Forging tolerances refer to the acceptable range of deviation for a forging's dimensions, shape, weight, and surface characteristics. Since forging involves heating metal, applying extreme pressure, and subsequent cooling, deviations caused by factors such as thermal contraction, die wear, and material flow are inevitable.

To ensure parts function correctly and assemble properly, forging tolerances are categorized into the following types:

1. Dimensional Tolerances

This is the most fundamental type of tolerance, defining the allowable deviation for linear dimensions such as length, width, diameter, and thickness. For example, a part designed with a length of 100 mm might have an allowable deviation of ±0.3 mm.

2. Geometric (Shape) Tolerances

These tolerances control the physical shape of the part rather than its linear dimensions. They include specifications such as flatness, straightness, roundness, and cylindricity—parameters significantly affected by material flow and uneven cooling during the forging process.

3. Positional Tolerances

Positional tolerances refer to the allowable deviation in the relative positions of specific features on the forging, such as the center-to-center distance between holes, slots, or bosses.

4. Misalignment (Offset) Tolerances

Misalignment tolerances refer to the specific allowable limits for the offset between the upper and lower dies during forging. Such lateral offsets can result from equipment wear or lateral thrust; this tolerance ensures the part remains functional even if the dies are not perfectly aligned.

5. Surface Finish Tolerances

Forged surfaces are typically rougher than machined surfaces due to scale formation and die texture. Surface tolerances define the acceptable range of surface roughness to ensure the part meets functional or aesthetic requirements prior to any secondary processing.

6. Weight Tolerances

Variations in initial billet size, material loss due to oxidation, and the formation of flash can all lead to deviations in the forging's final weight. Weight tolerances specify the acceptable range of positive or negative deviation between the actual weight and the nominal weight.

7. Draft Angle Tolerances

Draft angles refer to the taper applied to the vertical surfaces of a forging to facilitate its removal from the die. Tolerances for these angles account for errors in die manufacturing and material shrinkage.