Machining allowance

Material allowance on the raw casting which is removed during machining.

This refers to those allowances required on surfaces to be machined in order to remove casting-related inclusions in the casting skin or due to the effects of surface roughness and geometrical inaccuracies and to correct production-related dimensional variations. Machining allowances are standardized, e.g. in DIN ISO 8062 “General tolerances and machining allowances” for GJL and GJS.

The standard comprises 10 degrees of required machining allowances (A-K). Unless otherwise indicated, the required machining allowance applies to the entire raw casting, i.e. only one value is defined for all surfaces to be machined. This value must be chosen from the relevant dimensional range according to the largest overall dimension of the finished casting after finishing. The necessary machining allowances (for information only) for machine moldings made of flake graphite cast iron (EN-GJL) and nodular graphite cast iron (EN-GJS) are represented by the classes E–G. Generally, cast iron requires minimum machining allowances of 2.5mm due to technical reasons. In addition, the height of the allowance depends on the material, the largest nominal dimension of the casting and the type of machining.