Machining Allowance (Stock for Finish)
Definition
The total thickness of metal removed from a surface in turning a blank into a finished part is called the total machining allowance.
The metal layer removed in one individual operation is the operation (inter-pass) allowance.
Allowance types
Rotational features (shafts, holes): the value is specified on the diameter, so the actual radial stock is one-half of the quoted bilateral allowance.
Flat surfaces: the allowance is unilateral and equals the actual depth of cut.
Purpose of leaving allowance
Eliminate defects and errors inherited from previous stages: casting skin, chill layer, sand inclusions, oxide scale, decarburised skin, forging cracks, residual stresses and roughness from prior cuts.
Provide enough material for the current set-up to achieve the required dimensional accuracy and surface finish.
Influence on manufacturing
Excessive allowance increases cutting time, tool wear, energy consumption and cost; insufficient allowance cannot wipe out preceding errors or accommodate locating/clamping variations, leading to scrap.
Selection principle
Choose the smallest allowance that still guarantees the final quality. In general, the finer the operation, the smaller the allowance.
Scope of application
The concept applies to all metal-cutting processes on cast, forged, sintered or pre-machined blanks where at least one subsequent machining operation is required.
