How to specify tolerances for 5-axis machined parts
Tight tolerances are cheap when they sit on the features that matter and expensive everywhere else. A practical approach for designers and buyers.
Key takeaways
- Base tolerances on function and a datum frame that matches clamping and inspection.
- Use position and profile tolerances on critical features; a general tolerance for the rest.
- Avoid sharp internal corners, deep narrow pockets and thin walls where possible.
- Check measurability: uncertainty a small fraction of the tolerance, at 20 °C.
Start with function, then datums
Every tolerance should trace back to how the part works in the assembly: which faces locate it, which bores carry shafts, which surfaces seal. Those locating features become the datum reference frame. When the datums on the drawing match the way the part is clamped and inspected, a 5-axis machine can produce all related features in one setup — and the measured result means what you think it means.
Use geometric tolerances where they belong
- Position for hole patterns and bores relative to datums. Adding a material condition modifier (MMC) where assembly allows gives a bonus tolerance at no functional cost.
- Profile of a surface for freeform and blended surfaces — the natural tolerance for typical 5-axis geometry.
- Perpendicularity, parallelism, flatness for functional faces such as sealing or mounting surfaces.
- Follow one system consistently: ISO GPS (ISO 1101 and related) or ASME Y14.5.
Let general tolerances do the rest
Non-functional features should carry a general tolerance note — for example ISO 2768-mK, or ISO 22081 for GPS-based drawings — rather than individual tight values. ISO 2768-2 was withdrawn in 2021 and replaced by ISO 22081:2021, but many drawings still call it up and we work to either. A drawing where every dimension is ±0.01 mm costs more to make and to measure, and hides the features that actually matter.
What drives cost on a 5-axis part
| Feature | Why it costs | Cheaper alternative |
|---|---|---|
| Tight tolerance between features in different setups | Re-clamping adds a tolerance stack | Design so related features are reachable in one setup |
| Sharp internal corners | Needs very small tools or EDM | Corner radius ≥ tool radius + allowance |
| Deep narrow pockets | Long tools deflect and chatter | Limit depth to about 4–6× tool diameter |
| Thin walls | Vibration and distortion | Add ribs or increase wall thickness |
| Very low roughness everywhere | Extra finishing passes | Specify roughness only on functional faces |
Make sure it can be measured
A tolerance is only useful if it can be verified. As a rule of thumb, measurement uncertainty should be a small fraction of the tolerance (often quoted as 10–20 %). Tolerances refer to 20 °C (ISO 1), so very tight values on large aluminium parts require temperature-controlled measurement. Our Zeiss Contura G2 is specified at E0 = 1.5 + L/350 µm — see the metrology laboratory.
Checklist before you send the drawing
- Datums reflect function and can be used for clamping and inspection
- Critical features carry position or profile tolerances; the rest a general tolerance
- Internal radii and pocket depths suit standard tools
- Roughness specified only where it matters
- Material, heat treatment and coating are stated
- Inspection requirements (FAI format, CMM report, sampling) are stated
How we review drawings
At quotation, an engineer checks each critical tolerance against the machine, fixture and measuring method we would use. If a requirement adds cost without function, we say so and suggest an alternative before you place an order. Send a drawing.
Frequently asked questions
What tolerance can a 5-axis machine hold?
Machine positioning accuracy (for example 5 µm on a GF Mikron Mill P800 U) is not the same as part tolerance. Achievable part tolerance depends on geometry, material, temperature, fixturing and measurement. Send the drawing for a part-specific answer.
Should I use ISO 2768 or ISO 22081?
ISO 2768 is still widely used; ISO 22081 is the general tolerance standard within the ISO GPS system. Either is fine if it is stated clearly on the drawing.
Related pages
- Capabilities
CNC milling and 5-axis machining
3-, 4- and 5-axis milling: 5-axis to 800 × 800 × 550 mm, frames to 1,600 × 800 × 700 mm.
- Technical guides
3-, 4- or 5-axis machining
3+2 vs simultaneous, cost and accuracy trade-offs, which part goes where.
Send us your drawing.
STEP, IGES, DXF or PDF. Quotation within 2–15 business days, depending on size and complexity.