In everyday engineering drawings, many users like to cite phrases such as "unspecified tolerances are in accordance with ISO 2768-m" or "unspecified tolerances are in accordance with ISO 2768-mK". So what is ISO 2768?
ISO 2768 is divided into two parts: ISO 2768-1 and ISO 2768-2. ISO 2768-1 specifies unspecified tolerances for the linear and angular dimensions of parts, divided into four grades: high (f), medium (m), rough (c), and very rough (v). ISO 2768-2 specifies unspecified tolerances for the geometric shape of parts, with three grades: HKL. Simply put, ISO 2768-1 specifies dimensional tolerances, while ISO 2768-2 specifies shape tolerances.
The ISO2768-mK mentioned above, with the lowercase letter m, represents the m grade in the table of ISO2678-1 for the dimensions, angles, radii, bevels, etc. of the part, and the uppercase letter K represents the K grade in the table of ISO2768-2 for the geometric parameters of the part, such as straightness, perpendicularity, symmetry, and circular runout.
ISO 2768-1 standard
| Limiting dimensions of length and angle |
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According to ISO 2768-1
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| Nominal size |
Tolerance dimensions |
| scope |
f(fine) |
m(Intermediate) |
c(coarse) |
v(extremely thivk) |
| mm |
Tolerance mm |
Tolerance mm |
Tolerance mm |
Tolerance mm |
| 0.5-3 |
± 0.05 |
± 0.10 |
± 0.15 |
– |
| 3- 6 |
± 0.05 |
± 0.10 |
± 0.20 |
± 0.50 |
| 6- 30 |
± 0.10 |
± 0.20 |
± 0.50 |
± 1.00 |
| 30- 120 |
± 0.15 |
± 0.30 |
± 0.80 |
± 1.50 |
| 120- 400 |
± 0.20 |
± 0.50 |
± 1.20 |
± 2.50 |
| 400- 1000 |
± 0.30 |
± 0.80 |
± 2.00 |
± 4.00 |
| 1000- 2000 |
± 0.50 |
± 1.20 |
± 3.00 |
± 6.00 |
| 2000- 4000 |
– |
± 2.00 |
± 4.00 |
± 8.00 |
| Limits of angular dimensions |
| According to ISO 2768-1 |
| Nominal size |
Tolerance dimensions |
| scope |
f (fine) |
m (Intermediate) |
c (coarse) |
v (extremely thick) |
| mm |
Tolerance mm |
Tolerance mm |
Tolerance mm |
Tolerance mm |
| <10 |
± 10′ |
±1°30′ |
± 3° |
| 10- 50 |
± 30′ |
± 1° |
± 2° |
| 50- 120 |
± 20′ |
± 30′ |
± 1° |
| 120- 400 |
± 10′ |
± 15′ |
± 30′ |
| 400 or more |
± 5′ |
± 10′ |
± 20′ |
| Limiting dimensions of bending radius (radians) and slope height (angle). |
| According to ISO 2768-1 |
| Nominal size |
Tolerance dimensions |
| scope |
f (fine) |
m (Intermediate) |
c (coarse) |
v (extremely thick) |
| mm |
Tolerance mm |
Tolerance mm |
Tolerance mm |
Tolerance mm |
| 0.5-3 |
± 0.20 |
± 0.40 |
| >3-6 |
± 0.50 |
± 1.00 |
| >6 |
± 1.00 |
± 2.00 |
