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Why CNC machining should leave machining allowance on the workpiece

The thickness of the metal layer removed by CNC machining in each process is called the machining allowance between processes. For the torsion of the outer circle and the hole, the machining allowance is considered from the diameter, so it is called the symmetric allowance (that is, the bilateral allowance), that is, the thickness of the metal layer cut off is actually the machining allowance on the diameter half. The machining allowance of the plane is the one-sided allowance, which is the thickness of the metal layer cut off in reality.

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The goal of leaving the machining allowance on the workpiece is to remove the machining deviations and external shortcomings left by the previous process, such as the cold layer, pores, sand inclusions on the outside of the casting, the scale, decarburization layer, and external cracks on the outside of the forging , Inner stress layer and outer roughness after cutting. Thereby improving the accuracy and roughness of the workpiece. The size of the processing allowance has a greater impact on processing quality and production efficiency.

   The cnc machining allowance is too large, which not only increases the labor of mechanical processing, reduces productivity, but also increases data, objects, and power consumption. CNC processing improves the processing cost. If the processing allowance is too small, it is impossible to eliminate the various shortcomings and deviations of the previous process, and to compensate for the clamping deviations during the processing of this process, forming waste products. The selection criterion is to make the margin as small as possible under CNC processing under the premise of ensuring quality. Generally speaking, the more finishing, the smaller the process margin.

  When determining the CNC machining sequence, it is also necessary to clarify whether the parts are to be pre-machined before machining. Pre-machining is often done by ordinary machine tools. If the blank precision is high, the positioning is more reliable, or the machining allowance is sufficient and uniform, then it can be processed directly on the CNC machine without pre-processing. At this time, it is necessary to consider the division of CNC machine tool processes according to the accuracy of the rough rough reference, which can be completed in one process or divided into several processes.

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