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ReferenceTolerance reference by process
Typical is what shops hold routinely on the right machine in reasonable material. Achievable needs a slower process, more measurement and sometimes a different machine, and it is priced accordingly. Neither column is a promise on your part.
| Feature | Typical | Achievable with care | What decides it |
|---|---|---|---|
| Milled linear dimension | ±0.05 mm | ±0.02 mm | Depends on part size and how many setups the feature crosses |
| Turned diameter | ±0.02 mm | ±0.01 mm | Tighter in one chucking than across two |
| Drilled hole | +0.1 / 0 mm | Reaming needed below this | A drill is not a sizing tool |
| Reamed hole | ±0.01 mm | ±0.005 mm | Standard route to a precise bore without grinding |
| Ground flat or diameter | ±0.005 mm | ±0.002 mm | Separate operation, often a separate machine queue |
| Wire EDM profile | ±0.01 mm | ±0.005 mm | Corner radius limited by wire diameter, commonly 0.1 to 0.15 mm |
| Flatness over 100 mm | 0.05 mm | 0.01 mm ground | Thin parts relax after unclamping, so state free or constrained |
| Position of holes | 0.2 mm | 0.05 mm | Needs a CMM to prove at the tighter end |
| Angular | ±0.5 degree | ±0.1 degree | Tighter angles usually imply a rotary or 5-axis setup |
| Surface finish, milled | Ra 1.6 to 3.2 | Ra 0.8 | Below Ra 0.4 expect grinding, polishing or lapping |
| Surface finish, turned | Ra 0.8 to 1.6 | Ra 0.4 | Material and insert nose radius drive this as much as the machine |
| Plastics, general | ±0.1 mm | ±0.05 mm | Moisture, temperature and creep move the part after machining |
Three things that move every row
- MaterialAluminium and brass behave. Stainless work hardens and moves. Plastics expand with heat and creep over time, so a ±0.01 mm feature in unfilled nylon is not a realistic request.
- SetupsFeatures cut in one clamping hold to each other far better than features cut in two. A tolerance between faces is really a question about setups.
- MeasurementBelow ±0.01 mm, temperature matters. Aluminium moves about 0.005 mm per degree over a 200 mm part, so state the inspection condition or expect honest disagreement.
How to use this on a drawing
Set a general tolerance in the title block, then call out only the features that control function.
General tolerance ISO 2768-mK. Critical: bore A 25 H7, parallelism of face B to A within 0.03 mm, position of the four M6 holes 0.2 mm to A-B-C. Everything else general.
The full reasoning is in the guide on tolerances for buyers.
About tolerances
What tolerance can a CNC machine shop hold as standard?
Without a called-out tolerance most shops work to a general standard such as ISO 2768 medium, which allows roughly ±0.1 to ±0.3 mm depending on feature size. Called-out features are commonly held between ±0.05 and ±0.01 mm, and below that grinding or reaming usually enters the process.
Why is a blanket tight tolerance so expensive?
It changes three things at once: slower finishing passes on every surface, measurement of every dimension rather than the critical few, and a higher scrap risk that the shop prices into the quote. The scrap risk is usually the largest of the three.
Does a tolerance mean anything if the shop cannot measure it?
No. A tolerance is only enforceable if someone can prove it with an instrument that is calibrated and appropriate. Below about ±0.02 mm that normally means a CMM, so ask what a shop measures with before the order rather than after the first article.