Twelve years of experience in CNC machining

CNC machining, also known as NC machining, refers to the machining with NC machining tools. Because NC machining is controlled by computer after programming, CNC machining has the advantages of stable machining quality, high machining precision, high repetition precision, complex surface machining and high machining efficiency. In the actual processing process, human factors and operation experience will affect the final processing quality to a great extent. Next, let’s take a look at the twelve valuable experiences summarized by an old driver with ten years of CNC processing experience

1、 How to divide processing operations?

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

1. The centralized sequencing method of cutting tools (common cutting tools for CNC machining (I): milling cutters) is to divide the processes according to the tools used, and use the same tool to process all the parts that can be completed on the part. The second knife and the third knife are used to complete other parts they can complete. This can reduce the number of tool changes, compress the idle time and reduce unnecessary positioning errors.

2. For parts with many processing contents, the processing part can be divided into several parts according to its structural characteristics, such as inner shape, shape, curved surface or plane. Generally, the plane and positioning surface are processed first, and then the hole is processed; First process simple geometry, and then process complex geometry; First process the parts with low accuracy, and then process the parts with high accuracy requirements.

3. For the parts prone to machining deformation by rough and finish machining sequence method, the shape needs to be calibrated due to the possible deformation after rough machining. Therefore, generally speaking, the processes should be separated for rough and finish machining. To sum up, when dividing the process, we must flexibly grasp the structure and manufacturability of the parts, the function of the machine tool, the number of NC machining contents of the parts, the installation times and the production organization of the unit. In addition, it is suggested to adopt the principle of centralized process or decentralized process, which should be determined according to the actual situation, but we must strive to be reasonable.

2、 What principles should be followed in the arrangement of processing sequence?

The arrangement of processing sequence shall be considered according to the structure and blank condition of parts and the needs of positioning and clamping, with the focus on the rigidity of the workpiece not being damaged. Generally, the sequence shall be carried out according to the following principles:

1. The processing of the previous process shall not affect the positioning and clamping of the next process, and the general machine tool processing process interspersed in the middle shall also be comprehensively considered.

2. First carry out the machining procedure of inner shape and inner cavity, and then carry out the contour machining procedure.

3. It is better to connect the processes processed with the same positioning, clamping mode or the same knife, so as to reduce the times of repeated positioning, tool change and moving the pressing plate.

4. For multiple processes in the same installation, the process with small damage to the rigidity of the workpiece shall be arranged first.

3、 What aspects should be paid attention to in determining the clamping mode of workpiece? (recommended reading: [fooling machine] common tools for CNC machining (II): tool clamping)

When determining the positioning datum and clamping scheme, pay attention to the following three points:

1. Strive to unify the benchmark of design, process and programming calculation.

2. Reduce the clamping times as much as possible, and try to process all the surfaces to be processed after one positioning.

3. Avoid using manual adjustment scheme.

4. The fixture shall be open, and its positioning and clamping mechanism shall not affect the tool walking in processing (such as collision). In case of such circumstances, it can be clamped by using vise or adding base plate to extract screws.

4、 How to determine the tool setting point is more reasonable? What is the relationship between workpiece coordinate system and programming coordinate system?

1. The tool setting point can be set on the of the machined parts, but note that the tool setting point must be the reference position or the part that has been finished. Sometimes the tool setting point is damaged after the first process, which will make it impossible to find the tool setting point in the second process and after. Therefore, when setting the tool in the first process, pay attention to setting a relative tool setting position where there is a relatively fixed dimension relationship with the positioning reference, In this way, 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 tool workbench or fixture. The selection principles are as follows:

1) It’s easy to find it.

2) Convenient programming.

3) Small tool setting error.

4) The inspection during processing is convenient and can be checked.

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

5、 How to choose the cutting route?

The tool path refers to the motion path and direction of the tool relative to the workpiece in the process of NC machining. The reasonable selection of machining route is very important, because it is closely related to the machining accuracy and surface quality of parts. The following points are mainly considered when determining the cutting route:

1. Ensure the machining accuracy requirements of parts.

2. Facilitate numerical calculation and reduce programming workload.

3. Find the shortest machining route to improve machining efficiency.

4. Minimize the number of segments.

5. Ensure the requirements of the roughness of the workpiece contour surface after machining, and the final contour shall be processed continuously with the last tool.

6. The forward and backward (cut in and cut out) route of the tool should also be carefully considered, so as to minimize the knife marks caused by stopping the tool at the contour (elastic deformation caused by sudden change of cutting force) and avoid scratching the workpiece by cutting vertically on the contour surface.

6、 How to monitor and adjust during processing?

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

Monitoring the cutting process mainly considers the following aspects:

1. Machining process monitoring rough machining mainly considers the rapid removal of excess allowance on the workpiece surface. In the automatic machining process of the machine tool, the tool automatically cuts according to the predetermined cutting path according to the set cutting parameters. At this time, the operator should observe the change of cutting load in the automatic machining process through the cutting load table, and adjust the cutting parameters according to the bearing force of the tool to give full play to the maximum efficiency of the machine tool.

2. Monitoring of cutting sound in the process of cutting in the process of automatic cutting, the sound of the tool cutting the workpiece is stable, continuous and light at the beginning of cutting. At this time, the movement of the machine tool is stable. With the progress of the cutting process, when there are hard spots on the workpiece, tool wear or tool clamping, the cutting process will be unstable. The unstable performance is that the cutting sound will change, there will be mutual impact sound between the tool and the workpiece, and the machine tool will vibrate. At this time, the cutting parameters and cutting conditions shall be adjusted in time. When the adjustment effect is not obvious, the machine tool shall be suspended to check the condition of tools and workpieces.

3. The monitoring of finishing process is mainly to ensure the machining size and surface quality of the workpiece. The cutting speed is high and the feed rate is large. At this time, attention should be paid to the influence of chip accumulation on the machining surface. For cavity machining, attention should also be paid to over cutting and tool yield at the corner. To solve the above problems, first, pay attention to adjusting the spraying position of cutting fluid to keep the machining surface in cooling conditions at all times; Second, observe the quality of the machined surface and avoid the change of the machined surface as much as possible. If the adjustment still has no obvious effect, stop the machine to check whether the original program is reasonable. In particular, pay attention to the position of the tool when pausing the inspection or stopping the inspection. If the cutting tool stops in the cutting process, the sudden spindle stop will produce tool marks on the surface of the workpiece. Generally, shutdown should be considered when the tool leaves the cutting state.

4. The quality of the tool determines the quality of the tool. In the process of automatic machining and cutting, the normal wear condition and abnormal damage condition of the tool should be judged by means of sound monitoring, cutting time control, pause inspection in the cutting process, workpiece surface analysis and so on. According to the processing requirements, the cutting tools shall be handled in time to prevent the processing quality problems caused by the untimely handling of the cutting tools.

7、 How to reasonably select machining tools? How many factors are there in cutting parameters? How many kinds of cutting tools are there? How to determine the tool speed, cutting speed and cutting width?

1. Non regrinding carbide end milling cutter or end milling cutter shall be selected for plane milling. In general milling, try to use the second tool feeding for processing. For the first tool feeding, it is best to use the end milling cutter for rough milling and continuously feed the tool along the workpiece surface. The recommended width of each tool feeding is 60% – 75% of the tool diameter.

2. End mills and end mills with carbide inserts are mainly used to process bosses, grooves and box mouth surfaces.

3. Ball cutter and round cutter (also known as round nose cutter) are often used to process curved surface and variable angle contour. Ball knives are mostly used for semi finishing and finishing. The circular cutter with cemented carbide cutter is mostly used for roughing.

8、 What is the function of the processing program sheet? What should be included in the processing procedure sheet?

1. The machining program list is one of the contents of NC machining process design, and it is also a procedure that needs to be observed and executed by the operator. It is a specific description of the machining program. The purpose is to let the operator clarify the content of the program, the clamping and positioning mode, the problems that should be paid attention to by the tools selected in each machining program, etc.

2. The processing program list shall include: drawing and programming file name, workpiece name, clamping sketch, program name, tool used in each program, maximum cutting depth, processing nature (such as rough machining or finish machining), theoretical processing time, etc.

9、 What preparations should be made before NC programming?

After determining the processing technology, you should understand the following before programming:

1. Workpiece clamping mode;

2. Size of workpiece rough embryo — in order to determine the processing range or whether multiple clamping is required;

3. The material of the workpiece — in order to select which tool to use for machining;

4. What are the tools in stock? Avoid modifying the program because there is no such tool during processing. If you must use this tool, you can prepare it in advance.

10、 What are the principles for setting the safety height in programming?

Setting principle of safety height: generally higher than the highest surface of the island. Or set the programming zero point on the highest surface, which can also avoid the risk of knife collision to the greatest extent.

11、 After the tool path is compiled, why post-processing?

Because the address code and NC program format recognized by different machine tools are different, it is necessary to select the correct post-processing format for the machine tools used to ensure that the compiled program can run.

12、 What is DNC communication?

The mode of program transmission can be divided into CNC and DNC. CNC refers to that the program is transmitted to the memory of the machine tool through media media (such as floppy disk, tape reader, communication line, etc.) and stored. During processing, the program is called out from the memory for processing. Because the capacity of the memory is limited by the size, the DNC mode can be used for processing when the program is large. Because the machine tool directly reads the program from the control computer during DNC processing (that is, while sending and doing), it is not limited by the capacity of the memory.

1. There are three elements of cutting parameters: cutting depth, spindle speed and feed speed The general principle of selecting cutting parameters is: less cutting and fast feed (i.e. small cutting depth and fast feed speed)

2. According to the classification of materials, the cutting tools are generally divided into ordinary hard white steel cutting tools (the material is high-speed steel), coated cutting tools (such as titanium plating), alloy cutting tools (such as tungsten steel, boron nitride cutting tools, etc.).