CNC Machining
CNC Machining Tolerances: A Plain-English Guide
A tolerance is the range of sizes you will accept for a feature. Get it wrong in either direction and you pay, once in scrap or every time in price.
Key takeaways
- A tolerance is an allowed range, not a target size.
- ±0.1 mm is a normal CNC default. ±0.02 mm is precision work.
- Tighten only the features that mate with another part.
- Every extra decimal place adds cost, inspection time and lead time.
What a tolerance actually means
A tolerance is the range of sizes you will accept for a feature. If a drawing calls out a 25 mm bore at ±0.1 mm, any bore from 24.9 mm to 25.1 mm passes. Anything outside that window is scrap.
That is the whole idea. A tolerance is not a grade of quality. It is a promise about how much variation your assembly can live with.
The default tolerance trap
Most drawings carry a general tolerance in the title block, often ±0.1 mm for metal parts. That number applies to every dimension that does not carry its own tolerance.
The trap is treating it as a target. If the note says ±0.05 mm across the whole drawing, the shop has to hold ±0.05 mm on every feature, including the ones nobody will ever check. You pay for all of it.
Where tight tolerances earn their money
- Bearing seats and shafts. A seat that is 0.05 mm oversize will spin, rattle or wear early.
- Dowel pin holes. Position matters more than hole size here.
- Seal grooves. A few hundredths of a millimetre decides whether an O-ring seals.
- Alignment features. Anywhere two parts must line up without adjustment.
Everything else, including outside profiles, clearance holes and cosmetic edges, can usually sit at ±0.1 mm or looser.
What each process can hold
| Process | Typical tolerance | When you need tighter |
|---|---|---|
| CNC milling | ±0.1 mm | ±0.02 mm for bearing seats |
| CNC turning | ±0.05 mm | ±0.01 mm for shaft diameters |
| Surface grinding | ±0.005 mm | Flatness and parallelism callouts |
| Drilling | ±0.1 mm position | Ream or bore when you need a fit |
| SLA 3D printing | ±0.1 mm | Small features and smooth surfaces |
| FDM 3D printing | ±0.3 mm or ±0.3% | Not a precision process |
| Injection molding | ±0.1 mm | ±0.05 mm on critical features |
These are production numbers, not lab bests. A good shop can hit tighter on one part. Holding it across a thousand parts at a fair price is the real question.
Why tight costs more
Five things change when you tighten a tolerance:
- Speed. Finer cuts mean lighter passes and slower feed rates.
- Setup. More care in fixturing, more checks before the run starts.
- Heat. Aluminium grows about 23 µm per metre for every degree Celsius. Precision work needs temperature control.
- Inspection. A CMM report on every feature takes time, and you pay for that time.
- Scrap. A narrower window means more parts fall outside it.
On a simple bracket, moving from ±0.1 mm to ±0.02 mm can add 30% to 50% to the price. On a complex part it can double it.
How to write tolerances a shop can hit
- Give every critical feature a clear datum. Without datums, two inspectors can measure the same part two different ways.
- Check your stack-up. Three ±0.1 mm dimensions in a row can add up to ±0.3 mm at the end.
- Do not dimension the same feature twice. Pick one path and stay on it.
- Use a position tolerance for hole patterns instead of ± on X and Y. It says what you actually mean.
- Ask what happens if the feature sits at the edge of the range. If the answer is nothing, loosen it.
What to expect from a supplier
A serious shop sends a first article inspection report with the first batch. It lists every dimension, the measured value and a pass or fail. For tight parts, ask for CMM data as well.
If a supplier cannot tell you what tolerance they hold as standard, that is your answer.
Common questions
What is the standard tolerance for CNC machining?
±0.1 mm (about ±0.004 in) is a fair default for CNC milling and turning on metal. Many shops hold ±0.05 mm with little extra cost. Below ±0.02 mm, expect a clear price jump.
Can a shop hold ±0.01 mm?
Yes, with the right machine, a temperature-controlled room and the right cutter. Expect slower cycle times and a metrology report. Not every feature needs it.
Does a tighter tolerance make a better part?
No. It makes a more expensive part. A part is better when the tolerances match the way it is used.
The bottom line
Start loose. Tighten only what touches something else. Put the money into the features that decide whether the assembly works, and leave the rest alone.