Tolerance Is The Number Behind Every Other Number

Specifications are written as single values and manufactured as ranges. The permitted variation around a figure is what determines whether two parts fit, whether a rating is reliable, and how much the item cost to make.

Nothing is made to an exact dimension

Every process leaves variation, so a drawing specifies a nominal value together with the amount by which a part may deviate and still be acceptable.

Tighter tolerances require better machines, slower operations and more inspection, and the cost rises steeply rather than proportionally as the band narrows.

This is the underlying reason two apparently identical components can differ substantially in price, since the difference is in the variation rather than the material.

Fit is a relationship between two tolerances

Whether a shaft turns freely in a hole, or is held immovably in it, depends on the combined ranges of both parts rather than on either specification alone.

A clearance fit guarantees a gap even in the worst combination, while an interference fit guarantees contact, and both require the two tolerances to be chosen together.

Replacing a component with one made to a different tolerance can therefore produce a joint that is either loose or impossible to assemble, even at the correct nominal size.

Tolerances stack along an assembly

Where several parts are placed end to end, their individual variations add together, and the total possible error at the end is much larger than any single tolerance.

Designers manage this by identifying which dimensions matter to function and tightening only those, leaving the rest generous to keep costs down.

The consequence for repair is that fitting several replacement parts at once can produce a stack that does not work, even though each part is individually within specification.

Ratings have tolerances too

Electrical and mechanical ratings are also produced with variation, so a component sold at a nominal value may legitimately be some percentage above or below it.

Where the tolerance is wide, designers must ensure the circuit or mechanism works across the whole range, which constrains the design more than the nominal figure suggests.

A narrower tolerance part is more expensive for the same nominal rating, and substituting a wide-tolerance equivalent in a precision position is a common source of inconsistent behaviour.

Unstated tolerance is itself information

A specification given as a bare number with no stated variation is either governed by a standard that defines the default, or it is not a controlled figure at all.

Consumer specifications frequently fall into the second category, where the number is a typical value from a sample rather than a limit every unit meets.

The distinction between typical and guaranteed figures is one of the more useful things to look for, because it separates a description from a commitment.