CNC Machining Tolerances: What the Number Really Means
A tolerance is a limit, not a wish. This page explains how CNC machining tolerances are set, where the practical floor sits on a real machine, and how to read a drawing so tight callouts do not send your part cost through the roof.
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What CNC machining tolerances actually control
A tolerance is the allowed spread around a nominal dimension. Write 25.00 ±0.05 mm and you are telling the shop that anywhere between 24.95 mm and 25.05 mm is acceptable. Nothing outside that band matters, no matter how nice it looks.
The number does two jobs at once. It tells the machinist where to aim, and it tells the inspector when to stop measuring. Both people need the same band, which is why a blank tolerance block on a drawing is a problem. If nothing is stated, the shop either assumes a general block like ISO 2768-m or calls you to ask.
Tolerances also stack. Chain five dimensions in a row and each one carries its own deviation, so the gap at the end of the chain can drift far more than any single callout suggests. A part can pass every individual check and still not assemble. That is why datum selection matters as much as the numbers themselves.
One more thing. A tolerance is not a quality grade. A ±0.1 mm bracket and a ±0.01 mm bracket can both be perfectly good parts. The question is only whether the function needs the tighter band.
Types of CNC machining tolerances you will see on drawings
Dimensional tolerance is the plain one: length, width, hole diameter, slot width. These are the callouts most engineers picture first, and they are usually the easiest to hold because the machine is moving in a straight line and the tool is a known size.
Geometric tolerances come next. Flatness, parallelism, perpendicularity, position, runout, concentricity. These control shape and relationship rather than size. A shaft can measure 20.00 mm exactly and still vibrate if its roundness or runout is out of band.
Fit tolerance is the pairing of two tolerances. A bore and a shaft each get their own spread, and the assembly works only if the two spreads overlap the right way. Clearance fits leave a gap, interference fits leave material to press, transition fits do a little of both.
Surface finish travels with tolerance in practice. If a sealing face calls for Ra 0.8–1.6 μm and a ±0.02 mm profile, the finishing pass that hits one usually helps the other. Rough the part first, then take a light finishing cut at low feed. Push the feed rate and you trade finish for cycle time.
Where the practical floor of CNC machining tolerances sits
On a well-kept three-axis mill, ±0.05 mm is comfortable work. The machine can repeat that all day with normal tool wear and a reasonable setup. Most brackets, plates, and housings live in this band.
Move to ±0.02 mm and you are in the range where setup, fixturing, and thermal drift start to show up. Coolant temperature, spindle warm-up, and how firmly the vise grips all matter. A part that passes at 8 a.m. can drift by mid-afternoon if the shop floor heats up.
At ±0.005 mm, the tolerance approaches the machine's own positioning limit. This is where we slow down, use a temperature-controlled environment, and often finish on a five-axis machine in one setup so the part is not re-clamped. GreatLight holds ±0.005 mm (±0.0002 in) on qualifying features. It is not a default.
Below that, the process changes. Grinding, lapping, or jig boring take over from milling. If your drawing needs ±0.002 mm on a 200 mm aluminum plate, the honest answer is that milling will not get you there reliably, and the cost curve turns nearly vertical.
What pushes a tolerance out of reach
Material moves. Aluminum 6061 cuts clean and holds size well, but thin walls spring back after the clamps come off. Stainless 316 work-hardens, so a light finishing pass on a dull tool can rub instead of cut and pull the dimension off. Titanium TC4 (Ti-6Al-4V) generates heat fast and needs lower speeds and steady coolant.
Geometry is the second factor. A deep pocket with a 3:1 depth-to-diameter ratio needs a long, slender tool. That tool deflects. A tolerance on the floor of a 60 mm deep pocket is harder to hold than the same tolerance on an open face.
Feature size matters too. A Ø2 mm hole in a 40 mm deep bore is a different job than a Ø2 mm hole through a 3 mm plate. The drill or end mill has to reach, clear chips, and stay straight. Small and deep together is the hard combination.
Hardness is the third factor. Pre-hardened 4140 at 30 HRC machines fine with the right inserts. Tool steel at 60 HRC does not. It goes to grinding or EDM, and the tolerance conversation changes completely.
How to write callouts that a shop can actually hit
Start with function. Ask what the part really does. A bearing seat needs a controlled fit. A cover plate needs to sit flush. A mounting hole needs clearance for a bolt. Most features fall into one of a few buckets, and each bucket has a natural tolerance range.
Apply the tight number only where it earns its keep. Tolerance the whole drawing at ±0.01 mm and the shop has to treat every feature as critical. That means more setups, more in-process checks, slower feeds, and a higher price. It also means the truly critical feature gets no extra attention.
Pick your datums before you pick your numbers. A position tolerance on a hole is meaningless without a datum to measure from. Use functional faces and mounting surfaces as datums, not whatever was convenient in CAD.
Add a general tolerance block for everything else. ISO 2768-m is a common starting point for machined metal parts. Then the tight callouts stand out, and the shop knows where to spend time. Talk to us before you release the drawing if something feels uncertain.
Tolerance band, typical process, and the features it suits
Practical ranges for machined metal parts. Tighter than the bottom row means a different process.
| Tolerance band | Typical process | Best-fit features |
|---|---|---|
| ±0.10 mm | 3-axis milling, standard setup | Brackets, covers, clearance holes |
| ±0.05 mm | 3-axis or 4-axis, light finish pass | Housings, plates, general bores |
| ±0.02 mm | 4-axis or 5-axis, controlled setup | Bearing seats, mating faces, slots |
| ±0.01 mm | 5-axis, one-setup, warm-up cycle | Precision shafts, dowel holes, spigots |
| ±0.005 mm | 5-axis, temperature control, slow finish | Critical fits, aerospace and medical parts |
| Below ±0.005 mm | Grinding, lapping, EDM, jig boring | Gauge work, hardened tooling, reference faces |
When to tighten, when to loosen
Tolerance the two or three features that carry function at ±0.02 mm or tighter, and leave everything else on a general block. If the whole drawing is tight, the price climbs and the critical feature gets no more attention than the rest.
Common questions about CNC machining tolerances
What is the tightest tolerance GreatLight can hold?
We hold ±0.005 mm (±0.0002 in) on qualifying features, usually on a five-axis machine in a single setup with temperature control. That is the floor for milling.
Below that, the part moves to grinding, lapping, or EDM. Send the drawing and we will tell you which route your feature needs.
Does a tighter tolerance always cost more?
Almost always, but not in a straight line. Going from ±0.10 mm to ±0.05 mm is a small step. Going from ±0.02 mm to ±0.005 mm is a large one, because the shop has to slow down, control temperature, and often add an inspection step.
The cost jump is why we ask which features actually need the tight band.
How do I know if my tolerance is realistic?
Check the feature against the process. A 0.5 mm thick aluminum wall held at ±0.01 mm will move when the clamps release. A 60 mm deep pocket at ±0.01 mm needs a long tool that deflects.
If the tolerance is tighter than the tool or the material can support, the drawing needs a change, not a better machine.
Should I use a general tolerance block?
Yes, for everything that is not critical. ISO 2768-m is a common default for machined metal parts. It keeps the drawing readable and tells the shop where the tight callouts are.
Features outside the block should each carry their own number and, where needed, a datum.
How does surface finish relate to tolerance?
They travel together on most parts. A finishing pass at low feed improves both size and Ra. We routinely hit Ra 0.8–1.6 μm on machined surfaces, and Ra 0.2–0.8 μm when the drawing calls for it.
If you need both a tight profile and a fine finish, say so early. It changes the tool path and the cycle time.
Do you inspect every part against the tolerance?
We inspect 100% before shipment, with raw material checks, in-process monitoring, and a final inspection. Reports are available on request.
For critical features, we can add CMM reports so you have the numbers, not just a pass stamp.
Send the drawing and get a DFM review in 12 hours
Upload your files and we will flag the tolerances that drive cost and the ones that can relax. No minimum order quantity, from one prototype to 10,000+ parts.
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