When the Tolerance Is Tight

Many drawings carry one tight general tolerance in the title block, applied to every dimension on the sheet. It feels careful. In practice it makes every feature as costly as the hardest one, and it hides which features actually matter.

A tight tolerance belongs on the few features that do a job: the bore a bearing presses into, the faces that locate against another part, the hole pattern that has to line up with a mating part. Everything else can carry the general tolerance a shop holds without special effort. The part is no worse for it, and the effort goes where it counts.

Between 2020 and 2025 the shop machined parts for rugged electronics, where tight work is routine. The shop has taken contracts with tolerances down to fractions of a thousandth of an inch. Its measuring tools include calipers, micrometers and feeler gauges.

Which dimensions need it

Ask two questions of each tight dimension. What does this feature touch? What happens if it lands at the edge of its tolerance band? If the answer to the second is "nothing much", the tolerance can open up.

Watch the chains too. When dimensions are stacked one off the next, their tolerances add, and the last feature in the chain can wander further than any single number suggests. Dimensioning the critical features from the datum that matters, rather than from each other, keeps the relationship you care about under control. We work to GD&T callouts and help clients with tolerance stack-ups.

Heat moves the part

Metal grows as it warms. A 100 mm aluminum part grows by about 0.002 mm for every degree Celsius, and steel grows by about half that. A part measured warm off the machine and again the next morning in a cold shipping bay can read differently on a feature that never moved. That is why tight tolerances are measured at a controlled temperature, conventionally 20 degrees Celsius, and why the same tolerance is harder to hold in aluminum than in steel, and harder again in most plastics.

Rough, settle, then finish

Cutting releases stress locked into the material, so when a lot of metal comes off, the part moves. The usual practice is to rough the part, let it settle, then finish it, and to cut related critical features in a single setup, because every time a part is unclamped and clamped again, the relationship between features picks up a little error. The shop roughs first and then finishes the critical features, where that makes sense.

Surface finish is a separate number

Surface roughness, written as Ra, is its own requirement. A drawing that calls out Ra on the sealing face and leaves the rest at the default tells the shop where the effort goes. A finish that is easy on an open flat face can be much harder in a deep pocket, so say which faces matter.

On quality-sensitive work, parts ship with a certificate of compliance.

Where a general machine shop stops

The very tightest work belongs to grinding and lapping, usually in a temperature-controlled room. Not every machine shop is set up for it, and a buyer should ask any shop directly whether it grinds, and where. We have a surface grinder in-house, and send specialist grinding or lapping out.

If your drawing carries one tight tolerance on everything, mark the features that really need it before you send it out. The quote will be better, and the part will be right where it matters.

Related services: Aluminum tooling, molding and machined parts Shop capabilities