In forging production, due to the influence of various factors such as the difference in final forging temperature, forging equipment, tooling accuracy, and differences in the technical level of operators, the actual size of the forging cannot reach the nominal size, and a certain deviation is allowed. This deviation is called forging tolerance.

Parts with forging sizes larger than the nominal size are called upper deviations (positive deviations), and parts with sizes smaller than the nominal size are called lower deviations (negative deviations).

All forging parts, whether machined or not, should be marked with forging tolerances. Usually, the forging tolerance is about 1/4 to 1/3 of the machining allowance.

The specific values of forging machining allowances and tolerances can be found in relevant manuals or implemented according to factory standards. Special circumstances can also be negotiated with machining technicians.