General Tolerances to ISO 2768: What Drawing Entries Mean
Knowledge · CNC machining
How do you read ISO 2768-mK?
The standard has two parts. Part 1 covers general tolerances for linear dimensions, chamfers and angles in four classes: f (fine), m (medium), c (coarse), v (very coarse). Part 2 covers form and position (straightness, flatness, perpendicularity, symmetry) in classes H, K and L. “ISO 2768-mK” therefore means: lengths and angles to class medium, form and position to class K, the widespread standard in general machine building; class c and v serve welded structures and steelwork.
Which values sit behind the classes?
| Nominal range | Class f | Class m | Class c |
|---|---|---|---|
| 0.5 to 3 mm | ±0.05 | ±0.1 | ±0.2 |
| over 3 to 6 mm | ±0.05 | ±0.1 | ±0.3 |
| over 6 to 30 mm | ±0.1 | ±0.2 | ±0.5 |
| over 30 to 120 mm | ±0.15 | ±0.3 | ±0.8 |
| over 120 to 400 mm | ±0.2 | ±0.5 | ±1.2 |
| over 400 to 1000 mm | ±0.3 | ±0.8 | ±2.0 |
| over 1000 to 2000 mm | ±0.5 | ±1.2 | ±3.0 |
| over 2000 to 4000 mm | — | ±2.0 | ±4.0 |
Often overlooked: the tolerance grows with the nominal size. A 2,000 mm dimension in class m may vary by ±1.2 mm. If a hole position on a frame must sit within ±0.2 mm, it needs an explicit entry, otherwise the general tolerance applies and any complaint fails.
When do general tolerances stop being enough?
For everything that fits, guides or seals: fits (H7/g6 and relatives), bearing seats, sealing faces and hole-pattern positions need deliberate individual tolerances, possibly with form and position entries and datums. The art is dosage: every tighter entry costs machining and measuring effort, as the article on CNC milling costs shows. The economical rule: general tolerance as the basis, individual tolerances only where function demands them.
What applies to welded structures?
A separate standard: ISO 13920 governs general tolerances for welded structures and is far more generous, because welding scatters by nature. The usual combination: welded assembly to ISO 13920, machined functional faces to ISO 2768 or with individual tolerances. That is exactly why functional faces are milled after welding; see machining welded structures. A practical tip: before release, ask of every dimension “is this one truly functional?”. Experience says fewer than a fifth are, and everything else may live in the general tolerance, which is what keeps parts affordable.
Frequently asked questions
What does ISO 2768-mK on a drawing mean?
All dimensions without individual tolerance follow class m (medium) for lengths and angles and class K for form and position. The entry sits in the title block and keeps the drawing lean.
Which tolerance applies to a 100 mm dimension at ISO 2768-m?
±0.3 mm, because 100 mm falls in the range over 30 to 120 mm of the medium class.
Does ISO 2768 replace fit specifications?
No. Fits, bearing seats and function-critical positions need deliberate individual tolerances. The general tolerance only covers non-critical dimensions.
Does ISO 2768 apply to welded structures?
The welded assembly as a whole usually follows the more generous ISO 13920. Machined functional faces of the assembly are then dimensioned to ISO 2768 or individually.