Design for Piece Part Production

It is impossible to provide guidelines for every production process; new refined processes are constantly under development. However, Figure 14.7 provides guidelines for injection-molded parts (Xerox, 1(84), Figure 14.8 provides guidelines for sheet metal parts (Dayton Rogers), Figure 14.8 provides guidelines for cast parts adapted from Bradney, and Figure 14.9 provides guidelines for machined parts. Malloy, Wakil, Rosato and Rosato, Bradney, and Boothroyd and Knight provide additional reading. Each should be considered when designing a part requiring these processes.

http://www.youtube.com/watch?v=PHWoUx7hySQ

http://www.youtube.com/watch?v=nPIvLxFwiqY&feature=related

Example: Krups Toastronic Toaster

http://www.youtube.com/watch?v=b62OR9nOjjA

As an example of applying these design guidelines, a product design can be analyzed in terms of Figures 14.3-14.10 and suggestions for simplification made. There may be reasons for not adopting anyone of the suggestions, such as aesthetic considerations, but nonetheless they should be considered. Generally, failure to adopt the guidelines results in higher costs and/or reduced part quality.

An exploded view of the Krups toaster is shown in Figure 14.11. Generally speaking, each part in the toaster is designed well, adequate draft angles have been applied, uniform material thickness and adequate corner radii are all used. When considering each guideline, however, there are redesign suggestions that can be made.

For example, the toaster has over 25 separate parts, complicating its assembly. A theoretical minimum of six parts are needed: one as the housing, one for the thermally isolated heating element, one to connect these two, one for the toast pop-up-and-down motion, one for the knob adjustment motion, and one as the electrically isolated circuit board.

Examining further, the sheet metal housing is made of seven parts. It could be made with just one part as shown in Figure 14.12. Another example is that the toaster is held together with four screw fasteners that screw through the base up into internal standoffs that are a part of the outer shell. This requires four internal cams to be designed into the mold, so that the part can be removed from the mold. Each additional cam adds complexity, and internal cams are particularly difficult. The design could be simplified by eliminating the standoffs. Using snap fits, for example, could replace the entire screw fastening system, including the standoffs and screws as shown in Figure 14.12. The snap fits use external cams, simplifying the part production, and also assemble the toaster without fasteners, simplifying the assembly.

As with many design decisions, there are trade-offs with these suggestions. The snap-fit solution can impact the appearance, for example. Nonetheless, it is always worth examining a design concept for ways to decrease manufacturing complexity. This is especially true for assembly complexity-one should always first determine the theoretical minimum number of parts and then question every part beyond this minimum and how the design could be simplified to this minimum.

Potential Conflicts between DFA and DFM

Application of DFA and DFM guidelines can reduce manufacturing costs and increase quality. However, the guidelines do not necessarily always do this. For example, one can increase part functionality to the point where the parts become so complex they cannot be easily produced. For example, in Figure 14.13, the part shown was used in the Kodak single-use camera and was very complex, supplying three separate lens functions for aiming light, also a switch actuator, and also snap fits for attachment functions. This part was sufficiently complex that in the next generation of Kodak single-use camera, the part was separated into two parts: one providing the lens function and a separate part providing the switch actuation function. This violates the DFA guideline of minimizing parts but was nonetheless a more cost-effective design, since the original part was sufficiently complex that it was excessively difficult to produce reliably.

Typically, this situation does not occur; it is almost always better to simplify a design by eliminating parts. To address such trade-off decisions, we must explore actual concept costing methods. Then, numerical comparisons can be made between designs with more complex assembly versus designs with more complex piece parts.