Understanding General Tolerance: ISO 2768 & GB/T 1804
According to GB/T 1804-2000 national standard, dimensions without specified tolerances in mechanical processing (formerly called "free tolerances") are now uniformly referred to as "general tolerances". This standard applies to metal cutting and stamping processes, while non-metal processes may use it as reference. Technical drawings must indicate the tolerance grade standard in the title block (e.g., GB/T 1804-m).
What is General Tolerance (Open/Free Tolerance)?
NOTES ON GENERAL TOLERANCES (formerly Free Tolerances):
1. Application Scope:
Applies exclusively to machined surfaces
Not to be marked on working drawings
2. Deviation Principles:
Shaft dimensions: unilateral (-) deviation
Holes, hole depths, slot widths/depths/lengths: unilateral (+) deviation
All other dimensions: bilateral (±) deviation
- Exceptions requiring unilateral deviation must be explicitly specified
3. GB 1800-79 Revision Highlights:
- 20 tolerance grades (IT01, IT0, IT1 to IT18)
- IT = ISO Tolerance, with descending precision from IT01 to IT18
4. GB 1804-79 Key Provisions:
a) Applies to both machined and non-machined metal parts
b) Unspecified tolerances shall follow technical documentation
c) Default grades: IT12 to IT18 with these conventions:
- Holes: H (+) system
- Shafts: h (-) system
- Lengths: ±IT (Js/js)
- Optional: Uniformly apply Js/js system
Technical Notes:
1. "Free tolerance" is deprecated per current GB/T 1804-2000
2. Modern standards use "general tolerances for unspecified dimensions"
3. Current practice requires title block notation (e.g. "GB/T 1804-m")
Tolerance Classes and Grades (f, m, c, v)
Deviation value
According to GB 1804-2000 (ISO 2768-1:1989), general tolerances are classified into four grades:f (fine), m (medium), c (coarse), and v (very coarse). The tables below specify limit deviations for linear and angular dimensions.
General Tolerance Charts (Linear, Radius, Angular)
The following are tolerance specifications for three different dimension types under general tolerances:
Table 1: Limit Deviation Values for Linear Dimensions with Unspecified Tolerances
| Tolerance class | Basic Size Ranges(MM) | |||||||
| Dimension | 0.5~3 | 3～6 | 6～30 | 30～120 | 120～400 | 400～1000 | 1000～2000 | 2000～4000 |
| f(fine) | ±0.05 | ±0.05 | ±0.1 | ±0.15 | ±0.2 | ±0.3 | ±0.5 | - |
| m(Medium) | ±0.1 | ±0.1 | ±0.2 | ±0.3 | ±0.5 | ±0.8 | ±1.2 | 2 |
| c(coarse) | ±0.2 | ±0.3 | ±0.5 | ±0.8 | ±1.2 | ±2 | ±3 | ±4 |
| v(very coarse) | - | ±0.5 | ±1 | ±1.5 | ±2.5 | ±4 | ±6 | ±8 |
Table2：Limit Deviations for Fillet Radii and Chamfer Heights
| Tolerance class | Basic Size Ranges(MM) | |||||||||||||
| Dimension | 0.5~3 | 3～6 | 6～30 | 30～ | ||||||||||
| f(fine) | ±0.2 | ±0.5 | ±1 | ±2 | ||||||||||
| m(Medium) | - | - | - | - | ||||||||||
| c(coarse) | ±0.4 | ±1 | ±2 | ±4 | ||||||||||
| v(very coarse) | - | - | - | - | ||||||||||
Table 3: Limit Deviation Values for Angular Dimensions
| Tolerance class | Basic Size Ranges(MM) | ||||
| Length Ranges | ≤10 | 10～50 | 50～120 | 120～400 | 400～ |
| f(fine) | ±1° | ±30' | ±20' | ±10' | ±5' |
| m(Medium) | - | - | - | - | - |
| c(coarse) | ±1°30' | ±1° | ±30' | ±15' | ±10' |
| v(very coarse) | ±3° | ±2° | ±1° | ±30' | ±20' |
Notes:
1. For angular dimensions, the length refers to the shorter side length. For conical angles, use the generatrix length.
2. Medium (m) and very coarse (v) grades for Table 2/3 follow corresponding values from Table 1.
3. Dash (-) indicates the grade isn't recommended for that size range.
FAQ
What is tolerance class m?
Tolerance class "m" stands for "medium" grade in the ISO 2768-1 (or GB/T 1804) standard. It is the most common tolerance class used in standard CNC machining and metal fabrication.
What is the difference between ISO 2768-f and ISO 2768-m?
Class "f" is for "fine" precision (tighter tolerances), while class "m" is for "medium" precision. For example, for a 10mm dimension, class f allows ±0.05mm error, while class m allows ±0.1mm error.
What does "General Tolerance" mean on a drawing?
It refers to the standard tolerance applied to all dimensions that do not have a specific tolerance annotation directly next to them. It simplifies the drawing and inspection process.
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