Understanding Tolerances
Every machined dimension has a tolerance — a band within which the actual measurement must fall. No cutting process produces a perfectly exact size, so tolerances define what variation is acceptable for the part to work as intended.
A tolerance is written as a nominal size with upper and lower limits: for example, 40.00 +0.00 / -0.02 mm means the feature must measure between 39.98mm and 40.00mm. Getting this right on your drawing is one of the most effective things you can do to avoid rework and unnecessary cost.
The most common mistake engineers make is applying the same tight tolerance across an entire drawing. This drives up machining time, inspection time, and scrap risk — for no functional benefit on the non-critical features. A well-toleranced drawing applies tight limits only where the function of the part genuinely demands it, and uses a general tolerance standard for everything else.
The ISO Tolerance System (ISO 286)
ISO 286 provides a standardised framework for specifying tolerances on cylindrical features. A letter code sets the position of the tolerance zone relative to the nominal size, and a number (the IT grade) sets how wide that zone is. Together they define a complete tolerance — for example H7 for a bore, or f7 for a shaft.
IT Grade Tolerance Widths (µm) for Selected Nominal Sizes
Values in micrometres (µm). 1 µm = 0.001 mm.
| Nominal size | IT6 | IT7 | IT8 | IT9 | IT11 |
|---|---|---|---|---|---|
| up to 3 mm | 6 | 10 | 14 | 25 | 60 |
| 3 – 6 mm | 8 | 12 | 18 | 30 | 75 |
| 6 – 10 mm | 9 | 15 | 22 | 36 | 90 |
| 10 – 18 mm | 11 | 18 | 27 | 43 | 110 |
| 18 – 30 mm | 13 | 21 | 33 | 52 | 130 |
| 30 – 50 mm | 16 | 25 | 39 | 62 | 160 |
| 50 – 80 mm | 19 | 30 | 46 | 74 | 190 |
| 80 – 120 mm | 22 | 35 | 54 | 87 | 220 |
Standard Fits
A fit describes the relationship between a shaft and a bore. ISO 286 defines three categories — clearance, transition, and interference — each suited to different assembly requirements.
| Fit type | Example | Application |
|---|---|---|
| Clearance fit | H7/f7 | Shaft runs or slides freely inside the bore |
| Transition fit | H7/k6 | Location fit — could be slight clearance or slight interference |
| Interference fit | H7/p6 | Shaft is larger than bore — requires pressing or heating to assemble |
Worked Example: Specifying a Bearing Fit
A 30mm bore is to accept a rolling element bearing with an interference fit. The bearing outer ring is nominally 30mm. Specifying the bore as 30 H7 gives a tolerance of +0.021 / 0.000 mm — the bore may be between 30.000mm and 30.021mm. Pairing this with a shaft specified as 30 p6 (+0.035 / +0.022 mm) gives an interference of between 0.001mm and 0.035mm — enough to retain the bearing under load without risk of fracturing the housing.
General Tolerances — ISO 2768
When individual tolerances are not stated, a general tolerance standard covers everything else. ISO 2768 is the most commonly used. Two grades are relevant for CNC machined parts:
ISO 2768 Linear Tolerance Limits by Grade
All values in millimetres (mm).
| Dimension range | Fine (f) | Medium (m) | Coarse (c) | Very coarse (v) |
|---|---|---|---|---|
| 0.5 – 3 mm | ±0.05 | ±0.1 | ±0.2 | — |
| 3 – 30 mm | ±0.05 | ±0.1 | ±0.2 | ±0.5 |
| 30 – 120 mm | ±0.1 | ±0.2 | ±0.5 | ±1.0 |
| 120 – 400 mm | ±0.15 | ±0.3 | ±0.8 | ±1.5 |
| 400 – 1000 mm | ±0.2 | ±0.5 | ±1.2 | ±2.5 |
Drawing Checklist
- Tight tolerances cost more to achieve and inspect — apply them only where the function of the part genuinely demands it.
- State a general tolerance standard in the title block so every un-toleranced dimension has a defined limit.
- Use standard ISO 286 fits for mating features — they are well understood and avoid ambiguity.
- For assemblies with tight fits, account for thermal expansion — materials with different coefficients will behave differently in service.
- Consider tolerance stack-up in assemblies — individual part tolerances accumulate, and the worst-case combination must still allow the assembly to function.