A tolerance tells the manufacturer how far a dimension may vary while the part still works. Too loose and parts do not fit; too tight and they cost more than they need to. This guide covers the common systems and how to use them on a drawing.
General tolerances: ISO 2768
ISO 2768 sets default tolerances for dimensions that have no individual tolerance on the drawing. Part 1 covers linear and angular dimensions in four classes: f (fine), m (medium), c (coarse) and v (very coarse). Part 2 covers general geometrical tolerances (straightness, flatness, perpendicularity, symmetry, circular runout) in classes H, K and L. A note such as “ISO 2768-mK” on the drawing sets the defaults for the whole part.
| Nominal size (mm) | ISO 2768-f | ISO 2768-m |
|---|---|---|
| 0.5 to 3 | ±0.05 | ±0.1 |
| over 3 to 6 | ±0.05 | ±0.1 |
| over 6 to 30 | ±0.1 | ±0.2 |
| over 30 to 120 | ±0.15 | ±0.3 |
| over 120 to 400 | ±0.2 | ±0.5 |
| over 400 to 1000 | ±0.3 | ±0.8 |
Linear tolerances from ISO 2768-1 for the fine and medium classes.
Individual tolerances and fits
Dimensions that matter for function, such as bearing bores, mating diameters and locating holes, get their own tolerance or an ISO fit (for example H7 for a bore). These are the dimensions to discuss with your supplier.
GD&T
Geometric dimensioning and tolerancing (ASME Y14.5 or ISO GPS) controls form, orientation and location relative to datums: flatness of a sealing face, position of a hole pattern, runout of a shaft journal. It often describes function better than plus-minus dimensions and can allow looser size tolerances where position is what matters.
How tolerances affect cost
- As a rough guide (typical): ±0.1 mm is routine CNC work, ±0.02–0.05 mm needs finishing passes, stable fixturing and more inspection, and much below ±0.01 mm usually means grinding.
- Each step tighter needs slower cuts, temperature-stable conditions and more measuring time.
- Very tight tolerances may need grinding, honing or lapping after machining.
- Finishes such as anodizing or plating add thickness that must be allowed for on tight fits.
A sensible way to tolerance a drawing
- Set a general tolerance in the title block (for example ISO 2768-mK).
- Give individual tolerances or fits only to functional features.
- Use datums that match how the part is located in the assembly.
- Ask your supplier to review critical tolerances before production. See what drives CNC machining cost.
Frequently asked questions
What does ISO 2768-mK mean?
Medium class (m) for linear and angular dimensions without individual tolerances, and class K for general geometrical tolerances such as flatness and perpendicularity.
Is ±0.01 mm possible?
On suitable features, materials and sizes, yes, but it needs careful machining and inspection and costs more. Use it only where the function requires it.
Do tolerances apply before or after finishing?
State it on the drawing. By default dimensions apply to the finished part, so coating thickness must be allowed for during machining.
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