Insertion Guidelines
After being handled, the part must be inserted into the partially assembled product. Guidelines can be developed for this process in particular as shown in Figure 14.5.
The first guideline suggests adding chamfers to make parts easier to insert. One should design in allowances on all fits, so that if there is variation from part to part, it does not prevent assembly.
The second guideline suggests providing alignment features on the assembly so new parts are easily oriented without measurement. One should do this using a kinematic attachment scheme, such as the 3- 2-1 alignment process as discussed in Chapter 12. Here, first we provide "3" points on the assembly that a new part is placed against. The part is slid along the three points up against "2" more points that are in a perpendicular plane on the assembly. Then the part is slid along the five points up against a final sixth ("1") point, thereby kinematically constraining the new part into the assembly in a predictable way. Also, the geometry defining these six points is candidate geometry for tighter tolerance control compared to other points on the part and assembly.
The third guideline basically suggests that we not fight gravity when placing and maintaining parts for fastening. Make the first part large and wide to be stable and then assemble smaller parts on top of it sequentially. Having to grasp and hold parts from below or from the side while they are being fastened is poor design.
The fourth guideline refines this to suggest that if we cannot assemble parts from the top down exclusively, then apply as few insertion directions as possible. Assemble only from the top and have fasteners come in from only one side, for example. This eliminates reorientation of the product during assembly. One should also consider the assembly sequence and ensure that all initial assembly work is completed on one surface, and then only one reorientation is required to finish the assembly work on the other surface. Do not make the assembly system constantly reorient the product. The worst case is when the subassembly requires turning over to continue assembling parts. It is difficult to keep parts precisely located when they are partially fastened and turned over.
Joining Guidelines
After a part is inserted onto the assembly, it must be firmly attached through some joining process. This can be with fasteners, snap fits, welds, or adhesives. Again, guidelines can be developed to simplify this aspect of the assembly process as shown in Figure 14.6.
The first guideline suggests reducing the number of fasteners. This does not mean to reduce the factor of safety by reducing the attachment strength but rather to change a portion of the fasteners to be of a quick insert type, such as one side of a part being held down by a tongue-in-slot joint on the assembly.
The second guideline suggests that it is better to locate fasteners in places where one has access to the fastener. The third guideline is similar with respect to enclosed spaces.
The fourth guideline suggests designing the assembly of parts per the instructions of the fastener. Typically this means to not fasten against angled surfaces. The fifth guideline is similar: One should space fasteners sufficiently to permit socket or rivet tool heads access to the fastener.
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.