Machining allowance

Machining allowance

In the process of turning the blank into the finished product, the total thickness of the metal layer removed on a machined surface is called the total machining allowance of the surface. The thickness of the metal layer removed by each process is called the interprocess processing allowance. For rotating surfaces such as outer circles and holes, the machining allowance is considered from the diameter, so it is called the symmetric allowance (that is, the bilateral allowance), that is, the actual thickness of the metal layer cut is half of the machining allowance on the diameter. The machining allowance of the plane is the unilateral allowance, which is equal to the actual thickness of the metal layer cut. The purpose of leaving processing allowance on the workpiece is to remove the processing errors and surface defects left by the previous process, such as the casting surface chill layer, porosity, sand layer, the oxide skin on the surface of the forging, the decarburization layer, the surface crack, the internal stress layer and the surface roughness after cutting. Thus, the precision and surface roughness of the workpiece are improved. The size of machining allowance has great influence on machining quality and production efficiency. The processing allowance is too large, not only increases the amount of labor in mechanical processing, reduces productivity, but also increases the consumption of materials, tools and power, and increases the processing cost. If the processing allowance is too small, it can neither eliminate the various defects and errors of the previous process, nor compensate the clamping errors during the processing of this process, resulting in waste products. The selection principle is to make the margin as small as possible under the premise of ensuring the quality. Generally speaking, the more finishing, the smaller the process margin.