TABLE OF CONTENTS Customer: Why are these parts too big?
Supplier: It s hotter here than where you are, so we compensated.
Up to this point, the emphasis in this book has been on the nominal design of an assembly. By this we mean the creation of a design for an assembly that will put the parts in certain positions and orientations with respect to each other perfectly, achieving each KC perfectly and ignoring any errors in fabrication of individual parts or errors in assembling them. In this chapter and the next, we consider such errors for the first time.
The word we use when referring to these errors is variation.Variation is a physical result of manufacturing processes: Parts and assemblies that are supposed to be identical actually differ from each other and from what we want them to be. Another important word is tolerance. Tolerance refers to the amount of variation that we can tolerate in a part or assembly. A third word must be defined for completeness because it is so often confused with tolerance. That word is clearance. Clearance is empty space between two parts. One can place a tolerance on a clearance, and clearances can have variation.
Errors in parts and assemblies are inevitable. The actual value of each KC will therefore deviate from the desired value. Variation analysis seeks to ensure that the deviations are acceptable that is, that each KC lies within the desired range on all, or nearly all, actual assemblies. Two...
