The thickness of the metal layer cut off in each process of CNC machining is called inter process machining allowance. For the torsioned outer surfaces such as the outer circle and hole, the machining allowance is considered from the diameter, so it is called symmetrical allowance (i.e. bilateral allowance), that is, the thickness of the metal layer actually cut is half of the machining allowance on the diameter. The machining allowance of the plane is the unilateral allowance, which is the thickness of the metal layer cut off in reality.
The goal of leaving machining allowance on the workpiece is to remove the machining deviation and external defects left by the previous process, such as the external cold hard layer, air hole, sand inclusion layer of the casting, oxide skin, decarburization layer, external cracks outside the forging, internal stress layer and external roughness after cutting, etc. So as to improve the precision and external roughness of the workpiece. The size of machining allowance has a great impact on the processing quality and production efficiency.
The excessive machining allowance not only increases the amount of labor for machining, reduces productivity, but also increases the consumption of materials, objects and electricity. CNC machining improves the processing cost. If the machining allowance is too small, the defects and deviations of the previous process cannot be eliminated, and the clamping deviation of this process cannot be compensated, resulting in waste products. The selection criterion is to make the allowance as small as possible under the premise of ensuring the quality of CNC processing. Generally speaking, the more finishing, the smaller the process margin.
When determining the CNC processing sequence, it is also necessary to determine whether the parts need to be pre processed before processing. The pre machining is usually completed by ordinary machine tools. If the blank precision is high, the positioning is reliable, or the machining allowance is sufficient and uniform, it can be directly processed on the CNC machine tool without pre processing. At this time, it is necessary to consider the division of NC machine tool processes according to the accuracy of the rough benchmark of the blank, which can be completed in one process or divided into several processes.
