The total thickness of the metal layer cut off on the outside of a certain machining process during the process of turning a blank into a finished product is called the total machining allowance on that outside. The thickness of the metal layer cut during each CNC machining process is called the machining allowance between processes. When it comes to changing the outer surface of the outer circle and holes, the machining allowance is considered based on the diameter, so it is called symmetrical allowance (i.e. bilateral allowance), which means that the actual thickness of the metal layer cut is half of the machining allowance on the diameter. The machining allowance of a plane is the unilateral allowance, which is the actual thickness of the metal layer cut.

The policy of leaving machining allowance on the workpiece is to eliminate the machining errors and external defects left by the previous process, such as the cold hard layer, porosity, sand inclusion layer on the outside of the casting, the oxide skin, decarburization layer, external cracks on the outside of the forging, the internal stress layer and external roughness after cutting, etc., in order to improve the accuracy and external roughness of the workpiece. The size of the machining allowance has a significant impact on the machining quality and production power. Excessive machining allowance not only increases the amount of labor involved in machining, reduces production rates, but also increases material and power consumption. If the machining allowance is too small, it cannot eliminate the various shortcomings and errors of the previous process, nor can it compensate for the clamping errors during the processing of this process, forming waste. The selection criteria are to ensure the quality of Shanghai CNC processing and minimize the margin as much as possible. Generally speaking, the more precision machining, the smaller the process margin.