In the process of changing from a blank to a finished product, the total thickness of the metal layer cut on a machined surface is called the total machining allowance of the surface. The thickness of the metal layer removed in each process is called the machining allowance between processes. For rotating surfaces such as outer circles and holes, the machining allowance is considered from the diameter, so it is called symmetrical allowance (that is, bilateral allowance), that is, the actual thickness of the metal layer removed is the addition of the machining allowance on the diameter. Half. The machining allowance of the plane is the unilateral allowance, which is equal to the actual thickness of the metal layer removed. The purpose of leaving a machining allowance on the workpiece is to remove the machining errors and surface defects left by the previous process, such as chilled layer, pores, sand inclusion layer on the surface of the casting, oxide scale, decarburization layer, and surface cracks on the surface of the forging. , the internal stress layer and surface roughness after machining. Thereby improving the accuracy and surface roughness of the workpiece. The size of the machining allowance has a great influence on the machining quality and production efficiency. Excessive machining allowance not only increases the labor of machining and reduces productivity, but also increases the consumption of materials, tools and electricity, and increases the processing cost. If the machining allowance is too small, it can neither eliminate various defects and errors in the previous process, nor compensate for the clamping error during processing in this process, resulting in waste products. The selection principle is to make the margin as small as possible on the premise of ensuring the quality. Generally speaking, the more finishing, the smaller the process allowance.