CNC machining process planning

December 31, 2022

The processing 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 datums need to be processed accurately in the semi-finishing stage and even in the roughing stage. Sometimes, in order to avoid dimensional conversion, some minor surface semi-finishing can also be arranged during the finishing stage.

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

1. Process division method

In the processing of parts on CNC machine tools, the process is generally divided according to the principle of process concentration, and the following methods are divided:

(1) divided by the tool used

The process completed with the same tool is used as a process. This division method is applicable to the case where there are many surfaces to be machined. Machining centers often use this method.

(2) divided by the number of workpiece installation

The process that can be completed with one part clamping is used as a process. This method is suitable for the processing of parts with few contents. Under the premise of guaranteeing the quality of parts processing, all the processing contents are completed in one setup.

(3) divided by rough finishing

The part of the process completed in the roughing process is used as a process, and the part of the process completed in the finishing process is used as another process. This division method is suitable for parts with strength and hardness requirements, the need for heat treatment or high part accuracy requirements, the need to effectively remove the internal stress, as well as the greater deformation of the parts after processing, need to be divided according to the coarse and fine processing stages of parts processing.

(4) divided by processing area

The part of the process that completes the same profile is used as a process. For parts with many processing surfaces that are more complex, the order of numerical control processing, heat treatment, and auxiliary operations should be properly arranged, and the connection between the processes should be solved.

2. The principle of division of processing operations

Parts are made up of multiple surfaces. These surfaces have their own precision requirements, and there are also corresponding accuracy requirements between surfaces. In order to achieve the design accuracy requirements of parts, the processing sequence should follow certain principles.

(1) The principle of rough after finishing

The processing sequence of each surface is performed in the order of roughing, semi-finishing, finishing, and finishing. The purpose is to gradually improve the accuracy and surface quality of the part's machining surface.

If the entire surface of the part is processed by CNC machine tools, the process arrangement is generally performed in the order of roughing, semi-finishing, and finishing, that is, after the roughing is completed, semi-finishing and finishing are performed. During roughing, most of the machining allowance can be quickly removed, and then each surface can be finished in turn. This can increase the production efficiency and ensure the machining accuracy and surface roughness of the parts. This method is suitable for machining surfaces with high requirements for position accuracy.

This is not absolute, as for some machining surfaces with high dimensional accuracy, taking into account the requirements of the rigidity, deformation and dimensional accuracy of the parts, it is also possible to consider the order of rough machining, semi-finishing machining and finishing machining of these machining surfaces. carry out.

For machining surfaces with high precision, between the rough and finishing operations, the parts are preferably held for a period of time, so that the surface stress of the rough-finished parts is completely released, and the stress deformation level of the parts is reduced, which is beneficial to improvement. Parts machining accuracy.

(2) The principle of the first plane processing

At the beginning of machining, the surface used as a reference for finishing is always machined, because the surface of the positioning reference is accurate and the clamping error is small. Therefore, in any part machining process, the positioning reference plane is always roughed and semi-finished first. Machining, if necessary, also finishing, for example, shaft parts are always roughing and semi-finishing the positioning reference surface, and then finishing. For example, the shaft part always processes the center hole first, and then uses the center hole surface and the positioning hole as the fine reference machining hole system and other surfaces. If there is more than one datum plane, the datum planes should be arranged in accordance with the order of datum transformation and the principle of gradually improving the machining accuracy.

(3) Principle of front face back hole

For box, bracket, body and other parts, the plane contour size is larger, more stable and reliable with plane positioning, it should be processed first plane, after processing hole. In this way, not only does the subsequent machining have a stable and reliable plane as the positioning reference surface, but also the machining of the holes on the flat surface makes the machining easier and helps to improve the machining accuracy of the holes. In general, the process can be divided according to the parts to be processed, the general processing of a simple geometric shape, after the processing of complex geometric shapes; the first processing of the lower accuracy of the site, after the high precision of the site; first processing plane, after the processing hole.

(4) First inside and outside principle

For the precision sleeve, the concentricity of the outer circle and the hole is required to be relatively high. Generally, the principle of the first hole and the outer circle is generally adopted, that is, the outer circle is used as a positioning reference hole, and then the higher precision hole is used as a positioning reference. Round, this ensures a high degree of coaxiality between the outer circle and the hole, and the fixture structure used is also very simple.

(5) The principle of reducing the number of tool changes

In the numerical control processing, the processing sequence should be arranged in the order of the tool entering the processing position. This requires that the number of tool changes is reduced as much as possible, the idle stroke is reduced, and the auxiliary time is saved without affecting the machining accuracy. After the parts are clamped, use the same tool as much as possible to complete more machining surfaces. When a tool completes all parts that may be machined, try to do some pre-machining for the next process, and then change the tool to finish the finishing or to process other parts. For some unimportant parts, use the same tool as much as possible to complete multiple processes at the same station.

(6) The principle of continuous processing

When processing semi-enclosed or closed inside and outside contours, stoppages in CNC machining should be avoided as much as possible. Because the process system of parts, tools, and machine tools is temporarily in a dynamic equilibrium state during the machining process, if the equipment suddenly stops due to the NC program, the cutting force will be significantly reduced and the original process system will be lost. In a steady state, the tool leaves scratches or dents at the standstill. Therefore, in the contour processing should avoid the phenomenon of feed stalls to ensure the quality of parts processing.

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Author:

Ms. Mirada Kuo

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