加工余量
Machining allowance
由毛坯变成成品的过程中，在某加工表面上切除的金属层的总厚度称为该表面的加工总余量。每一道工序所切除的金属层厚度称为工序间加工余量。对于外圆和孔等旋转表面而言，加工余量是从直径上考虑的，故称为对称余量（即双边余量），即实际所切除的金属层厚度是直径上的加工余量之半。
In the process of turning a blank into a finished product, the total thickness of the metal layer removed on a machined surface is called the total allowance of the machined surface. The thickness of metal layer removed in each process is called inter-process margin. For rotating surfaces such as cylindrical surfaces and holes, the machining allowance is considered from the diameter, so it is called symmetrical allowance (bilateral allowance). That is to say, the thickness of the metal layer actually removed is half of the machining allowance on the diameter.
平面的加工余量则是单边余量，它等于实际所切除的金属层厚度。在工件上留加工余量的目的是为了切除上一道工序所留下来的加工误差和表面缺陷，如铸件表面冷硬层、气孔、夹砂层，锻件表面的氧化皮、脱碳层、表面裂纹，切削加工后的内应力层和表面粗糙度等。
The processing allowance of a plane is a unilateral allowance, which is equal to the thickness of the metal layer actually removed. The purpose of reserving machining allowance on workpiece is to remove the machining errors and surface defects left by the previous process, such as the surface hardening layer, blowhole, sand layer, oxide skin, decarburization layer, surface crack, internal stress layer and surface roughness after cutting.