Why Should CNC Parts Machining Leave A Machining Allowance On The Workpiece

The thickness of the metal layer removed in each process of CNC parts processing is called the machining allowance between processes. For changes such as outer circle and hole, the machining allowance is measured from the diameter, so it is called symmetry allowance (i.e., bilateral allowance), that is, the thickness of the metal layer actually removed is half of the machining allowance on the diameter. The machining allowance of the plane is the one-sided allowance, which is the thickness of the metal layer that is actually removed.

The policy of leaving machining allowance on the workpiece is to remove the machining error and external defects left by the previous process, such as the cold hard layer, porosity, and sand layer outside the casting, the oxide scale, decarburization layer, and external cracks outside the forging, and the internal stress layer and external roughness after cutting. Then advance the accuracy of the workpiece and the external roughness. The size of the machining allowance has a great impact on the processing quality and the output power.

The machining allowance is too large, which not only increases the labor volume of machining, reduces the yield, but also increases the consumption of materials, objects and electricity, and CNC machining advances the processing cost. If the machining allowance is too small, it is not enough to eliminate all kinds of defects and errors in the previous process, and it is not enough to compensate for the clamping error during the processing of this process, forming waste. The extraction criterion is to ensure the quality of the premise of CNC machining, so that the margin is as small as possible. Generally speaking, the more refined the machining, the smaller the process allowance.

When determining the CNC machining sequence, it is also necessary to clarify whether the part is to be pre-machined before machining. Pre-machining is often done by ordinary machine tools. If the blank has high precision, the positioning is more reliable, or the machining allowance is sufficient and uniform, it can be processed directly on the CNC machine without pre-machining. At this time, the division of the CNC machine tool process should be considered according to the accuracy of the rough benchmark of the blank, so one process or several processes can be completed.