While there is no fixed formula for calculating machining allowances, a general guideline accounting for its main factors may be as follows:
Machining Allowance = Surface Variation + Tool Access Margin + Finish Requirement Buffer
Let us consider the case of the engineering drawing below and make a rough calculation of the machining allowance. Assuming the part is to be machined from Aluminum a conservative machine allowance of 0.5-1 mm would be OK, considering its simple geometry and no tolerancing requirements on the main dimensions.
For positioning the hole, however, larger allowances might be necessary. There is a tolerance on the position (±0.05mm) and a unilateral tolerance on the hole diameter (+0.1 – +0.3mm); the surface variation allowance can be 0.5 mm. There is no problem with tool access, so that allowance can be ignored. However, we need the hole to be smooth and might use finishing operations with reaming or polishing. Thus, the finish requirement buffer can be 0.1 mm.
Therefore, the machining allowance for positioning and drilling the hole would be:
Machining Allowance = Default (0.5) + Surface Variation (0.5) + Finish Requirement Buffer (0.1) = 1.1 mm
