one cone (although variation of its diameter will cause axial variation) and clamped from the other. The design shown in Fig 3.113 includes a cylindrical feature that can be used for location and clamping and that in Fig 3.114 includes a cylindrical hole that can be used for location and a flange that can be used for seating and clamping. 3.7.3 Design to suit cutting tool approach: Fig 3.115 shows a part of a component that includes a large conical hole. The hole must be produced by a cutting tool approach as shown, but the component also has a shallow cylindrical recess that obstructs the tool approach. The version shown in Fig 3.116 allows the tool to approach and that shown in Fig 3.117 combine unobstructed tool approach with a shallow recess.

Fig 3.115 Design that obstructs tool

Fig 3.116 Design that eliminate obstruction

Fig 3.117 Further Improved design

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Fig 3.118 Drilling into inclined wall

114

Fig 3.119 Modification to remove inclined effect

Approach requires special attention when a hole is to be drilled in an inclined wall as shown in Fig 3.118. As indicated by ‘x’ and ‘y’ in the fig., the drill will tend to run upon entry and, if of small diameter, be deflected at entry and exit because it will supported at only one side. This problem can be overcome by use of Drilling Jigs. If the hole is simply for the sake of oiling or ventilation, the orientation of hole can be altered as shown in Fig 3.119.

Fig 3.121 Modified form Fig 3.120 Boss to to produce a flat surface produce a flat surface A local flat surface can be produced by introducing a boss as illustrated in Fig 3.120. But this will cause a structural variation because of the local increase in thickness. The design shown in Fig 3.121 eliminates the local area of thickness and the problem of drill deflection at the exit.

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Fig 3.122 Design that Fig 3.124 Design for Fig 3.123 Design with obstructs drill path unobstructed approach alternative approach The nature of drilling operation demands that the approach path of the drill must not be obstructed. Fig 3.122 illustrates a design in which the opening at the top of the casting is too small to allow the drill to enter without drilling the surrounding material, and it is impractical to approach from the other side because of the sloping surface effect discussed earlier. The problem can be solved by either enlarging the opening, as shown in Fig 3.123 or by re-designing the wall as shown in Fig 3.124 to enable the drill to drill from the opposite side.

Fig 3.125 Design with obstructed milling cutter path

Fig 3.126 Design with clear path for milling cutter

Fig 3.125 shows an approach problem often met when surfaces are to be machined by milling. In this example the top surface pf flange A is to be milled, but flang B and C prevent the use of a roller cutter, which is more convenient cutter to use. By raising the flange A or lowering the flange B a clear path for a roller cutter is obtained as shown in Fig 3.126.

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Fig 3.127 Design with a bore that will not accept tool assembly

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Fig 3.128 Design with a bore that will accept tool assembly

When a recess is to be machined using a boring tool or boring bar arrangement, the design must be such that the main bore is large enough to allow the tool assembly to enter and be fed out to produce the recess. Fig 3.127 shows an unsuitable design and Fig 3.128 shows a suitable design. 3.7.4 Design to allow cutting tool over-run: Except in a few instances, it is necessary to allow the cutting tool to over –run. Over-run may be necessary to severe the chip from the workpiece, to provide body clearance for the cutting tool, to obtain better cutting action or to eliminate the need to stop the cutter at a precise point.

Fig 3.129 Over-run recess for slotting operations Fig 3.129 illustrates a blind, slotted hole. The recess at the bottom of the hole ensures that the chips will be severed from the workpiece and also allow the operator some latitude when setting the stroke of the slotting machine used to produced the slot.

Fig 3.130 Design with inadequate drill over-run

Fig 3.131 Design with adequate drill over-run

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Fig 3.130 does not allow adequate over-run. But by a slight modification as shown in Fig 3.131 an adequate over-run is obtained.

Fig 3.132 Design without grinding over-run groove

Fig 3.133 Design with grinding over-run groove

When a shouldered, or multi diameter, cylindrical component is ground using a traverse, a groove should be introduced so that a precise stopping point is not required and so that the best cutting action is obtained. Fig 3.132 shows an unsuitable design and Fig 3.133 shows a design with a over-run groove. Fig 3.134 and 3.135 shows how over run can be introduced in a conical section.

Fig 3.135 Conical Fig 3.134 Conical feature Feature with over-run without over-run 3.7.5 Design to minimize tool deflection:

Fig 3.136 Design to enable boring bar to be supported A cutting tool of cantilever design will deflect during cutting if the over hang is large compared with its depth and this deflection will cause in accuracies. The situation illustrated by Fig 3.127 and 3.128 includes the problem caused by the main bore being too small to permit the use of boring bar that will not deflect. Prepared by Prof.R.Panneer, Assistant Professor.

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The boring of long workpieces causes problem because of the necessity of long boring-bar overhang to reach far enough; horizontal boring machines incorporate means of supporting the boring bar at its far end so that it becomes a beam, thereby reducing the deflection. The support cannot be used when a blind hole is bored and so a cored hole should, if possible, be introduced as shown in Fig 3.136. to permit the support of the boring bar. 3.7.6 Design to cut cost of machining:

Fig 3.137 Hole drilled to a depth

Fig 3.139 Facing in diff. planes

Fig 3.138 Through Hole

Fig 3.140 Facing in same plane

Fig 3.142 Facing parallel to main Fig 3.141 Angular Facing surface Machining costs can be minimized by reducing the setting costs. For example the drilled hole shown in Fig 3.137 almost breaks through and the time taken to drill is the same as making a through hole. Unless a blind hole is necessary it would be better to alter the design as shown in Fig 3.138 eliminating the equipment and time required to set the machine to give the required depth of the hole ‘D’. A change of setting during an operation increase the time taken and may also lead to error. Fig 3.139 shows two facings whose heights differ by a height ‘H’. A change of setting is eliminated by making them same height as shown in Fig 3.140. The need to reposition the workpiece or to split an operation can be eliminated by making features lie in the same plane or at least parallel to each other as illustrated by Fig 3.141 and 3.142.

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Fig 3.143 Design with extensive seating surface

Fig 3.145 Design with large machined surface

Fig 3.147 Design with unrelieved bore

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Fig 3.144 Design with relieved seating surface

Fig 3.146 Design with relieved machined surface

Fig 3.148 Design with bore relieved by coring

Costs can also be reduced by minimizing the surface area to be machined and the amount of metal to be removed from them. Fig 3.143 to Fig 3.148 illustrate three examples of casting design to minimize the surface areas to be machined. Minimizing the area of location surfaces satisfies the principle of location because large areas are more prone to geometric errors during machining or when the part is in service.

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Fig 3.149 Application of spot Facing

Fig 3.150 Application of Boss It is often necessary to machine the workpiece adjacent to a drilled hole to provide a seating surface for a bolt head or other component. This can either be done by localizing the machining (Fig 3.149 spot facing) or by localizing the metal that surround the hole (Fig 3.150 use of Boss). 3.8 DESIGN FOR ASSEMBLY: The simple design considerations based on assembly are already discussed vide section 3.2.5.

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