Tolerance reference
Most drawings do not need a tolerance on every dimension. They need the right general tolerance, the right fits, and a few critical callouts that tell the machinist where to spend the time.
ISO 2768: the general tolerance
ISO 2768-1 defines four tolerance classes for linear and angular dimensions without individual tolerances: f (fine), m (medium), c (coarse) and v (very coarse). If the drawing says “ISO 2768-mK”, the m class applies to linear dimensions and K to geometric tolerances.
| Nominal range (mm) | f | m | c | v |
|---|---|---|---|---|
| 0.5–3 | ±0.05 | ±0.1 | ±0.2 | ±0.5 |
| >3–6 | ±0.05 | ±0.1 | ±0.3 | ±0.5 |
| >6–30 | ±0.1 | ±0.2 | ±0.5 | ±1.0 |
| >30–120 | ±0.15 | ±0.3 | ±0.8 | ±1.5 |
| >120–400 | ±0.2 | ±0.5 | ±1.2 | ±2.5 |
| >400–1000 | ±0.3 | ±0.8 | ±2.0 | ±4.0 |
ISO 2768-1 linear tolerances, in millimetres. Chamfers, radii and angular dimensions have separate tables.
ISO 286: fits and IT grades
ISO 286 defines the tolerance zones for shafts and holes. The IT grade is the width of the zone: IT6 is tight, IT8 is a common machined fit, IT11 is coarse. For a 20 mm shaft, IT6 is 13 µm and IT7 is 21 µm — a difference that changes the machining route and the inspection plan.
| Nominal range (mm) | IT6 (µm) | IT7 (µm) | IT8 (µm) |
|---|---|---|---|
| 3–6 | 8 | 12 | 18 |
| 6–10 | 9 | 15 | 22 |
| 10–18 | 11 | 18 | 27 |
| 18–30 | 13 | 21 | 33 |
| 30–50 | 16 | 25 | 39 |
| 50–80 | 19 | 30 | 46 |
| 80–120 | 22 | 35 | 54 |
| 120–180 | 25 | 40 | 63 |
ISO 286-1 standard tolerance grades. Always specify the fit (e.g. H7/g6), not just the grade.
GD&T: say what matters
GD&T tells the machinist what the part must do, not just how big it is. The callouts that change a CNC route most often are:
- Flatness: a large plate that must sit without rocking. It may need a stress-relieved plate and a finish pass on both sides.
- Position: hole patterns that must line up with a mating part. The CMM checks the true position, not just the hole diameter.
- Perpendicularity: a bore that must be square to a face. It often means one setup, not two.
- Profile: a curved surface that must follow the CAD model. It may need a ball-nose finish pass and a CMM scan.
What a 3/4-axis shop can hold
On critical milled features we hold ±0.002 mm; on turned parts, IT6–IT8 fits are routine. That does not mean every dimension on the drawing should be ±0.002 mm. Tightening a non-functional face adds inspection and slower cuts without adding function. Mark the critical few, use ISO 2768-mK for the rest, and let the DFM note tell you where the cost is.
If you are unsure which callouts to use, send the drawing and the mating part. We will mark the features that drive cost and the ones that need a CMM check. Quality and inspection · CNC machining services · Free DFM review.
Written by the engineering team at GY Machining, Dongguan. We machine prototypes and production parts in aluminum, stainless, titanium, brass and engineering plastics.