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).
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")
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.
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.