5 Axis Machining (5 Axis Machining), as the name implies, is a mode of CNC machining. The linear interpolation movement of any 5 coordinates among X, Y, Z, A, B, and C is adopted. The machine tool used for five-axis machining is usually called a five-axis machine tool or a five-axis machining center. But do you really understand five-axis machining?
The development of five-axis technology
For decades, people have generally believed that five-axis CNC machining technology is the only way to process continuous, smooth, and complex curved surfaces. Once people encounter unsolvable problems in the design and manufacture of complex curved surfaces, they will turn to five-axis machining technology. but.
Five-axis linkage CNC is the most difficult and most widely used technology in CNC technology. It integrates computer control, high-performance servo drive and precision machining technology, and is used for efficient, precise and automated machining of complex curved surfaces. Internationally, the five-axis linkage numerical control technology is regarded as a symbol of the automation technology level of a country's production equipment. Due to its special status, especially its important influence on aviation, aerospace, military industry, and technical complexity, western industrialized countries have always implemented the export license system for five-axis CNC systems as strategic materials.
Compared with three-axis CNC machining, from the perspective of technology and programming, five-axis CNC machining for complex curved surfaces has the following advantages:
1) Improve processing quality and efficiency
2) Expand the process range
3) Meet the new direction of composite development
However, due to interference and position control of the tool in the machining space for five-axis CNC machining, its CNC programming, CNC system and machine tool structure are far more complex than those of a three-axis machine tool. Therefore, the five-axis is easy to say, but it is really difficult to realize it! In addition, it is more difficult to operate well!
Speaking of five-axis, do you have to talk about true and false five-axis? The main difference between true and false 5-axis is whether there is an RTCP function. For this reason, I specifically searched for the term!
RTCP, explain, Fidia's RTCP is the abbreviation of "Rotational Tool Center Point", literally means "rotating tool center", the industry tends to slightly escape it as "turn around the tool center", some people literally translate it as "rotating tool Central Programming", in fact, this is only the result of RTCP. The RTCP of PA is the abbreviation of the first few words of "Real-time Tool Center Point rotation". Heidenhain refers to the similar so-called upgrade technology as TCPM, which is the abbreviation of "Tool Centre Point Management", tool center point management. Other manufacturers call the similar technology TCPC, which is the abbreviation of "Tool Center Point Control", tool center point control.
From the literal meaning of Fidia's RTCP, assuming that the RTCP function is executed manually, the tool center point and the actual contact point between the tool and the workpiece surface will remain unchanged. At this time, the tool center point falls at the actual contact point between the tool and the workpiece surface. The tool holder will rotate around the tool center point. For ball-end tools, the tool center point is the target track point of the CNC code.
In order to achieve the purpose of allowing the tool holder to simply rotate around the target track point (ie the tool center point) when executing the RTCP function, it is necessary to compensate for the offset of the linear coordinates of the tool center point caused by the rotation of the tool holder in real time. It can change the angle between the tool holder and the normal at the actual contact point of the tool and the workpiece surface while keeping the tool center point and the actual contact point of the tool and the workpiece surface unchanged, so as to play the best cutting of the ball nose tool Efficiency, and effectively avoid interference. Therefore, RTCP seems to be more standing on the tool center point (that is, the target trajectory point of the numerical control code) to deal with the change of the rotation coordinate.
Five-axis machine tools and CNC systems that do not have RTCP must rely on CAM programming and post-processing, and plan the tool path in advance. For the same part, if the machine tool is changed, or the tool is changed, CAM programming and post-processing must be performed again. It is called fake five-axis, and many domestic five-axis CNC machine tools and systems belong to this kind of fake five-axis. Of course, it is understandable that people insist on calling themselves five-axis linkage, but this (fake) five-axis is not the other (true) five-axis!
The editor therefore also consulted industry experts. In short, the true five-axis is five-axis and five linkage, the fake five-axis may be five-axis and three linkage, and the other two axes only play a positioning function!
This is a popular saying, not a standard one. Generally speaking, five-axis machine tools are divided into two types: one is five-axis linkage, that is, all five axes can be linked simultaneously, and the other is five-axis positioning processing. It is a five-axis three-linkage: that is, two rotating shafts are rotated and positioned, and only three axes can be simultaneously processed. This five-axis machine tool commonly known as 3+2 mode can also be understood as a fake five-axis.
The current form of five-axis CNC machine tools
In the mechanical design of 5-axis machining centers, machine tool manufacturers have always been unremittingly committed to developing new motion modes to meet various requirements. In summary, there are various types of five-axis machine tools currently on the market. Although their mechanical structures are in various forms, there are mainly the following forms:
Two rotating coordinates directly control the direction of the tool axis (double swing head form)
The two coordinate axes are at the top of the tool,
But the rotation axis is not perpendicular to the linear axis (pitch type swing head type)
Two rotating coordinates directly control the rotation of the space (double turntable form)
The two coordinate axes are on the worktable,
But the rotation axis is not perpendicular to the linear axis (pitch type table)
Two rotating coordinates, one acting on the tool and one acting on the workpiece (one swing and one rotation)
Having seen these five-axis machine tools, I believe we should understand what and how the five-axis machine is moving.
Difficulties and resistances in developing five-axis CNC technology
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 only have 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 also coordinates related calculations of rotational motion, such as rotation angle stroke inspection, non-linear error check, tool rotational motion calculation, etc. The amount of information processed is 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 machine tool utilization rate is very low. In many cases, it is better to use three-axis machine tools.
Very strict requirements for NC interpolation controller and servo drive system
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 rotating coordinates will greatly reduce the processing accuracy. Therefore, the controller is required to have higher calculation 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 cut" 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, 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.
Post processor
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 path, 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 X, Y, Z, B, and C five-axis linkage, 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; just clamp the workpiece on the worktable, measure the position and direction of the workpiece coordinate system, and input these data into the post processing After processing the tool path, the appropriate NC program can be obtained.
Nonlinear 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 path while processing, but they usually perform linearization in the post processor.
Another problem caused by the rotating shaft 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 CAD/ CAM system
For the operation of pentahedron processing, users 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 five-axis machine tools is only 30% to 50% higher than that of three-axis machine tools.
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
The control mode and human-machine interface of smart equipment will have great changes. 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.