Predicting Assembly Quality (Six Sigma
Methodologies to Optimize Tolerances)
Dale Van Wyk
Raytheon Systems Company
Mr. Van Wyk has more than 14 years of experience with mechanical tolerance analysis and mechanical
design at Texas Instruments’ Defense Group, which became part of Raytheon Systems Company. In addition
to direct design work, he has developed courses for mechanical tolerancing and application of statistical
principles to systems design. He has also participated in development of a U.S. Air Force training class,
teaching techniques to use statistics in creating affordable products. He has written several papers and
delivered numerous presentations about the use of statistical techniques for mechanical tolerancing. Mr.
Van Wyk has a BSME from Iowa State University and a MSME from Southern Methodist University.
We introduced the traditional approaches to tolerance analysis in Chapter 9. At that time, we noted
several assumptions and limitations that (perhaps not obvious to you) are particularly important in the
root sum of squares and modified root sum of squares techniques. These assumptions and limitations
introduce some risk that defects will occur during the assembly process. The problem: There is no way to
understand the magnitude of this risk or to estimate the number of defects that will occur. For example, if
you change a tolerance from .010 to .005, the RSS Model would assume that a different process with a
higher precision would be used to manufacture it. This is not necessarily true.
What Is Tolerance Allocation?
In this chapter, we will introduce and demonstrate methods of tolerance allocation. Fig. 11-1 shows how
tolerance allocation differs from tolerance analysis. Tolerance analysis is a process where we assign
11-2 Chapter Eleven
tolerances to each component and determine how well we meet a goal or requirement. If we don’t meet the
goal, we reassign or resize the tolerances until the goal is met. It is by nature an iterative proc