Understanding CNC Tolerances

A tolerance of ±0.005mm means the finished feature has to fall within a five-micron band either side of its nominal size, and that it has to be measured and proven to do so. It is a promise about the real, physical part, not just a number on a drawing.

Holding that band reliably, part after part, is a different thing from hitting it once. That distinction is where precision machining lives.

Tolerance is a cost dial. Standard work around ±0.025mm is comfortable for most machines. Precision work at ±0.005mm tightens every variable in the process. Ultra-precision at ±0.002mm, with surface finishes down to Ra 0.1µm, demands climate-controlled cells, careful tool selection and in-process measurement.

Each step down that curve adds process control, and process control adds time and cost. Asking for the tightest band everywhere is one of the most expensive things a drawing can do.

Precision is worth paying for on the features that do real work: mating faces, sealing surfaces, bearing bores, press fits and the datums everything else references. Get those right and the part performs.

Everywhere else, clearance holes, cosmetic faces, non-critical edges, an open tolerance is not a compromise, it is good engineering. It lets the shop concentrate effort and cost where the function actually depends on it.

Precision comes from machine rigidity, thermal stability, the right tooling and measurement built into the process. In-cycle probing catches error before it is cut; CMM inspection verifies the finished part against the model; statistical process control holds capability in production, typically at Cpk ≥ 1.67.

A tolerance you cannot measure is a tolerance you cannot promise. The inspection is as much a part of the job as the cut.