Finishing
Surface Finish Ra: What the Number Means and How Machining Controls It
Why Ra alone does not describe a surface, how cutter path, stepover, and nose radius drive finish, and where a surface grinder is the only right tool.
Reference
CNC Machining Tolerance Chart
A practical guide to what tolerance you should put on the drawing, and what each process can actually hold — general tolerances, per-process achievable tolerances, and surface finish.
General Tolerances
Start With ISO 2768, Then Get Specific
If a dimension has no tolerance, ISO 2768-1 supplies a default. Most drawings we see use the medium class; fine class is for features that genuinely matter. Here is what each class means for a linear dimension.
| Nominal size (mm) | Fine (f) | Medium (m) | Coarse (c) |
|---|---|---|---|
| 0.5 – 3 | ±0.05 | ±0.1 | ±0.2 |
| 3 – 6 | ±0.05 | ±0.1 | ±0.3 |
| 6 – 30 | ±0.1 | ±0.2 | ±0.5 |
| 30 – 120 | ±0.15 | ±0.3 | ±0.8 |
| 120 – 400 | ±0.2 | ±0.5 | ±1.2 |
| 400 – 1000 | ±0.3 | ±0.8 | ±2.0 |
Per Process
Achievable Tolerance by Process
These are the tolerances we hold in normal production, not best-case lab demos. They assume a sensible drawing and a competent setup; a tolerance tighter than the "production" column costs extra in setup, inspection and scrap.
| Process | Typical achievable | Notes |
|---|---|---|
| CNC milling (3/4-axis) | ±0.05 mm typical | Holes, pockets, faces, plates |
| CNC turning | ±0.05 mm typical | Bores and ODs, one-setup concentricity |
| Swiss / sliding-head | ±0.05 mm typical | Long slender parts, sub-spindle work |
| 5-axis machining | ±0.05 mm typical | Compound angles, one setup |
| Turn-mill | ±0.05 mm typical | Turned + milled in one clamping |
Surface Finish
Ra by Process
Surface finish is specified as Ra in micrometres. Lower is not always better — a sealing face needs a specific Ra band, not just "the smoothest you can make".
| Process | Typical Ra (µm) | Notes |
|---|---|---|
| Milling (finishing pass) | 0.8 – 1.6 | Ball nose with fine stepover |
| Turning | 0.4 – 0.8 | Finish pass with correct nose radius |
| Swiss turning | 0.4 – 0.8 | High-pressure coolant |
| As-milled (rough) | 3.2 – 6.3 | Default when no finish is called out |
Geometric
GD&T Quick Reference
Geometric tolerances behave differently from size tolerances. These are the values we routinely hold on a production part; tighter is possible on individual features with the right setup.
| Characteristic | Routinely held | What it costs to tighten |
|---|---|---|
| Flatness | Drawing-driven | More finishing passes, lighter cuts |
| Parallelism | Drawing-driven | Square setups, datum discipline |
| Perpendicularity | Drawing-driven | Location vs datums |
| Position (true position) | Confirmed by optical measuring | Probe / 2D inspection |
| Concentricity / runout | Single-setup turning | One-setup turning or turn-mill |
Guidance
How to Tolerance a Drawing Without Overpaying
- Put a general tolerance (ISO 2768) in the title block, then add specific callouts only on the features that actually need them.
- Tolerance only what drives function — a clearance hole does not need ±0.01 mm.
- Specify a datum scheme before tightening GD&T; position without datums is meaningless.
- If a bore and an OD must be concentric, say so — it changes the process to one-setup turning.
- State the finish (Ra) only where it matters: sealing faces, bearing seats, mating surfaces.
Tolerance is only half the story — proving the process holds it is the other. See how we verify with quality assurance and what our machining capabilities actually cover.
Insights
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