The thickness of the metal layer cut off in each CNC machining process is called the machining allowance between processes. For twisted outer surfaces such as outer circles and holes, the machining allowance is considered based on the diameter, hence 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 thickness of the metal layer that is cut in reality.

The goal of leaving machining allowance on the workpiece is to remove the machining deviation and external defects left by the previous process, such as the cold hard layer, pores, and sand layer on the outside of the casting, the oxide skin, decarburization layer, and external cracks on the outside of the forging, as well as the internal stress layer and external roughness after cutting. Thus improving the accuracy and external roughness of the workpiece. The size of processing allowance has a significant impact on processing quality and production efficiency.

Excessive machining allowance not only increases the labor volume of mechanical processing, reduces productivity, but also increases the consumption of materials, objects, and electricity. CNC machining has improved processing costs. If the machining allowance is too small, it cannot eliminate the various shortcomings and deviations of the previous process, nor can it compensate for the clamping deviation during the processing of this process, resulting in waste. The selection criterion is to minimize the margin while ensuring quality during CNC machining. Generally speaking, the more precision machining, the smaller the process margin.

When determining the CNC machining sequence, it is also necessary to first clarify whether the parts need to undergo pre processing before machining. Pre processing is often completed by ordinary machine tools. If the accuracy of the blank is high, the positioning is also reliable, or the machining allowance is sufficient and uniform, it is not necessary to perform pre processing and can be directly processed on the CNC machine tool. At this point, the division of CNC machine tool processes should be considered based on the accuracy of the rough benchmark of the blank, which can be completed in one process or divided into several processes.