CNC Machining Process Explanation

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

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

1. Methods of Dividing Machining Processes

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

1) Division by the Tool Used

The process completed by the same tool is regarded as a process. This division method is suitable for situations where there are many surfaces to be processed on the workpiece. This method is often used in machining centers.

2) Division by the Number of Workpiece Installations

The process that can be completed by one clamping of the part 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) Division by Roughing and Finishing

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

4) Division By Processing Part: The Part Of The Process That Completes The Same Profile Is Regarded As One Process

For parts with many and complex processing surfaces, the sequence of CNC processing, heat treatment and auxiliary processes should be arranged reasonably, and the connection problem between processes should be solved. 2. Principles of division of processing procedures Parts are composed of multiple surfaces, which have their own precision requirements, and there are corresponding precision requirements between each surface.

In order to achieve the design precision requirements of parts, the processing sequence arrangement should follow certain principles.

1) The Principle Of Roughing First And Then Finishing

The processing sequence of each surface is carried out in the order of roughing, semi-finishing, finishing and finishing, with the aim of gradually improving the precision and surface quality of the part processing surface. If all surfaces of a part are processed by CNC machine tools, the process arrangement is generally carried out in the order of roughing, semi-finishing, and finishing, that is, semi-finishing and finishing are carried out after all roughing is completed. Most of the machining allowance can be quickly removed during roughing, 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 machining surfaces with high position accuracy requirements. This is not absolute. For example, for some machining surfaces with high dimensional accuracy requirements, considering the requirements of the rigidity, deformation and dimensional accuracy of the parts, these machining surfaces can also be considered to be completed in the order of roughing, semi-finishing, and finishing. For machining surfaces with high accuracy requirements, it is best to leave the parts for a period of time between the roughing and finishing processes so that the surface stress of the parts after roughing 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 machining accuracy of the parts.

2) Principle Of Machining The Datum Surface First

At the beginning of machining, the surface used as the finishing datum is always machined out, because the surface of the positioning datum is accurate and the clamping error is small. Therefore, in the machining process of any part, the positioning datum surface is always rough-machined and semi-finished first, and if necessary, fine-machined. For example, shaft parts always perform rough-machined and semi-finished machining on the positioning datum surface, and then fine-machined. For example, shaft parts always machine the center hole first, and then use the center hole surface and the positioning hole as the fine datum to machine the hole system and other surfaces. If there is more than one fine datum surface, the machining of the datum surface should be arranged according to the order of datum conversion and the principle of gradually improving machining accuracy.

3) Principle Of Machining The Surface First And Then The Hole

For parts such as boxes, brackets, and bodies, the plane contour size is large, and it is more stable and reliable to use the plane for positioning, so the plane should be machined first and the hole should be machined later. In this way, not only does the subsequent machining have a stable and reliable plane as the positioning datum surface, but also machining the hole on a flat surface makes the machining easier, which is also conducive to improving the machining accuracy of the hole. Usually, the process can be divided according to the processing part of the part. Generally, simple geometric shapes are processed first, and complex geometric shapes are processed later; parts with lower precision are processed first, and parts with higher precision are processed later; planes are processed first, and holes are processed later.

4) The principle of inside first and outside later

For precision sleeves, the coaxiality requirements of the outer circle and the hole are higher. Generally, the principle of hole first and outer circle later is adopted, that is, the hole is processed first with the outer circle as the positioning reference, and then the outer circle is processed with the hole with higher precision as the positioning reference. This can ensure that the outer circle and the hole have higher coaxiality requirements, and the fixture 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 much as possible according to the order in which the tool enters the machining position.