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. Processing allowance can be divided into process allowance and total allowance. The thickness of the layer of metal that needs to be removed in a certain process is called the machining allowance of that process. The remaining amount that needs to be cut from the blank to the finished product is called the total remaining amount, which is equal to the sum of the remaining amounts of each process on the corresponding surface.
The purpose of leaving machining allowances on the workpiece is to remove the machining errors and surface defects left by the previous process, such as the cold hard layer, pores, and sand layers on the casting surface, the oxide scale, decarburization layer, and surface cracks on the forging , The internal stress layer and surface roughness after cutting. So as to improve the precision and surface roughness of the workpiece.
The amount of machining allowance has a great impact on the quality of processing and production efficiency. Excessive machining allowance not only increases the labor of mechanical processing and reduces productivity, but also increases the consumption of materials, tools and electricity, and increases processing costs. If the machining allowance is too small, it can not eliminate various defects and errors of the previous process, nor can it compensate for the clamping error during processing of this process, resulting in waste. The selection principle is to keep the margin as small as possible on the premise of ensuring quality. Generally speaking, the more finishing, the smaller the process margin.