Why should CNC machining leave machining allowance on workpieces?

The thickness of the metal layer cut in each process of CNC processing is called the inter-process processing allowance. For the twisting outside of the outer circle and hole, the processing allowance is considered from the diameter, so it is called the symmetric allowance (i.e., the bilateral allowance), that is, the thickness of the actually cut metal layer is the processing allowance in the diameter. Half. The processing allowance of the plane is the single-sided allowance, which is the thickness of the metal layer cut by reality.

The goal of leaving machining allowance on the workpiece is to cut off the processing deviations and external shortcomings left by the previous process, such as the cold hard layer, air holes, sand sandwich layer outside the casting, the scale, decarbonization layer outside the forging, and cracks outside the forging , the inner stress layer and outer roughness after cutting. This improves the accuracy and outer roughness of the workpiece. The size of the processing allowance has a great impact on processing quality and production efficiency.

The excessive processing allowance not only increases the labor volume of mechanical processing, reduces productivity, but also increases data, objects and power consumption, and CNC processing improves processing cost. If the processing allowance is too small, it will not be able to eliminate all kinds of shortcomings and deviations of the previous process, nor will it be able to compensate for the clamping deviation during processing of this process, resulting in waste products. The extraction criterion is to ensure the quality of CNC processing to make the margin as small as possible. Generally speaking, the more thorough the process, the smaller the process allowance.

When determining the CNC machining sequence, it is also necessary to first clarify whether the parts need to be pre-processed before processing. Pre-processing is often done by ordinary machine tools. If the blank has high precision, reliable positioning, or the processing allowance is sufficient and uniform, pre-processing can be done directly on CNC machine tools without pre-processing. At this time, the division of CNC machine tools must be considered based on the accuracy of the rough reference, and it can be completed in one process or into several processes.