Dfma

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Design For Manufacturing/Assembly (DFM, DFA, DFMA) Much of the early and significant work on DFM and DFA was done in the early 1970s by Boothroyd and Dewhurst. Traditionally, product development was essentially done in several stages. The designer(s) (who usually had very good knowledge of materials, mechanisms, etc.) would design the product, and sometimes would construct working prototypes. Once the prototype was tested and approved, the manufacturing team would then construct manufacturing plans for the product, including the tooling etc. Often, different materials (e.g. different thickness or type of sheet metal), and different components (e.g. different sized screws etc), would be substituted by the manufacturing team. Their goal was to achieve the same functionality, but make mass production more efficient. However, the majority of the design remained unchanged, since the manufacturing engineers could never be sure whether a change would affect some functional requirement. Two things changed in the 1970’s: (i) Many new types of plastics were developed, and injection moulding technology became widely available, resulting in the possibility of low cost plastic components. An advantage of these new plastic materials was that they provided different material behaviour (e.g. many cycles of large elastic deformations without failure – a property useful in making snap-fit mating components). Thus pats that had to be made form metal and screwed together could just be made out of plastic and snap fitted. This reduced assembly time, assembly components, and production costs. (ii) Several companies were trying to bring their products to the market faster. One problem with the earlier method of doing things was that each time there was a design change made by the manufacturing engineer, product development was held up, waiting for the engineering change notice (ECN) to be approved by the designer. Often, this process introduced delays because the design engineer would be busy with other tasks, or unavailable. To avoid this, the concept of concurrent engineering (CE) became popular. The idea here was that a combined team of engineers and management would be assigned to each new product. This team may consist of mechanical designers, electrical engineers, software engineers, production engineers, marketing and sales, and management. Thus, as the design was being generated by the designers, the production people would give feedback about feasibility to manufacture, more economical alternatives etc. At the same time, sales people would negotiate of product outlook and features, and so on. The biggest advantage of CE was that the

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product was designed in a way that manufacturing cost and time would be low during production. At this time, working with many different companies, Boothroyd’s team analysed existing designs of hundreds of products, and suggested design improvements based on manufacturing and assembly ease. Using the experience of these projects, they then developed a very large set of guidelines on how to estimate whether a design was designed well (from a manufacturing point of view), and potential methods to improve the designs. In subsequent years, the idea of DFM and DFA were extended to include other aspects of better designs – including design for maintenance, design for environment, design for cost, etc. Often, this application of CE is referred to as DFX, where X is a variable selected from the set {manufacture, assembly, …}. We shall restrict our study of manufacturing decisions to those related to fabrication and assembly. Manufacturing Assembly Operations

Fabrication Operations

Material

Geometry

Tolerance

Figure 1. Effects of design (materials, geometry, tolerances) on manufacturing Effects of Tolerance on Fabrication Tight tolerances will require More care to be taken during machining Î larger processing time Higher reject rates More expensive measurement/inspection systems Restricted set...
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