Why should CNC machining leave a machining allowance on the workpiece:
The thickness of the metal layer removed in each process of CNC machining is called the machining allowance between processes. For the torsion surface of the outer circle and the hole, the machining allowance is considered from the diameter, so it is called the symmetry allowance (that is, the allowance for both sides), that is, the thickness of the actually removed metal layer is the machining allowance on the diameter Half. The machining allowance of the plane is the unilateral allowance, which is the thickness of the metal layer actually removed.
The goal of leaving a machining allowance on the workpiece is to remove the machining errors and external defects left by the previous process, such as the chilled layer, pores, sand layer on the outside of the casting, and the oxide scale, decarburized layer, and external cracks on the outside of the forging. , The internal stress layer and external roughness after cutting. Then improve the accuracy and surface roughness of the workpiece. The size of the machining allowance has a great influence on the processing quality and production power.
Excessive machining allowance not only increases the amount of machining labor and reduces productivity, but also increases material, target and power consumption. CNC machining increases the processing cost. If the machining allowance is too small, it will not be able to eliminate various defects and errors in the previous process, nor can it compensate for the clamping error during the processing of this process, which constitutes a waste product. The selection criterion is to make the margin as small as possible under the condition of ensuring the quality of CNC machining. 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 need to be pre-processed before machining. Pre-processing is often completed by ordinary machine tools. If the precision of the blank is high, the positioning is more reliable, or the machining allowance is sufficient and uniform, it can be processed directly on the CNC machine without pre-processing. At this time, it is necessary to consider the distinction of CNC machine tool processes according to the accuracy of the rough rough reference, so one process may be divided into several processes to complete.
