USE OF A QUALITY LOSS FUNCTION TO SELECT STATISTICAL TOLERANCES

In this paper, we present a method for the selection of processes to m anufacture various parts of an assembly by establishing a compromise b etween product quality and part manufacturing cost. We quantify the im pact the precision of a part characteristic has on the overall quality of a product by using a standard Taguchi loss function. Part manufact uring cost is modeled as a function of process precision (i.e., standa rd deviation of the output characteristic) as opposed to previous mode ls where manufacturing cost is a function of part tolerance. This appr oach is more realistic and does not assume, a priori, a relationship b etween conventional tolerance and process spread. Rather than allocati ng conventional tolerances on the assembly parts, we use statistical t olerances that are more pertinent when using a quality loss function. The model adopted makes it possible to investigate the relationship be tween optimum quality loss and tolerance variations. As expected the o ptimum quality loss generally decreases when the tolerance increases. Exceptions may be encountered when changes of process occur. The manuf acture of a simple three component assembly is studied to illustrate t he findings.