Variation and controlling variability.

Why do We Need Tolerances?

Specs and Work QuotasIf it was possible to make all items alike, we would not need tolerances. But no two things are exactly alike. So, we must make allowances for the differences. Tolerances are these allowances. They provide a range over which a product can vary and still be accepted by the customer.

Of course, “accepted” is not “desired,” and in the battle over width of tolerances the idea of a specific target has been lost. For example, ask almost any machinist what size a part is supposed to be, and you will get a range as the answer — “from 2.048 to 2.052 inches.” Why isn’t the answer “2.050 inches” if that is the target? Because with tolerances, we tend to aim for the acceptable range instead of the exact target. This means we are less likely to hit the target. It also means our variance will most likely be greater than if we aimed for the target.

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That leaves us with a bit of a dilemma.

Aim for Perfect, Tolerate (Some) Variance

Variance is a natural occurrence, but that doesn’t mean we shouldn’t try to control it. If we aim for a specific target value, instead of a range, we have more control of the variance. In archery, if you shoot for a bull’s eye, your arrow is likely to land near it, but if you broaden your aim to encompass the entire target, your arrow could land anywhere.

The same is true for manufacturing. A perfect product is one that is on target, so aiming for the target will improve product quality. Specifying a target value and controlling the variability makes more sense than focusing on tolerances.

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This content is adapted from Zontec’s The Book of Statistical Process Control.
You can download a free copy here.