Tolerances, the Heart of Quality

When a box of parts hits the QA inspector’s bench, the drawing is referenced and the first thing looked at is the tolerances. Often, the design engineer just uses "boiler plate" tolerances overlooking this important part of the design. The most common error is that the tolerances will far exceed what is required, adding time and even forcing extra steps in the machining processes. These parts aren’t necessarily better, just more expensive. Remember, tolerances that are too tight don’t show up in the prototyping process.

What does show up in the prototyping process is if the part functions as designed. If a part fails to function as designed, then the first thing that should be looked at is the tolerances. Often, the expense of the tighter tolerance is money well spent, but just as often the extra expense of an adjustment fixture, such as sliding brackets, is less expensive. Such expense must be weighed in the manufacturing world. Parts that are too expensive are out of tolerance. Cost is critical in all production processes, but a part that is too expensive will not fail on the QA inspector’s bench.

Every engineer should take a close look at the tolerances of each part. Just doing a cut and paste on a drawing does not work. Machinists should be alert for parts that require a tighter tolerance than necessary. QA inspectors should alert for things like high rejection rates or malfunctioning parts on the production floor. Assembly personnel should also be alert making sure they don’t have to “fit” the parts. In all these cases the engineer should be informed and the engineer should respond. Quality is a team effort and tolerances are the heart of quality.