What a tolerance actually promises

The difference between a nominal value and the range a part may occupy.

A tolerance promises that a part will be accepted as correct as long as its actual measured size falls anywhere within a stated range around the target value, rather than promising that the part will measure exactly that target value, because no process can hit an exact number every single time and a drawing that demanded exactness would reject good parts for no real reason.

A single number is only a target

A dimension written as a single number, twenty millimetres, states a target, but no manufacturing process, however good, produces that exact number every single time, since every real process has some variation built into it. Even a hand-cut piece of wood, sawn and sanded with real care, will measure a fraction of a millimetre one way or the other from what was aimed for, and a tolerance turns that single target into a workable instruction by stating how far above and below it the actual part is still allowed to measure and still count as correct. A hole dimensioned as twenty millimetres with a tolerance of two tenths either way is accepted at nineteen point eight, at twenty point two, and at every value between them, and rejected only once it strays outside that band. The whole band is four tenths of a millimetre wide, about the thickness of four sheets of printer paper stacked together, so two parts can both pass inspection and still differ from each other by that much.

Why a car anywhere inside the lines is parked

A car parked between two painted lines does not need its wheels at the exact middle of the bay to count as parked correctly. Any position with all four wheels comfortably inside the lines is equally acceptable, and a driver who stops dead centre has done no better, in any way that matters, than one who stopped a little to one side. The moment a wheel crosses a line, though, the car is parked badly, even if it missed by a hair's width. A toleranced dimension works the same way: every value inside the stated band is equally correct, and a part is rejected only for actually crossing a limit. Nobody inspecting a parked car measures the clearance each wheel has to the line and marks down the driver who left less room, and nobody inspecting a toleranced part should mark it down for measuring closer to the edge of the band than another part did.

This is the part of a tolerance people most often misread. A part measuring right at the edge of its band, a hair's width from being rejected, is every bit as acceptable as one measuring dead on the target, because the drawing states a band of equally valid outcomes with no best answer inside it. Worrying over a part that landed near the edge treats a perfectly good result as a marginal one, a distinction the drawing itself never drew.

Loose where it costs nothing, tight where parts meet

Because every value within the tolerance is equally acceptable, a workshop is free to aim anywhere inside that band that suits its process best. A designer who specifies a tolerance far tighter than the part needs gets the same part at a higher price, since the extra precision takes slower cutting, more careful setting up and more inspection, and buys nothing the part will ever use. A tolerance that matches what the part requires, wide where slight variation is harmless and tight only where two parts must fit together, keeps a design both correct and affordable, while a drawing toleranced equally tightly everywhere usually means nobody stopped to ask which numbers mattered. Asking, for each dimension in turn, what goes wrong if the part lands at the very edge of a looser band is usually enough to sort the few critical measurements from the much larger number that were only ever toleranced tightly out of habit.