What tolerance can a CNC shop actually hold?
Every quotation request we receive has a tolerance somewhere. A minority of them put the tolerance where it belongs — on a specific feature, with a datum, and with a reason. The rest say "general tolerance ±0.01 mm" across a drawing that includes a 200 mm plate, and then the first article fails and everyone blames the machine.
A machine tool has a positioning accuracy. A shop has a process capability, which is lower and depends on the material, the fixture, the feature and the temperature of the room. The number on your drawing has to survive all four.
The number on the machine is not the number on your part
A Japanese Swiss-type lathe is usually quoted at around ±0.005 mm positioning, and a five-axis turn-mill at a similar figure. That is the machine holding a tool at a coordinate, in a temperature-controlled room, with a warm spindle, on a short rigid part. It is a real number and it is not the tolerance your part will hold.
Once you put a workpiece in a chuck and start cutting, four things move:
- Thermal growth. A spindle warms up over the first hour. On a long run the first fifty parts and the last fifty parts are not the same size unless the machine is compensated.
- Tool wear. A carbide insert wears measurably over a few hundred parts. Either the operator compensates on a schedule or the size drifts inside the tolerance band.
- Clamping distortion. A thin wall deflects under chuck pressure and springs back when released. The machine did nothing wrong.
- Fixture and datum error. Every re-clamp adds stack-up. A part machined in three setups has three datum transfers, and each one costs you tolerance.
What the numbers look like in practice
In our shop the honest split is this. On Japanese machines, ±0.02 mm is the standard tolerance we quote without discussion. On domestic machines it is ±0.05 mm. ±0.005 mm is achievable on a specific feature — a bore, a spigot, a bearing seat — when it is short, supported, and machined in the same setup as its datum. It is not achievable across a whole drawing, and any shop that says otherwise is either pricing in a second process or will fail the first article.
That gives you a practical rule: put the tight callout on the features that actually need it, and let the rest sit at a general tolerance. A bearing seat at ±0.005 mm and the rest of the part at ±0.1 mm is cheaper than the whole drawing at ±0.02 mm and equally functional.
Material changes the answer
The same machine holds different numbers on different materials. Brass H59 and 6061 aluminium cut freely and hold a tight callout easily. 304 is gummy and work-hardens; 316L is worse. Ti-6Al-4V moves under tool pressure and springs back, so a ±0.005 mm callout on a long titanium journal is a design change waiting to happen — expect ±0.01 mm on a short supported feature.
This is why a shop that quotes the same tolerance for brass and titanium is telling you something about how it quotes, not about its machines.
How to write the callout so it survives
- Put the tight tolerance on the feature, not on the title block.
- Give every tight feature a datum, and make the datum a machined surface rather than a cast or a folded edge.
- Say which dimension is functional. A 0.02 mm error on a cosmetic diameter is invisible; on a bearing bore it is a reject.
- If a feature is only tight because the CAD defaulted to it, relax it. It is the cheapest cost reduction available and it costs you nothing functionally.
- State the inspection method you will accept. A callout measured on a CMM and a callout measured with a caliper are two different specifications.
What to send us
Send a STEP or a DWG with the tolerance callouts on the features that matter, the material grade, the finish, the annual quantity and any inspection report format your quality team needs. If the drawing has a general tolerance block, we will quote to it — but we will also tell you which features will drive the price and which ones you could relax.
That conversation takes one email and it usually takes 10–20% off the quote. It is worth having before the drawing is frozen, not after.