How to Report Tolerances for a CNC Machine
This page is for design engineers and buyers who write or approve CNC drawings. It shows how to report tolerances for a cnc machine so the shop reads the same limits you intended, from a single datum callout to a full GD&T frame.
In this article
- 1
- 2
- 3
- 4
- 5
- 6
- 7
- 8
Key takeaways
What Report Tolerances for a CNC Machine Actually Means
A tolerance is the allowed spread around a nominal dimension. If a drawing calls a bore Ø10 mm with a tolerance of +0.02/-0 mm, any hole between 10.000 mm and 10.020 mm passes. Anything at 10.021 mm is scrap, even if it fits on the bench.
Reporting that tolerance is the act of writing it in a form the machinist, the CAM programmer, and the inspector can all read the same way. Vague wording is the root cause of most tolerance disputes. It is also the reason for most rework.
Three things travel with every tolerance: the nominal, the allowable deviation, and the datum or feature it applies to. Miss any one of them and the shop will guess. Guessing is expensive.
GreatLight machines to ±0.005 mm (±0.0002 in) on 5-axis centers. That capability only matters if your drawing states the limit clearly enough for the operator to verify it.
- 1NominalThe ideal size on the drawing, before any deviation.
- 2DeviationThe plus and minus allowance, or the upper and lower limits.
- 3DatumThe reference feature the tolerance is measured from.
Pick the Right Reporting Method for the Feature
Direct limits are the default on turned and milled parts. You write 15 ±0.05 mm next to the dimension. The machinist knows the target is the middle of the band, not the edge. On a 15 mm shaft, that means aiming near 15.00 mm, not 15.05 mm.
Limit dimensions are a variant. Instead of 15 ±0.05 mm you write 14.95/15.05 mm. It removes any ambiguity about which side of nominal is preferred, but it also removes the operator's freedom to bias the cut for tool wear.
GD&T is the better tool once a feature has orientation, location, or runout requirements. A position callout like ⌖ Ø0.1 M A B C tells the shop how far a hole center may drift from true position, and which datums define true position.
For a shaft or a bore that runs against a mating part, you may need a fit class instead of plain limits. A 20 mm shaft in a 20 mm bearing bore is not a fit. It is a collision.
- 1Direct limitsBest for simple size control on a single feature.
- 2Limit dimensionsBest when both limits are absolute and no bias is allowed.
- 3GD&T framesBest for position, orientation, profile, and runout.
- 4Fit classesBest for mating shafts, bores, and bearings.
Build the Datum Scheme Before You Place Callouts
A datum is the origin of measurement. If you dimension a hole from the left edge on one view and from the right edge on another, you have created two truths. The inspector cannot satisfy both if the part is at the low end of the outer length tolerance.
A clean scheme uses three datums: primary A, secondary B, tertiary C. A is usually the largest flat face that sits on the fixture. B is a long edge. C is a short edge or a second hole. Together they lock all six degrees of freedom.
Place the datum feature symbols on the drawing before you add any tolerance frames. Every position, profile, or runout callout should reference A, B, C in that order. If a callout references only A, ask why.
On 5-axis work the part may be re-fixtured between operations. A clear datum scheme lets the second setup pick up the same origin from a previously machined feature, not from a raw casting surface.
- 1One origin per directionNever mix left-edge and right-edge dimensioning on the same axis.
- 2Reference in orderWrite A B C, not C B A. The order defines the constraint sequence.
- 3Re-fixture carefullyUse a machined feature as the datum for second-operation setups.
Set Tolerances That the Process Can Actually Hold
A tolerance is a promise about capability. On a 3-axis mill in aluminum, ±0.05 mm on a milled pocket is routine. On a deep bore in 17-4PH stainless, ±0.02 mm may need a second operation and a reamer.
Surface finish travels with tolerance. A Ra 0.8–1.6 μm finish on a sealing face is a normal machined finish. Ra 0.2–0.8 μm usually requires a finishing pass at lower feed, and sometimes a lap or polish step.
Tighter than ±0.005 mm on a 300 mm long part is possible on our 5-axis centers, but it rarely survives thermal drift across a 10-hour run. If the feature is that critical, ask for in-process inspection, not a tighter number.
The wrong tolerance is worse than a loose one. A ±0.005 mm callout on a non-functional cosmetic edge forces the shop to slow the cycle, inspect every part, and price the risk into the quote.
- 1Size drives costA 4,000 mm part holds ±0.05 mm more easily than ±0.005 mm.
- 2Material drives riskTitanium and Inconel move more from heat than aluminum.
- 3Finish follows toleranceTight finish at tight tolerance means slower feed and more passes.
Use General Notes to Remove Drawing Noise
Most drawings repeat the same tolerance twenty times. Put it in the title block once. A note such as unless otherwise stated: linear ±0.1 mm, angular ±0.5°, edges deburred, sharp corners 0.3 mm max removes a large share of individual callouts.
State the units. Mixed inch and metric drawings cause more scrapped parts than tight tolerances do. If you work in inches, say so in the title block and keep it consistent on every view.
Add a note for threads and tapped holes. A typical M6 tapped hole needs a thread depth and a drill depth. Without them the operator picks a depth, and it may not match your screw length.
Finally, name the inspection expectation. If a feature requires a CMM report, say it. If a visual check is enough, say that too. We inspect 100% before shipment and can supply reports on request.
- 1Title block notesCover linear, angular, edges, and threads in one place.
- 2UnitsState mm or inches once. Do not mix.
- 3InspectionSay which features need a recorded measurement.
How to Report Tolerances for a CNC Machine: Step by Step
Follow these steps in order when you prepare or review a drawing.
- 11. List the functional features firstWrite down every surface that touches another part: bearing bores, seal faces, dowel holes, mating flanges. These carry the real tolerances. Cosmetic surfaces go on the general note list.
- 22. Assign the datum schemeChoose primary A, secondary B, tertiary C. Mark them on the drawing before placing any dimension. A is the largest stable face, B the longest edge, C the shortest reference.
- 33. Choose the reporting method per featureUse direct limits for simple size. Use a GD&T frame for position, profile, or runout. Use fit class callouts for shafts and bores that mate with bearings or bushings.
- 44. Set the tolerance value from function, not habitA clearance hole needs ±0.2 mm, not ±0.02 mm. A press-fit bore needs 0 to +0.02 mm. Match the number to the load the part will carry.
- 55. Add the general tolerance noteUnless otherwise stated: linear ±0.1 mm, angular ±0.5°, surface finish Ra 3.2 μm. This covers everything you did not call out individually.
- 66. State units and inspection requirementsPut mm or inches in the title block. Mark features that need a recorded CMM or gauge reading. Note if a full dimensional report is required at shipment.
- 77. Review for conflicts before releaseCheck that no feature has two conflicting tolerances. Check that every GD&T frame references the datum scheme. Check that the tightest tolerance is reachable by the chosen process.
Which Tolerance Reporting Method to Use
Match the method to the feature type and the process that will cut it.
| Method | Best for | Typical value | Watch out for |
|---|---|---|---|
| Direct limit (size) | Simple holes, slots, turned diameters | 15 ±0.05 mm | Operator may bias to one edge of the band |
| Limit dimensions | Absolute limits, no preferred side | 14.95/15.05 mm | No room to compensate for tool wear |
| GD&T position frame | Hole patterns, mating bores | ⌖ Ø0.1 M A B C | Always requires a stated datum scheme |
| GD&T profile | Contoured 5-axis surfaces | Profile 0.2 A B | Very expensive on large thin walls |
| Fit class | Shafts in bearings, bores for bushings | H7/h6, or stated clearance | Needs both parts measured, not one |
| General note | Non-functional edges, clearance corners | ±0.1 mm linear | Does not override a specific callout |
The drawing is the contract
A tolerance is only useful if the shop can read the same number you intended. State the limit, the datum, and the units once, and the part has a chance to be right the first time.
Frequently asked questions
Should I use ± tolerances or limit dimensions?
Use ± when the midpoint of the band is the preferred target and the operator may bias the cut to manage tool wear. Use limit dimensions when both ends are absolute and no bias is allowed.
For a bearing seat, limit dimensions are usually safer because you do not want the shop aiming at the low side of a press fit.
When is GD&T worth the extra drawing time?
When a feature has position, orientation, or runout requirements that cannot be expressed as a simple size band. Hole patterns, flange faces, and rotating shafts are the common cases.
For a single hole in a bracket, a direct limit is faster and just as clear.
How tight a tolerance can a CNC machine hold?
Our 5-axis centers hold ±0.005 mm (±0.0002 in). On a 4,000 mm part that limit is harder to hold across a long run because of thermal drift.
If a feature truly needs ±0.005 mm, plan for in-process inspection rather than assuming the number alone will hold.
What happens if my drawing has no general tolerance note?
The shop has to ask, or it picks a value. Both outcomes slow the job. A single title block note removes that ambiguity for the whole drawing.
Include units in the same note. Mixed inch and metric callouts are a common cause of scrapped features.
Can you check my tolerance scheme before quoting?
Yes. We review drawings and give DFM feedback with the quote, usually within 12 hours. That review flags conflicting callouts, unreachable tolerances, and missing datums.
Uploads are secure and confidential, and an NDA is available on request.
Do I need to mark every feature for inspection?
No. Mark the features that carry function or safety. Everything else can be covered by the general note and a visual check.
We inspect 100% of parts before shipment and can supply dimensional reports on request for the features you flag.
Send your drawing for a tolerance review
Quotation and free DFM analysis within 12 hours. No minimum order quantity, from one prototype to 10,000+ parts.
12-hour quote100% inspection±0.005 mmNDA on request
More tolerance and machining notes
We publish setup notes, tooling trials and inspection data from the factory floor.
