Principles of CNC machining process

CNC machining process

Process planning refers to the entire process, and cannot be judged by the nature of a process and the processing of a surface. For example, some positioning datums need to be processed very accurately in the semi-finishing stage or even in the roughing stage. Sometimes in order to avoid dimensional chain conversion, in the finishing stage, semi-finishing of some minor surfaces can also be arranged.

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When the processing method and processing stage of the surface of the part are determined, the processing of each surface in the same processing stage can be combined into several working steps.

1. Method of dividing processing steps

Parts processed on CNC machine tools are generally divided into processes according to the principle of process concentration. The methods of division are as follows:

1) Divided by the tools used

Taking the process completed by the same tool as a process, this division method is suitable for the situation where the workpiece has many surfaces to be machined. Processing plants often use this method to complete.

2) Divided by the number of workpiece installations

The process that can be completed by one-time clamping of parts is regarded as a process. This method is suitable for parts with few processing contents. Under the premise of ensuring the processing quality of the parts, all the processing contents can be completed in one clamping.

3) Divided by roughing and finishing

The part of the process completed in the roughing process is regarded as one process, and the part of the process process completed in the finishing process is regarded as another process. This division method is suitable for parts that have strength and hardness requirements, need heat treatment or parts with high precision requirements, need to effectively remove internal stress, and large deformation of parts after processing, and need to be divided according to roughing and finishing.

4) According to the processing part, the part of the process that completes the same profile is regarded as a process.

For parts with many machining surfaces and more complex, the sequence of CNC machining, heat treatment and auxiliary processes should be reasonably arranged, and the connection problem between the processes should be solved. 2. The principle of the division of processing steps The parts are composed of multiple surfaces, these surfaces have their own precision requirements, and each surface also has corresponding precision requirements. In order to meet the design accuracy requirements of the parts, the processing sequence arrangement should follow certain principles.

1) The principle of first coarse and then fine

The processing sequence of each surface is carried out in the order of roughing, semi-finishing, finishing and finishing, in order to gradually improve the precision and surface quality of the machined surface of the part. If the entire surface of the part is processed by CNC machine tools, the process arrangement is generally carried out in the order of roughing, semi-finishing, and finishing, that is, after all roughing is completed, semi-finishing and finishing are performed. During rough machining, most of the machining allowance can be quickly removed, and then each surface can be finished in turn, which can improve production efficiency and ensure the machining accuracy and surface roughness of the parts. This method is suitable for machined surfaces that require high positional accuracy. This is not certain. For example, for some machined surfaces that require high dimensional accuracy, considering the rigidity, deformation and dimensional accuracy of the parts, these machined surfaces can also be considered in the order of roughing, semi-finishing, and finishing. Finish. For the machined surface with high precision requirements, the parts should be put on hold for a period of time between the roughing and finishing processes, so that the surface stress of the parts after rough machining can be completely released, and the degree of stress deformation on the surface of the parts can be reduced, which is conducive to improving the quality of the parts. machining accuracy.

2) The principle of datum surface first processing

At the beginning of processing, the surface used as the finishing reference is always processed. Because the surface of the positioning reference is accurate, the clamping error is small. Therefore, in the processing of any part, the positioning reference surface is always roughed and semi-finished first. Machining and, if necessary, finishing, for example, shaft parts are always roughing and semi-finishing the positioning reference plane, and then finishing. For example, shaft parts always machine the middle hole first, and then use the middle hole surface and the positioning hole as the fine reference to machine the hole system and other surfaces. If there is more than one fine datum plane, the machining of the datum plane should be arranged according to the order of datum conversion and the principle of gradually improving the machining accuracy.

3) The principle of face first, hole later

For parts such as boxes, brackets, and airframes, the plane outline size is large, and the plane positioning is more stable and reliable. Therefore, the plane should be processed first, and then the holes should be processed. In this way, not only a stable and reliable plane can be used as a positioning reference surface for subsequent processing, but also the hole is processed on a flat surface, which makes the processing easier, and also helps to improve the processing accuracy of the hole. Usually, the process can be divided according to the processing part of the part. Generally, the simple geometric shape is processed first, and then the complex geometric shape is processed; the parts with lower precision are processed first, and then the parts with higher precision are processed; the plane is processed first, and then the hole is processed.

4) The principle of internal and external

For the sleeve, the coaxiality between the outer circle and the hole is relatively high. Generally, the principle of first hole and then outer circle is adopted, that is, the outer circle is used as the positioning reference to process the hole, and then the hole with higher precision is used as the positioning reference to process the outer circle. , which can ensure high coaxiality requirements between the outer circle and the hole, and the clamp structure used is also very simple.

5) The principle of reducing the number of tool changes

In CNC machining, the machining sequence should be arranged as far as possible according to the order in which the tool enters the machining position.