In order to process qualified parts, the thickness of the layer of metal that must be cut from the blank is called the machining allowance. Machining allowance can be divided into process allowance and total allowance. The thickness of the metal layer that needs to be removed in a process is called the machining allowance of the process. The total allowance that needs to be removed from the blank to the finished product is called the total allowance, which is equal to the sum of the allowances of each process on the corresponding surface.
The purpose of the machine tool to leave the machining allowance on the workpiece is to remove the machining errors and surface defects left by the previous process, such as the chill layer, pores, sand inclusion layer on the surface of the casting, oxide scale, decarburization layer, surface Cracks, 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.
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