Tolerance: Why Can Allowing Variation Make a Fit More Reliable?

Tolerance is a specified range of acceptable variation: a dimension, time or performance measure may vary within that range while the required function still holds. It does not mean that “close enough” is good enough; it makes the boundary of acceptability explicit.

Consider a bolt that must pass through a metal hole. If a drawing says only that both must be exactly ten millimetres, ordinary manufacturing variation may make the bolt jam. If each is given a suitable range, parts from different batches can still fit. A tolerance that is too narrow raises manufacturing and inspection costs; one that is too wide may allow looseness. The right range therefore depends on the function.

Tolerance differs from a safety margin, which keeps normal operation away from a failure boundary. It also differs from robustness: tolerance specifies an allowed interval, while robustness describes how well a system keeps functioning under disturbance. Reliable design does not pretend variation can disappear. It places variation within limits that can be checked.