Understanding CNC Machining Tolerances

Tolerance is the allowable range on a dimension — how far a real, physical part is allowed to drift from the number on the drawing and still be correct. No process makes anything exactly to size, so every dimension carries a tolerance whether you write one or not. Getting tolerances right is one of the highest-leverage things an engineer does: too loose and parts do not fit; too tight and the price climbs for no reason.

How tolerance is written

You will see tolerance expressed a few ways: a bilateral band (25.00 ±0.05), a limit pair (25.05 / 24.95), or a fit class for holes and shafts. For anything involving orientation, position, or form — not just size — the right tool is GD&T, which controls how features relate to each other rather than just their size.

Standard machining tolerances

A good rule of thumb for milled and turned metal parts:

• Default / general: about ±0.005" (±0.125 mm) — achieved with no special effort.
• Precision: ±0.001" (±0.025 mm) on selected features by a capable shop.
• Tight / ground: ±0.0002–0.0005" on critical bores or mating faces, often needing grinding or EDM.

If a drawing has no callout on a dimension, the shop falls back to a general standard — usually ISO 2768 (medium or fine) or a title-block default. Never assume; if it matters, write it.

Why tight tolerances cost money

Every tolerance band tighter than default adds real work: lighter cuts, more passes, temperature control, in-process gauging, tighter workholding, and more parts rejected at inspection. A feature at ±0.001" can cost several times one at ±0.005" even though the drawing looks almost the same. We put numbers on this in the hidden tax of over-tolerancing.

Specify tight only where it earns its keep

The winning strategy is simple: set a sensible general tolerance for the whole part, then tighten only the handful of features that actually control fit or function — the bearing bore, the sealing face, the locating hole. Everything else rides on the general band. This is also why engineering fits exist: standard clearance, transition and interference classes tell the shop exactly what a hole-and-shaft pair needs to do.

Tolerances stack up

Individual tolerances add up across an assembly. Four features each at ±0.1 mm can combine into ±0.4 mm of drift at the end of the chain — enough to make a "correct" set of parts refuse to assemble. Before releasing a design, run a quick tolerance stack-up analysis on the dimensions that matter.

How tolerances get verified

Tight-tolerance features are checked, not trusted. Depending on the part that means calipers and gauges, a CMM, or a First Article Inspection report on the first production run. If you need documented dimensional evidence, ask for it up front so it is quoted in.

Not sure what to put on your drawing? Send the model with your critical features flagged and we will advise on realistic tolerances with your quote.