10 CNC Experience Summaries, What Is CNC Machining

CNC machining, also known as Numerical Control processing, refers to the processing with CNC machining tools. Because CNC machining is programmed and controlled by a computer, CNC machining has the advantages of stable machining quality, high machining accuracy, high repeatability, complex surface machining, and high machining efficiency. In the actual process of processing, human factors, and operating experience, to a large extent, will affect the final processing quality. Following, let’s have a look, at a seasoned professional with ten years of CNC processing experience, summed up the 10 valuable experiences.

1. How to Divide the CNC Machining Process?

1.1 The Method of Processing Process Division

The division of CNC machining processes can generally be carried out according to the following methods:

◆Divided By The Tool Used

Taking the process completed by the same tool as a processing procedure, this division method is suitable for situations where there are many workpiece surfaces to be machined. Machining centers often use this method to complete.

◆Divided By The Number Of Workpiece Installations

The technological process that can be completed by the parts in one clamping 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 processing contents can be completed in one clamping.

◆Divided By Roughing And Finishing

The part of the process completed in rough machining 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 that have strength and hardness requirements, require heat treatment or require high accuracy of parts, need to effectively remove internal stress, and have large deformation after machining, which needs to be divided into rough and finish machining stages.

◆Divided By Processing Part

The part of the process that completes the same profile as a process.

For the parts with a more complicated processing surface, the sequence of CNC processing, heat treatment, and auxiliary working procedure should be reasonably arranged, and the connection problem between working procedures should be solved.

1.2 Principle Of Process Division

Parts are made up of multiple surfaces that have their own precision requirements and also have corresponding precision requirements between the surfaces.

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

◆Roughing First, Finishing Later

The processing sequence of each surface is carried out in the order of roughing, semi-finishing, finishing and finishing, with the purpose of gradually improving the accuracy and surface quality of the processed surface of the part. If all surfaces of the 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 performed after all roughing are completed. During rough machining, most of the machining allowance can be quickly removed, and then each surface can be finished in sequence, 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 position accuracy. This is not absolute, such as for some higher dimensional accuracy requirements of the machining surface, taking into account the stiffness of parts, deformation, and dimensional accuracy requirements can also consider these machining surfaces respectively according to rough machining, semi-finishing, finishing process order to complete. For machining surfaces with high precision requirements, it is best to leave the parts for a period of time between roughing and finishing processes, so that the surface stress of the parts after roughing can be completely released, and the degree of stress and deformation on the surface of the parts can be reduced, which is beneficial to improve the machining accuracy of the parts.

◆Datum Surface First Processing Principle

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. Machining, and finishing if necessary, for example, shaft parts always rough and semi-finish the positioning reference surface before finishing.

For example, for shaft parts, the center hole is always processed first, and then the whole system and other surfaces are processed on the center hole surface and the positioning hole as a precision reference. If there is more than one precision reference surface, the processing of the reference surface should be arranged in accordance with the sequence of reference conversion and the principle of gradually improving the processing accuracy.

◆Surface Process First, Hole Process Later

For the box, bracket, machine body, and other parts, the plane outline size is larger, and the plane positioning is more stable and reliable, so the plane surface should be processed first, and then the hole should be processed. In this way, not only the subsequent processing has a stable and reliable plane as the positioning reference surface, but also the processing of holes on a flat surface makes the processing easier, and it is also conducive to improving the processing accuracy of the holes. Generally, the processes can be divided according to the processing positions of the parts, and the simple geometric shapes are processed first, and then the complex geometric shapes are processed; the lower precision parts are processed first, and the higher precision parts are processed first; the plane is processed first, and the hole is processed later.

◆Inner Hole First, Outer Circle Later

For precision sleeves, the coaxiality between the outer circle and the hole is required. Generally, the principle of the first hole and the outer circle is adopted, that is, the outer circle is used as the positioning reference to machine the hole, and then the hole with higher precision is used as the positioning reference to process the outer circle. Round, which can ensure a high coaxiality requirement between the outer circle and the hole, and the fixture structure used is also very simple.

◆Reduce The Number Of Tool Changes Principle

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.

2. What Principles Should Be Followed in the Arrangement of CNC Machining Sequence?

The arrangement of the processing sequence should be considered according to the structure of the part and the condition of the blank, as well as the need of positioning and clamping, and the important point is that the workpiece is not destroyed. The sequence of CNC machining generally follows the following rules:

◆The CNC machining of the previous process cannot affect the positioning and clamping of the next process, and the general-purpose machine tool processing procedure should also be considered comprehensively.

◆First, carry out the inner shape and cavity processing process, and then carry out the shape processing process.

◆The processes of CNC machining with the same positioning and clamping method or the same tool are best connected, so as to reduce the number of repeated positioning, the number of tool changes, and the number of times moving the platen.

◆For multiple processes carried out in the same installation, the process with less damage to the rigidity of the workpiece should be arranged first.

3. Which Aspects Should Be Paid Attention to When Determining the Clamping Method of the Workpiece?

The following three points should be paid attention to when determining the positioning reference and clamping scheme:

◆Strive to unify the design, technology, and programming calculation benchmarks.

◆As far as possible to reduce the number of clamping, as far as possible to be able to CNC machining all the surfaces after one positioning.

◆Avoid using manual adjustment schemes that occupy the machine tool.

◆The clamp should be opened smoothly, its positioning, and clamping mechanism can not affect the CNC processing in the knife (such as collision), encounter this kind of situation, can be used with a vise or add the bottom plate to draw a screw clamping.

4. How to Determine the Tool Setting Point is More Reasonable?

What is the Relationship Between the Workpiece Coordinate System and the Programming Coordinate System?
The tool setting point can be set on the part to be machined, but note that the tool setting point must be a reference position or a part that has been finished. Sometimes the tool setting point is destroyed by CNC machining after the first process, which will lead to the second process and the subsequent tool setting points cannot be found. Therefore, in the first process of the toolset, pay attention to setting up a relative tool setting position where there is a relatively fixed size relationship with the positioning datum so that the original tool setting point can be retrieved according to the relative position relationship between them. This relative tool setting position is usually set on the machine table or fixture. The selection principles are as follows:

◆It is easy to keep the relevant surface and datum in the right position.

◆Easy to program.

◆Small tool setting error.

◆Processing inspection is convenient and can be checked.

The origin position of the workpiece coordinate system is set by the operator. After the workpiece is clamped, it is determined by the tool. It reflects the distance position relationship between the workpiece and the zero of the machine tool. Once the workpiece coordinate system is fixed, it is generally not changed. The workpiece coordinate system and programming coordinate system must be unified, that is, during machining, the workpiece coordinate system and programming coordinate system are consistent.

5. How to Choose Tool Feed Route?

The tool feed route is the trajectory and direction of the tool relative to the workpiece in the CNC machining process. Reasonable choice of machining route is very important because it is closely related to CNC machining accuracy and surface quality of parts. The following points are mainly considered when determining the tool feed route:

◆Ensure the machining accuracy requirements of the parts.

◆Facilitate numerical calculation, and reduce programming workload.

◆Seek the shortest CNC machining route, and reduce idle cutting time to improve CNC machining efficiency

◆As far as possible minimize the program segment.

◆To ensure the roughness requirements of the workpiece contour surface after CNC machining, the final contour should be continuously processed by the last tool feed cutting.

◆The advance and retreat (cut-in and cut-out) route of the tool should also be carefully considered, to minimize the tool stops at the contour (a sudden change in cutting force causes elastic deformation) and leave tool marks, and avoid vertical cutting on the contour surface and scratch the workpiece.

6. How to Monitor and Adjust CNC Machining Process?

After the workpiece is aligned and the program is debugged, it can enter the automatic processing stage. In the automatic machining process, the operator should monitor the cutting process to prevent abnormal cutting caused by workpiece quality problems and other accidents.

Monitoring the cutting process mainly considers the following aspects:

◆The main consideration of machining process monitoring rough machining is the rapid removal of excess allowance on the workpiece surface. In the automatic machining process of the machine tool, according to the set cutting amount, the tool automatically cuts according to the predetermined cutting path. At this time, the operator should pay attention to observing the cutting load changes in the automatic machining process through the cutting load table, and adjust the cutting amount according to the bearing capacity of the tool to maximize the efficiency of the machine tool.

◆Monitoring of cutting sound during cutting in the automatic cutting process, generally when cutting is started, the sound of the tool cutting the workpiece is stable, continuous, and brisk. At this moment, the movement of the machine tool is stable. With the process of cutting, when there are hard spots on the workpiece or tool wear or tool clamping, the cutting process becomes unstable. The unstable performance is that the cutting sound changes and the tool and the workpiece collide with each other sound and the machine tool will vibrate.

At this time, the cutting amount and cutting conditions should be adjusted in time. When the adjustment effect is not obvious, the machine tool should be suspended to check the condition of the tool and the workpiece.

◆Finishing process monitoring finishing, mainly to ensure the workpiece processing size and processing surface quality, cutting speed is higher, larger feed. At this time, attention should be paid to the impact of built-up edge on the machined surface. For cavity processing, attention should be paid to overcutting and cutting at corners.

◆To solve the above problems, one is to pay attention to adjusting the spray position of the cutting fluid to keep the machined surface in a cooling condition at all times; The second is to pay attention to the quality of the processed surface of the workpiece and to avoid quality changes as much as possible by adjusting the cutting amount. If the adjustment still has no obvious effect, the machine should be shut down to check whether the original program is reasonable. Special attention should be paid to the position of the tool when the inspection is suspended or stopped. If the tool stops during the cutting process, a sudden stop of the spindle will cause tool marks on the surface of the workpiece. Generally, consider stopping the tool when it leaves the cutting state.

◆Tool monitoring the quality of the tool largely determines the processing quality of the workpiece. In the automatic machining and cutting process, it is necessary to judge the normal wear condition and abnormal damage condition of the tool through methods such as sound monitoring, cutting time control, pause inspection during cutting, and workpiece surface analysis. According to the processing requirements, the cutting tools should be processed in time to prevent the processing quality problems caused by the cutting tools not being processed in time.

7. How to Choose Machining Tool Reasonably?

How to Determine the Tool Rotate Speed, Cutting Speed, Cutting Width?

◆Non – heavy – grinding carbide end milling cutter or end milling cutter should be used in flat milling. For general milling, try to use the second tool feed processing, the first tool feed is best to use end milling cutter rough milling, along with the workpiece surface continuous tool. The recommended width of each cutting tool feed is 60%~75% of the tool diameter.

◆End milling cutters and cemented carbide insert end milling cutters are mainly used for processing boss, groove, and box face.

◆Ball cutter, round cutter (also known as round nose cutter) is often used for machining curved surfaces and bevel contour shapes. And the ball cutter is used for semi-finishing and finishing. Round cutter inlaid with carbide cutting tools is mostly used for roughing.

8. What is the Function Of the Processing Procedure Sheet?

What Should Be Included in the Processing Program List?

◆The machining procedure sheet is one of the contents of the CNC machining process design. It is also a procedure that needs to be followed and executed by the operator. It is a specific description of the machining procedure. The purpose is to let the operator clarify the content of the procedure, the method of clamping and positioning, and the location of each machining procedure. Attention should be paid to the selected tool.

◆In the processing program list, including drawing and programming file name, clamping sketches, program name, tools for each program, maximum depth of cutting, processing properties( such as roughing or finishing), theoretical processing time, etc.

9. How to Prepare Before CNC Programming?

◆After determining the processing technology, before the programming to understand:

◆Workpiece clamping method;

◆The size of workpiece blank–in order to ensure the processing scope or whether multiple clamping is required;

◆Workpiece materials–in order to determine the processing tools;

◆What tools are in stock–Avoid modifying the program during machining because there is no such tool. If you must use this tool, you can prepare in advance.

10. What Are the Principles for Setting Safety Height in Programming?

Setting principles of safety height: Generally higher than the highest surface of the island or programming zero at the highest surface can also minimize the risk of tool collision.

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