Why should CNC machining leave a machiningallowance on the workpiece
The thickness of the metal layer removed in each process of CNC turning is called the machiningallowance between processes. For the torsion surface of the outer circle andthe hole, the machining allowance is considered from the diameter, so it iscalled the symmetry allowance (that is, the bilateral allowance), that is, thethickness of the metal layer that is actually removed is the diameter of themachining allowance. half. The machining allowance of the plane is theunilateral allowance, which is the thickness of the metal layer that isactually removed
The goal of leaving a machining allowanceon the workpiece is to remove the machining deviation and external defects leftby the previous process, such as the chilled layer, pores, and sand layer onthe outside of the casting, and the oxide scale, decarburized layer, andexternal cracks on the outside of the forging. , The internal stress layer andexternal roughness after cutting. Thereby improving the accuracy and surfaceroughness of the workpiece. The size of the machining allowance has a greatinfluence on the processing quality and production efficiency.
The machining allowance is too large, which not only increases thelabor of machining, reduces the productivity, but also increases theconsumption of materials, objects and electricity. CNC medical machining increases the processing cost.If the machining allowance is too small, it will not be able to eliminate thevarious shortcomings and deviations of the previous process, nor can itcompensate for the clamping deviation during the processing of this process,resulting in waste products. The selection criterion is to make the margin assmall as possible under the premise 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 toclarify whether the parts need to be pre-processed before processing.Pre-processing is often done by ordinary machine tools. If the precision of theblank is higher, the positioning is more reliable, or the machining allowanceis sufficient and uniform, it is not necessary to perform pre-processing, anddirectly process it on the CNC machine tool. At this time, the division of CNC numerical contro processes shouldbe considered according to the accuracy of the rough reference of the blank,which can be completed in one process or divided into several processes.
