Machine Design

Design For Assembly

Design-for-assembly (DFA) and design-for-manufacture (DFM) techniques can be applied to products assembled manually or automatically or manufactured by specific techniques, such as machining, die casting or injection molding. Research at the University of Massachusetts, University of Rhode Island, and by industry has been a source of information, spreadsheets, and software for those interested in DFA and DFM techniques.

Design-for-assembly analysis will suggest the optimal assembly system and degree of automation for many applications. The design is analyzed for its overall efficiency and suitability for the chosen assembly method. For example, three steps can be used to determine if a product is suitable for automatic assembly:

  • Estimate the cost of handling the part automatically in bulk and delivering it in the correct orientation for insertion on an automatic-assembly machine.
  • Estimate the cost of inserting the part into the assembly automatically, and any extra operations.
  • Decide if the part must be separate from all others in the assembly.

Design for manufacture can be analyzed alongside design for assembly. DFM analysis helps compare materials and manufacturing processes for the component parts of the assembly. It also determines how the most efficient use of selected materials and processes impacts the component design cost.

DFM and especially DFA techniques can easily be used with automated assembly because of their similar goals. Used properly, automated assembly, DFA, and DFM lower a product's cost and increase its reliability. To achieve these goals, designs should have fewer, simpler parts, and such designs often lend themselves to automated assembly. Moreover, DFA suggests when orienting and insertion features need to be designed into the component.

The cost of assembling a product is typically proportional to its number of parts. Such parts as fasteners, clips, and washers may be small in size, but they often account for the majority of assembly cost. Small parts particularly influence the cost of automatic assembly, because each part requires a feeding and orienting device, a workhead, at least one extra work carrier, a transfer device, and an increase in size of the basic machine structure.

Three criteria, established by Geoffrey Boothroyd and Peter Dewhurst, have become standards for judging whether a part is necessary:

  • Does the part move with respect to other parts already assembled?
  • Must the part be made of a different material or be isolated from all other parts already assembled? (Only fundamental reasons concerned with material properties may be considered here.)
  • Must the part be separate from all other parts already assembled because necessary assembly or disassembly would otherwise be impossible?

Decisions on the feasibility of a design should take both DFA and DFM analyses into account. Reducing the number of parts may simplify the total assembly. However, combining several functions in one component may result in a complicated part that is prohibitively expensive to manufacture. In addition, such complicated parts might be too difficult for automated systems to handle. Balancing the results of DFA and DFM analyses can reduce both the total number and total complexity of parts, resulting in efficient product designs.

TAGS: Fasteners
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