Machining Tolerance Standards: ISO 2768, ISO 286, ASME Y14.5
Quick-reference tables for the tolerance systems buyers cite on drawings, plus what we hold in practice and how each class is verified.
ISO 2768: General Tolerances (m grade, most cited)
| Nominal size (mm) | ISO 2768-f | ISO 2768-m | ISO 2768-c |
|---|---|---|---|
| 0.5 to 3 | +/-0.05 | +/-0.1 | +/-0.2 |
| 3 to 6 | +/-0.05 | +/-0.1 | +/-0.3 |
| 6 to 30 | +/-0.1 | +/-0.2 | +/-0.5 |
| 30 to 120 | +/-0.15 | +/-0.3 | +/-0.8 |
| 120 to 400 | +/-0.2 | +/-0.5 | +/-1.2 |
ISO 286: IT Grades for Fits (typical machining range)
| IT grade | Typical use | Example on a 50 mm bore |
|---|---|---|
| IT6 | Precision spindles, gauge seats | Tolerance zone 0.016 mm |
| IT7 | Standard fits, bearing seats | 0.025 mm |
| IT8 | General machined fits | 0.039 mm |
| IT9 | Wide commercial fits | 0.062 mm |
ASME Y14.5: GD&T Callouts We Work With
| Control | What it constrains | How we verify |
|---|---|---|
| Position | Feature location to datums | CMM best-fit datum simulation |
| Flatness | Face flatness without datums | CMM scanned plane |
| Profile of a surface | 3D surface boundary | Point cloud comparison |
| Runout / total runout | Rotating feature to datum axis | CMM or bench center inspection |
| Concentricity | Feature axes to datum axis | CMM axis evaluation |
What We Hold in Practice
| Feature class | We hold | Verified by |
|---|---|---|
| General machined features | +/-0.05 mm (ISO 2768-m compatible) | Calibrated hand gauges |
| Milled and turned precision features | +/-0.005 mm | Hexagon / Zeiss CMM |
| Critical 5-axis features | +/-0.002 mm | CMM with datum simulation |
| Polymer parts, stable geometry | +/-0.005 mm | CMM after aging and finishing |
| Surface roughness | Ra 0.4 to Ra 3.2 by application | Profilometer |
FAI / FAIR Reporting Capability
First article inspection per AS9102 on request, the same block we apply across quality pages: Form 1 (product accountability), Form 2 (material and process accountability), Form 3 (characteristic accountability with ballooned drawings), backed by CMM measurement data. Reports are issued in English and filed per part number, so repeat orders reuse the same FAI package.
Related: Inspection & QC, AQL sampling, quality system.
Tolerance FAQ
What is your standard tolerance without a drawing callout?
+/-0.05 mm on general dimensions per ISO 2768-m. Tighter callouts on the drawing are planned and verified; our process capability floor is +/-0.005 mm on milled and turned features, with +/-0.002 mm on specific critical features of 5-axis parts.
Which tolerance standard should I specify?
ISO 2768 for general linear and angular dimensions, ISO 286 (IT grades) for fits on bores and shafts, ASME Y14.5 for GD&T when your design uses datums and position callouts. Mixed standards are fine; tell us which governs each feature class.
Can you really hold +/-0.005 mm on plastic parts?
On stable geometries, yes: our polymer process (custom tooling, cold air cooling, roughing plus 24 hour aging plus finishing) holds +/-0.005 mm on stable plastic part features. Movement-prone materials like virgin PTFE get honest looser numbers in the quote.
Want Tolerances Reviewed Before You Order?
Send the drawing; our quote flags every feature we would verify on CMM, within 24 hours.
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