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Postby subham » Fri Oct 12, 2012 11:28 am

For the past 20 years engineers have seen a large amount of effort devoted to the integration of design and manufacture, with the goals of reducing manufacturing cost and improving product quality. The processes and procedures that have been developed have become known as design for manufacture or design for manufacturability (DFM). Associated with this is the closely related area of design for assembly (DFA). The field is often simply described by the abbreviation DFM/DFA or DFMA. DFMA methods should be applied during the embodiment stage of design.
Design for manufacture represents an awareness of the importance of design as the time for thoughtful consideration of all steps of production. To achieve the goals of DFM requires a concurrent engineering team approach in which appropriate representatives from manufacturing, including outside suppliers, are members of the design team from the start.

DFM Guidelines :

DFM guidelines are statements of good design practice that have been empirically derived from years of experience. Using these guidelines helps narrow the range of possibilities so that the mass of detail that must be considered is within the capability of the designer.

1. Minimize total number of parts: Eliminating parts results in great savings. A part that is eliminated costs nothing to make, assemble, move, store, clean, inspect, rework, or service. A part is a good candidate for elimination if there is no need for relative motion, no need for subsequent adjustment between parts, and no need for materials to be different. However, part reduction should not go so far that it adds cost because the remaining parts become too heavy or complex.
The best way to eliminate parts is to make minimum part count a requirement of the design at the conceptual stage of design. Combining two or more parts into an integral design architecture is another approach. Plastic parts are particularly well suited for integral design. 20 Fasteners are often prime targets for part reduction.
Another advantage of making parts from plastics is the opportunity to use snap-fi ts instead of screws.

2. Standardize components: Costs are minimized and quality is enhanced when standard commercially available components are used in design. The benefits also occur when a company standardizes on a minimum number of part designs (sizes, materials, processes) that are produced internally in its factories. The life and reliability of standard components may have already been established, so cost reduction comes through quantity discounts, elimination of design effort, avoidance of equipment and tooling costs, and better inventory control.

3. Use common parts across product lines: It is good business sense to use parts in more than one product. Specify the same materials, parts, and subassemblies in each product as much as possible. This provides economies of scale that drive down unit cost and simplify operator training and process control. Product data management (PDM) systems can be used to facilitate retrieval of similar designs.


4. Standardize design features. Standardizing on design features like drilled hole sizes, screw thread types, and bend radii minimizes the number of tools that must be maintained in the tool room. This reduces manufacturing cost.

5. Aim to keep designs functional and simple: Achieving functionality is paramount, but don’t specify more performance than is needed. It is not good engineering to specify a heat-treated alloy steel when a plain carbon steel will achieve the performance with a little bit more careful analysis. When adding features to the design of a component, have a firm reason for the need. The product with the fewest parts, the least intricate shapes, the fewer precision adjustments, and the lowest number of manufacturing steps will be the least costly to manufacture. Also, the simplest design will usually be the most reliable and the easiest to maintain.

6. Design parts to be multifunctional: A good way to minimize part count is to design such that parts can fulfill more than one function, leading to integral architecture. For example, a part might serve as both a structural member and a spring, Fig. 13.14 b . The part might be designed to provide a guiding, aligning, or self-fix turing feature in assembly. This rule can cancel out guideline 5 and break guideline 7 if it is carried too far.

7. Design parts for ease of fabrication: As discussed in Chap. 11, the least costly material that satisfies the functional requirements should be chosen. It is often the case that materials with higher strength have poorer workability or fabricability. Thus, one pays more for a higher-strength material, and it also costs more to process it into the required shape. Since machining to shape tends to be costly, manufacturing processes that produce the part to near net shape are preferred whenever possible so as to eliminate or minimize machining. It is important to be able to visualize the steps that a machine operator will use to make a part so that you can minimize the manufacturing operations needed to make the part. For example, clamping a part before machining is a time consuming activity, so design to minimize the number of times the operator will be required to reorient the part in the machine to complete the machining task. Re-clamping also is a major source of geometric errors. Consider the needs for the use of fixtures and provide large solid mounting surfaces and parallel clamping surfaces. Rough evaluations for how easily specific materials can be processed by different manufacturing methods are given in Fig. 13.17.

8. Avoid excessively tight tolerances: Tolerances must be set with great care. Specifying tolerances that are tighter than needed results in increased cost; recall Fig. 13.13. These come about from the need for secondary finishing operations like grinding, honing, and lapping, from the cost of building extra precision into the tooling, from longer operating cycles because the operator is taking finer cuts, and from the need for more skilled workers. Before selecting a manufacturing process, be sure that it is capable of producing the needed tolerance and surface finish. As a designer, it is important to maintain your credibility with manufacturing concerning tolerances. If in doubt that a tolerance can be achieved in production, always communicate with manufacturing experts. Never give a verbal agreement to manufacturing that they can loosen a tolerance without documentation and making the change on the part drawing. Also, be careful about how the statement for blanket tolerances on the drawing is worded and might be misinterpreted by manufacturing.

9. Minimize secondary and finishing operations: Minimize secondary operations such as heat treatment, machining, and joining and avoid finishing operations such as deburring, painting, plating, and polishing. Use only when there is a functional reason for doing so. Machine a surface only when the functionality requires it or if it is needed for aesthetic purposes.

10. Utilize the special characteristics of processes: Be alert to the special design features that many processes provide. For example, molded polymers can be provided with “built-in” color, as opposed to metals that need to be painted or plated. Aluminum extrusions can be made in intricate cross sections that can then be cut to short lengths to provide parts. Powder-metal parts can be made with controlled porosity that provides self-lubricating bearings. These rules are becoming the norm in every engineering design course and in engineering practice.
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Postby nainauppalapati » Tue Jan 22, 2013 3:38 pm

Material Science and Engineering1.pdf
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