Theoretical Minimum Number of Parts

The thought behind the first two guidelines in Table 14.1-to modularize multiple parts into a single part-is so important it requires special emphasis. There is no better way to simplify an assembly step than to eliminate it. Fundamentally stated, the most effective DFA guideline is to "Simplify your design by eliminating all unnecessary separate parts: This idea goes all the way back to Henry Maudsley, the father of precision engineering, who said, "Eliminate all material not needed. …put to yourself the question, 'What business has it to be there?' Make everything as simple as possible:' Kelly Johnson, Chief engineer of Lockheed's famed Skunk Works development site, whose products include the F-80 Shooting Star fighter aircraft, the U-2 spy plane, the SR-71 Blackbird spy plane, and the F-117 A Nighthawk stealth fighter, was the first to express this succinctly as "KISS-Keep It Simple Stupid" (Garmon 1999). The question then becomes whether a part needs to be a separate part or whether it can be modularized into other parts. Chapter 9 provides means to combine parts and functions into modules.

Assembly modules (Chapter 9) can always be defined; the module is simply a subassembly. The real question is how to define assembly modules, how to determine whether it is possible to combine parts into a larger, more-complex part. At a basic level, some tests of neighboring parts can be applied. These include:

• Must the parts move relative to one another?
• Must the parts be electrically isolated?
• Must the parts be thermally isolated?
• Must the parts be of different materials?
• Does combining the parts prevent assembly of other parts?
• Will servicing be adversely affected?

If one can make the answer to these questions "No," then one should find a way to combine the two parts.

The part simplification concept may seem obvious, but at first it was not. The first to analyze it in detail and highlight its importance were Boothroyd, who argued against other design guidelines in the literature that suggested using more parts that are individually simpler to fabricate. Whereas others such as Iredale and Tipping had previously noted the importance of part reduction as a design heuristic, Boothroyd developed a systematic methodology and comparison standard. Generally speaking, it is better to make more complex parts that are individually more expensive but with that added individual part cost more than made up for in less assembly cost and also typically less total part cost due to the less number of parts. Every part requires documentation, control, and inventory.

These thoughts lead to the concept of the theoretical minimum number of parts for a product, originally proposed by Boothroyd. During assembly of the product, generally a part is required only when a kinematic motion of the part is required, when a different material is required, or when assembly of other required parts would otherwise be prevented. If none of these statements are true for a part in question, then the part is not needed to be a separate entity and design-for-assembly suggests combining it with another part.

Date: 2016-03-03; view: 1706