Tolerances & Fits 101
ISO 286 fits, general tolerances, and how to specify mating parts that assemble
Tolerancing is the contract between design and manufacturing: a general tolerance (ISO 2768-m, ±0.127 mm) covers most features, ISO 286 defines hole/shaft fit systems (clearance, transition, interference) by IT grade, and GD&T (ASME Y14.5 / ISO 1101) controls geometry beyond size. Fits — H7/g6 sliding, H7/h6 location, H7/p6 press — are the standard vocabulary for mating parts.
1. General tolerances
ISO 2768-1 defines four classes (f/m/c/v) for linear and angular dimensions: class m (medium, ±0.1 mm for 0.5–3 mm, ±0.2 mm for 3–6 mm, scaling up) is the manufacturing default. Class f (fine) for precision work; c/v for fabrication. State the class on the drawing — an unspecified tolerance invites interpretation.
For machined parts, ±0.127 mm (≈ ±0.005 in) is the practical standard; precision features hold ±0.025 mm; below ±0.01 mm requires grinding/EDM. Every tightening multiplies cost, so tolerance only what mates.
2. ISO 286 fits
ISO 286 defines fundamental tolerances (IT grades IT01–IT18) and deviations (A–ZC holes, a–zc shafts). Standard fits: H7/g6 loose sliding, H7/h6 location, H7/js6 transition, H7/k6 light press, H7/p6 press fit. Each specifies hole and shaft deviations for guaranteed clearance or interference.
Use the standard fit table rather than inventing tolerances: H7/g6, H7/h6, H7/p6 cover 90% of mechanical fits, and machines/inspection know them cold. Temperature and finish (Ra) change effective fits — a pressed bearing bore should account for expansion.
3. Stack-up thinking
Tolerances accumulate through an assembly: a 3-plate stack with ±0.1 mm per plate stacks to ±0.3 mm at the end. Use arithmetic (worst case) for critical clearances and statistical (root-sum-square) for mass production; GD&T position at MMC is the standard way to give assembly bonus tolerance.
Keep the tightest tolerances on the datum scheme, not on every feature: position holes at ⌀0.1 mm to the datum and let the rest run general. This is both cheaper and more reliable than blanket precision.
4. Specifying on the drawing
Write: general tolerance class (ISO 2768-m), fits by standard symbol (H7/g6), GD&T frames for critical geometry (position, flatness, perpendicularity), and surface finish per face. Declare ASME vs ISO so inspection agrees.
When quoting, the platform applies your tolerance class to the engine; precision classes raise the multiplier visibly — verify the class matches the function before ordering.
Quick reference
| Fit | Hole | Shaft | Use |
|---|---|---|---|
| Loose running | H8 | f7 | Thermal expansion, oil film |
| Sliding | H7 | g6 | Guided sliding, spindles |
| Location | H7 | h6 | Bolt patterns, alignment |
| Transition | H7 | js6/k6 | Balanced, light press |
| Press | H7 | p6 | Permanent assembly |
| Interference | H7 | s6/r6 | High torque transfer |
Sources & references
- ISO 286-1:2010 — Fits
- ISO 2768-1:1989 — General tolerances
- ASME Y14.5-2018 / ISO 1101 — GD&T
- Xometry Learning Center — Tolerances & Fits (public engineering reference)