Why tighter tolerance is not always better
Tolerances on a drawing determine the machining method, the inspection approach and ultimately the cost. Tightening an OD tolerance from ±0.05 mm to ±0.01 mm usually means slower speeds, smaller feeds and more inspection — the cost can double. Specifying realistic tolerances is the cheapest step in the whole project.
What CNC turning can actually hold
Based on our production experience (stainless steel, carbon steel, brass and aluminium, roughly Ø5–Ø80 mm):
| Process | Typical achievable tolerance | Typical use |
|---|---|---|
| Standard CNC turning | ±0.05 mm | General structural dimensions |
| Finish turning | ±0.02 mm | Mating surfaces, shoulder diameters |
| Precision turning (small feed, small depth of cut) | ±0.01 mm | Bearing seats, sealing faces, precision pins |
| Swiss-type turning (small slender parts) | ±0.005 mm (size dependent) | Small-diameter, high-precision long parts |
Larger diameters are harder: holding ±0.005 mm on Ø5 is a different problem from holding it on Ø80.
Mapping to IT grades
- IT6–IT7 — precision turning or grinding; roughly 0.018–0.025 mm for Ø10–50
- IT8–IT9 — normal finish turning; roughly 0.027–0.062 mm
- IT10–IT11 — standard turning; roughly 0.070–0.160 mm
- IT12 and looser — roughing range, lowest cost
Geometric tolerances matter too
- Concentricity / coaxiality — may require single-setup machining or a dedicated fixture
- Roundness / cylindricity — linked to machine rigidity and tool wear
- Face runout — driven mainly by workholding
- Surface roughness Ra — finish turning reaches Ra1.6–3.2, small-feed finish turning Ra0.8; lower values need grinding or polishing
Practical advice
- Tolerance only the dimensions that affect assembly or function
- Flag critical dimensions so the shop knows where to focus
- Keep surface finish consistent with the tolerance — avoid pairing an IT9 tolerance with Ra0.4
- State datums explicitly when they matter, to avoid inspection disputes
Send us the drawing and we will first confirm whether the tolerances can be held in stable production, then quote.