23 Assembly capability

When considering assembly processes, many problems are due to relatively minor design details. It must be remembered that a product is often an assembly of sheet metal, pipes, wires and a variety of other components held together by nuts, bolts and rivets. The attention to detail cannot be overemphasized at this stage of product design (Jones, 1978). The basic question we need to ask is, 'Can we assemble it every time, not just the once, and if not, why?'

Assembly variation (along with manufacturing variation) is a major contributor to poor quality and increased costs. For example, when the assembly of a poorly designed and poorly made product is attempted, faults such as accumulated tolerance error, incompatible dimensions and difficult part installation become apparent. At the same time, the cost of recovering from these problems during the late phase of production is high (Kroll, 1993). Industry has recognized the need to reduce assembly variations (Craig, 1992).

The three main sources of variation in mechanical assemblies are (Chase et al., 1997):

• Dimensional variations (lengths, angles)

• Form and feature (flatness, roundness, angularity)

• Kinematic variations (small adjustments between mating parts).

The above are all closely linked to manufacturing variability depending on the characteristic associated with the product. Manufacturing is an important aspect of assembly too. The design of products for assembly requires careful consideration of many factors that influence the functionality and manufacturability. For example, while stability and relative precision of part positions are often essential for the functional performance of assemblies, these same requirements may make the product difficult to manufacture. At the same time, dimensional clearance among parts is essential to create paths for assembly operations (Sanderson, 1997). In general, close (or numerically small) tolerances must be maintained on parts that are to be assembled with other parts, and the closer the tolerances, the greater the ease of assembly (Kutz, 1986).

'Assemblability' is a measure of how easy or difficult it is to assemble a product. Tolerances affect the assemblability of a product, which in turn affects the cost of the product because of the scrap cost, and wasted time and energy. It is important to predict the probability of successful assembly of the parts so that the tolerance specifications can be re-evaluated and modified if necessary to increase the probability of success and lower the production cost associated with assembly (Lee et al., 1997). While close tolerances reduce assembly costs, they increase the cost of manufacturing the individual parts. A balance of the two types of costs must be achieved, with the objective of minimizing their sum (Kutz, 1986).

When tolerances are mentioned in terms of mechanical assemblies, one considers tolerance or assembly stack models used to determine the final assembly tolerance capability for a given set of component tolerances. Worst case or statistical equations are commonly used for optimization purposes, as discussed in detail in Chapter 3. However, this can be confusing, as a tolerance can exist on a component characteristic, but the component itself may not necessarily be capable of being assembled to form the assembly tolerance. This may be due to one or more features of the components being assembled preventing effective and repeatable placement. If this occurs then the product suffers not only from an assembly problem, but also a tolerance problem.

CA provides assembly capability predictions which can highlight potential assembly problems. We must also have a means of estimating the failure costs associated with component assembly processes in addition to the tolerance process capability predictions. These additional failure costs are independent of whether the tolerances assigned to an assembly stack or the single characteristic are capable. By identifying components with high assembly risks and potentially high failure costs, further design effort is highlighted and performed in order to identify the associated tolerances, for example clearance for the optimal fit and function of the components.

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