Machining allowance refers to the difference between the actual size of the blank and the drawing size of the part. Generally, the machining of the part can only meet the requirements of the drawing after roughing and finishing.

Therefore, the total cnc machining allowance of the part should be equal to the sum of the machining allowance of the intermediate process. When selecting the processing allowance, the following factors should be considered.

• (1) The size of the parts is different. The deformation caused by cutting force and internal stress is also different. Generally, the larger the workpiece, the larger the deformation, so the machining allowance of the workpiece is correspondingly larger.
• (2) The parts must be deformed after heat treatment. Therefore, such parts should be adapted to increase the processing margin a little.
• (3) The machining method, clamping method and rigidity of the process equipment will also cause deformation of the parts, so the machining allowance should also be considered.

The cutting amount mainly includes the spindle speed (cutting speed), the feed amount (feed speed) and the back-feeding amount. The amount of cutting amount directly affects the performance of the machine tool, tool wear, processing quality and production efficiency. Reasonable selection of cutting amount is of great significance for giving full play to machine tool performance and cutting performance, improving cutting efficiency, and reducing processing costs.
Determine the amount of back-eaten knife. The choice of back-feeding amount (cutting speed) should be determined according to the machining allowance. It is mainly restricted by the rigidity of the machine tool, tool and workpiece system. When the system rigidity permits, try to choose a larger amount of back-eating knife. When roughing, without affecting the machining accuracy, the amount of back-feeding can be equal to the machining allowance of the part, which can reduce the number of passes.