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.

Date: 2016-03-03; view: 944