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In the process from blank to finished product, the total thickness of the metal layer cut off on a machined surface is referred to as the total processing margin of the surface. The thickness of the metal layer cut in each process is called the inter-process machining allowance. For rotating surfaces such as outer circles and holes, the machining allowance is considered from the diameter, so it is called the symmetry margin (ie, the bilateral margin), that is, the thickness of the actual cut metal layer is the machining allowance in diameter. half. The machining allowance of the plane is the unilateral margin, which is equal to the thickness of the metal layer actually removed. The purpose of leaving the machining allowance on the workpiece is to remove the machining errors and surface defects left in the previous process, such as the chilled layer on the surface of the casting, the pores, the sand layer, the scale on the surface of the forging, the decarburization layer, and the surface crack. , internal stress layer and surface roughness after cutting. Thereby improving the accuracy and surface roughness of the workpiece. The size of the machining allowance has a great influence on the processing quality and efficiency. Excessive machining allowance not only increases the labor of machining, reduces the rate, but also increases the material, tool and power consumption, and increases the processing cost. If the machining allowance is too small, it will not eliminate the various defects and errors of the previous process, and it will not compensate for the clamping error during the processing of this process, resulting in waste. The principle of selection is to make the margin as small as possible under the premise of ensuring quality. In general, the more finished, the smaller the process margin.