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What the letters and numbers mean
- Upper-case letters (H, G, K…) apply to holes; lower-case letters (h, g, k, p…) apply to shafts.
- The letter sets the position of the tolerance zone relative to the nominal size: H has a lower deviation of 0 (the hole is never smaller than nominal); h has an upper deviation of 0 (the shaft is never larger than nominal).
- The number (6, 7…) is the IT grade – the smaller the number, the narrower the zone and the costlier the machining.
Most drawings use the hole-basis system: an H7 hole (easy to produce with standard reamers), with the shaft letter chosen to create the desired fit.
Worked values for Ø20 (ISO 286)
| Callout | Deviations (mm) | Allowed size (mm) |
|---|---|---|
| Hole Ø20 H7 | +0.021 / 0 | 20.000 – 20.021 |
| Shaft Ø20 g6 | −0.007 / −0.020 | 19.980 – 19.993 |
| Shaft Ø20 h6 | 0 / −0.013 | 19.987 – 20.000 |
| Shaft Ø20 k6 | +0.015 / +0.002 | 20.002 – 20.015 |
| Shaft Ø20 p6 | +0.035 / +0.022 | 20.022 – 20.035 |
Choosing a fit by function
| Fit | Type | Behaviour | Typical use |
|---|---|---|---|
| H7/g6 | Clearance | Rotates or slides with lubrication | Plain bushings, pivot pins, guide pistons |
| H7/h6 | Locational clearance | Assembled by hand, accurate location | Fixture locating pins, change gears |
| H7/k6 | Transition | Snug; light mallet assembly | Bearings on shafts, pulleys, couplings |
| H7/p6 | Interference | Press-fit or shrink-fit | Pressed bushings, collars, fixed gears |
Tips for your drawings
- Apply tight tolerances only to real mating surfaces; use a general tolerance (e.g. ISO 2768-m) elsewhere to control cost.
- Specify a matching surface finish: IT6–IT7 fits usually need Ra 0.8–1.6 µm.
- For rolling bearings, follow the bearing maker's shaft/housing recommendations for your load case.
- For plated or anodized parts, state whether tolerances apply before or after finishing.
Duc Huy Intech CNC-turns and mills mating surfaces to IT6–IT7 and can provide inspection reports on request. Send your drawing for advice on the right machining approach for your tolerances.