Everyone has long recognized the superiority and importance of five-axis CNC technology. But so far, the application of five-axis CNC technology is still limited to a few well-funded departments, and there are still unresolved problems.
Five-axis CNC programming is abstract and difficult to operate
This is a headache that every traditional CNC programmer feels deeply. Three-axis machine tools have only linear coordinate axes, while five-axis CNC machine tools have various structures; the same NC code can achieve the same processing effect on different three-axis CNC machine tools, but the NC code of a five-axis machine tool cannot be applied All types of five-axis machine tools. In addition to linear motion, CNC programming must also coordinate related calculations of rotational motion, such as rotation angle stroke inspection, non-linear error check, tool rotation calculation, etc. The amount of information processed is very large, and CNC programming is extremely abstract.
The operation of five-axis CNC machining is closely related to programming skills. If the user adds special functions to the machine tool, the programming and operation will be more complicated. Only by repeated practice, programming and operating personnel can master the necessary knowledge and skills. The lack of experienced programming and operators is a major obstacle to the popularization of five-axis CNC technology.
Many domestic manufacturers have purchased five-axis CNC machine tools from abroad. Due to inadequate technical training and services, it is difficult to realize the inherent functions of five-axis CNC machine tools, and the utilization rate of the machine tools is very low. In many cases, it is better to use three-axis machine tools.
The requirements for NC interpolation controller and servo drive system are very strict. The movement of a five-axis machine tool is a combination of five coordinate axis movements. The addition of rotating coordinates will not only increase the burden of interpolation calculations, but also the small errors of the rotating coordinates will greatly reduce the processing accuracy. Therefore, the controller is required to have higher arithmetic accuracy.
The motion characteristics of the five-axis machine tool require the servo drive system to have good dynamic characteristics and a large speed range.
NC program verification of five-axis CNC is particularly important
To improve the efficiency of mechanical processing, it is urgent to eliminate the traditional “trial cutting method” verification method. In five-axis CNC machining, the verification of NC programs has also become very important, because workpieces processed by five-axis CNC machine tools are usually very expensive, and collisions are common problems in five-axis CNC machining: the tool cuts into the workpiece; It collides with the workpiece at a very high speed; the tool collides with the machine tool, fixture and other equipment within the processing range; the moving part on the machine tool collides with the fixed part or the workpiece. In five-axis CNC, collisions are difficult to predict, and the calibration program must comprehensively analyze the machine tool kinematics and control system.
If the CAM system detects an error, the tool path can be processed immediately; but if an NC program error is found during the machining process, the tool path cannot be modified directly as in the three-axis CNC. On a three-axis machine tool, the machine operator can directly modify the tool radius and other parameters. In five-axis machining, the situation is not so simple, because the change of the tool size and position has a direct impact on the subsequent rotary motion trajectory.
Tool radius compensation
In the 5-axis NC program, the tool length compensation function is still valid, but the tool radius compensation is invalid. When using cylindrical milling cutters for contact forming milling, different programs need to be programmed for cutters of different diameters. The current popular CNC systems are unable to complete the tool radius compensation, because the ISO file does not provide enough data to recalculate the tool position. The user needs to change the tool frequently or adjust the exact size of the tool when performing CNC machining. According to the normal processing procedure, the tool path should be sent back to the CAM system for recalculation. As a result, the efficiency of the entire processing process is very low.
In response to this problem, Norwegian researchers are developing a temporary solution called LCOPS (Low Cost Optimized Production Strategy). The data required for tool path correction is sent to the CAM system by the CNC application, and the calculated tool path is directly sent to the controller. LCOPS requires a third party to provide CAM software, which can be directly connected to the CNC machine tool, during which the CAM system file is transmitted instead of the ISO code. The final solution to this problem depends on the introduction of a new generation of CNC control system, which can recognize workpiece model files in common formats (such as STEP, etc.) or CAD system files.
The difference between a five-axis machine tool and a three-axis machine tool is that it also has two rotating coordinates. The tool position is transformed from the workpiece coordinate system to the machine tool coordinate system, and several coordinate transformations are required in the middle. Using the popular post processor generator on the market, you only need to input the basic parameters of the machine tool to generate the post processor of the three-axis CNC machine tool. For five-axis CNC machine tools, there are currently only some improved post processors. The post processor of the five-axis CNC machine tool needs further development.
When three-axis linkage, the position of the workpiece origin on the machine tool table does not need to be considered in the tool trajectory, and the post processor can automatically process the relationship between the workpiece coordinate system and the machine coordinate system. For five-axis linkage, for example, when machining on a horizontal milling machine with five-axis linkage of X, Y, Z, B, and C, the position and size of the workpiece on the C turntable and the position and size between the B and C turntables are generated when the tool path is generated Must be considered. Workers usually spend a lot of time dealing with these positional relationships when clamping workpieces. If the post-processor can process these data, the installation of the workpiece and the processing of the tool path will be greatly simplified; only the workpiece is clamped on the worktable, the position and direction of the workpiece coordinate system are measured, and these data are input to the post-processing After processing the tool path, the appropriate NC program can be obtained.
Non-linear errors and singularity problems
Due to the introduction of rotating coordinates, the kinematics of a five-axis CNC machine tool is much more complicated than that of a three-axis machine tool. The first problem related to rotation is nonlinear error. The non-linear error should be attributed to the programming error, which can be controlled by reducing the step distance. In the pre-calculation stage, the programmer cannot know the size of the non-linear error. Only after the machine tool program is generated by the post-processor can the non-linear error be calculated. Tool path linearization can solve this problem. Some control systems can linearize the tool trajectory while processing, but it is usually linearized in the post processor.
Another problem caused by rotating shafts is singularity. If the singular point is at the extreme position of the rotating shaft, a small oscillation near the singular point will cause the rotating shaft to flip by 180°, which is quite dangerous.
Requirements for CA/ CAM system
For the operation of pentahedron processing, the user must rely on a mature CAD/CAM system, and must have experienced programmers to operate the CAD/CAM system.
Large investment in the purchase of machine tools
There was a huge price gap between five-axis machine tools and three-axis machine tools in the past. Now, the addition of a rotating axis to a three-axis machine tool is basically the price of an ordinary three-axis machine tool, which can realize the functions of a multi-axis machine tool. At the same time, the price of a five-axis machine tool is only 30% to 50% higher than the price of a three-axis machine tool.
In addition to the investment in the machine tool itself, the CAD/CAM system software and post-processor must also be upgraded to adapt to the requirements of five-axis machining; the verification program must be upgraded to enable it to simulate the entire machine tool .
The future trend of intelligent five-axis processing machine tools
There will be great changes in the control mode and human-machine interface of smart equipment. With the improvement of network performance such as WiFi broadband and Bluetooth short-range communication, network-based mobile control methods such as tablets, mobile phones and wearable devices will become more and more popular. . The time-advanced touch screen and multi-touch graphical man-machine interface will gradually replace buttons, switches, mouse and keyboard. People, especially young people, have become accustomed to the operation of smart electronic consumer products. They can react quickly, switch screens, upload or download data, which greatly enriches the content of human-computer interaction and significantly reduces the misoperation rate.