The most important thing in the design and manufacture of each high-quality CNC machined part is the tolerance. How to clarify the tolerance value of a specific CNC machined part? This is inseparable from the help of ISO international standards. This article will use ISO 2768 standard and ISO 2768 tolerance chart to introduce you relevant knowledge.
What is the ISO 2768 tolerance standard?
The ISO 2768 tolerance standard is an international tolerance standard specified by the ISO International Organization for Standardization. It is mainly divided into one part and two parts. The international standard ISO2768-1 was drafted by the ISO/TC3 “Limits and Coordination” Technical Committee. ISO 2768-Part One (First Edition) and ISO 2768-Part Two (1989 Edition) replace ISO 2768 (1973 Edition) together.
ISO2768 consists of the following parts under the main heading “General Tolerance”:
Part 1: Tolerances for linear and angular dimensions without tolerances, divided into 4 levels (high precision f medium m rough c very rough v)
Part 2: The geometric tolerances of elements without tolerances are divided into 3 different grades of shape and position tolerance (H, K, L)
What do the letters in ISO 2768-mK, ISO 2768-fH, etc. mean?
It is a tolerance standard carried out by the grade of m in the first part and K in the second part. It is usually implemented by the m-level specified in ISO 2768-1 and the K-level tolerance level specified in ISO 2768-2. The same is true for ISO 2768–fH, ISO 2768–ck, ISO 2768–vL, etc.
How to choose tolerance standard?
Regarding this issue, you can determine the tolerance level you need based on your machining needs and the manufacturer’s production capacity. Normally, ISO 2768-fH is the ideal choice for CNC machining.
ISO 2768-1 range (general tolerances for linear and angular dimensions)
Suitable for the following sizes:
a) Linear dimensions (such as external dimensions, internal dimensions, step length, diameter, radius, distance, external radius and the height of the chamfer of the hemming);
b) Angular dimensions, including angular dimensions that are not usually indicated, such as right angles (90°). Unless referring to ISO 2768-2 or the angle of a unified polygon;
c) Linear and angular dimensions produced by processing assembled parts.
Not applicable to the following sizes:
a) Refer to the linear and angular dimensions covered by other general tolerance standards;
b) Auxiliary dimensions marked in brackets;
c) The theoretically accurate size shown in the rectangular frame.
ISO 2768-2 tolerance range (geometric tolerance of elements without tolerance)
Suitable for the following sizes:
a) Linear dimensions (such as outer dimensions, inner dimensions, step dimensions, diameter, radius, distance, rounding radius and chamfer height).
b) Angular dimensions, including angle dimensions for which no angle values are usually stated, such as right angles (90) (except for the angles specified in the second part of ISO 2768) or equal polygonal angles.
c) Linear and angular dimensions of the machined assembly.
Not applicable to the following sizes:
a) Linear and angular dimensions of other general CNC machining parts tolerance standard sand
b) Reference size in brackets
c) The theoretical correct size of the rectangular frame
ISO2768 tolerance standard table
| Limit dimensions of length and angle dimensions | ||||
| According to ISO 2768-1 | ||||
| Nominal size | Tolerance size | |||
| Scope | f (fine) | m (Intermediate) | c (coarse) | v (extremely thick) |
| 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 |
| Limit size of angular size | ||||
| According to ISO 2768-1 | ||||
| Nominal size | Tolerance size | |||
| 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 以上 | ± 5′ | ± 10′ | ± 20′ | |
| Limit size of bending radius (radian) and slope height (bevel angle) | ||||
| According to ISO 2768-1 | ||||
| Nominal size | Tolerance size | |||
| 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 | ||