Five-axis machining (5 Axis Machining), a mode of CNC machine tool machining.
According to ISO regulations, when describing the movement of CNC machine tools, a right-handed rectangular coordinate system is used; the coordinate axis parallel to the main axis is defined as the z axis, and the rotation coordinates around the x, y, and z axes are A, B, and C, respectively. The movement of each coordinate axis can be realized by the worktable or the movement of the tool, but the direction is defined by the movement direction of the tool relative to the workpiece. Usually five-axis linkage refers to the linear interpolation motion of any 5 coordinates in x, y, z, A, B, and C. In other words, five axes refer to the three moving axes of x, y, and z plus any two rotating axes. Compared with the common three-axis (x, y, z three degrees of freedom) machining, five-axis machining means that when machining parts with complex geometry, the machining tool needs to be able to be positioned and connected in five degrees of freedom.
The machine tools used in five-axis machining are usually called five-axis machine tools or five-axis machining centers. Five-axis machining is often used in the aerospace field to process body parts, turbine parts and impellers with free-form surfaces. The five-axis machine tool can process different sides of the workpiece without changing the position of the workpiece on the machine tool, which can greatly improve the processing efficiency of prismatic parts.
The development of five-axis technology
For decades, it was widely believed that five-axis CNC machining technology was the only means of machining continuous, smooth, complex surfaces. Once people encounter unsolvable problems in designing and manufacturing complex curved surfaces, they will turn to five-axis machining technology for help.
Five-axis linkage numerical control is the most difficult and widely used technology in numerical control technology. It integrates computer control, high-performance servo drive and precision machining technology, and is applied to efficient, precise and automatic 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, and military industries, as well as its technical complexity, western industrialized countries have always used the five-axis CNC system as a strategic material to implement an export license system.
Compared with three-axis CNC machining, from the perspective of technology and programming, the use of five-axis CNC machining for complex surfaces has the following advantages:
- Improve processing quality and efficiency
- Expand the scope of the process
- Meet the new direction of compound development
However, due to the interference and the position control of the tool in the machining space, the 5 axis CNC machining is much more complicated than the three-axis machine tool in its CNC programming, CNC system and machine tool structure. Therefore, the five-axis is easy to say, but it is really difficult to realize it! In addition, it is even more difficult to operate and use it well!
Speaking of five-axis, I have to talk about true and false five-axis? The difference between true and false 5-axis mainly lies in whether there is RTCP function. For this reason, the editor specifically searched for this word!
RTCP, let me explain, Fidia’s RTCP is the abbreviation of “Rotational Tool Center Point”, which literally means “rotating tool center”. Central programming”, in fact, this is just the result of RTCP. PA’s RTCP 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 Center Point Management”. Other manufacturers call similar technology TCPC, which is the abbreviation of “Tool Center Point Control”, which means tool center point control.
From the literal meaning of Fidia’s RTCP, assuming that the RTCP function is manually executed at a fixed point, the tool center point and the actual contact point between the tool and the workpiece surface will remain unchanged, and the tool center point falls at the actual contact point between the tool and the workpiece surface On the normal line, the tool handle will rotate around the center point of the tool. For the ball nose tool, the center point of the tool is the target trajectory point of the NC code.
In order to achieve the purpose of allowing the tool handle to simply rotate around the target trajectory point (that is, the tool center point) when executing the RTCP function, it is necessary to compensate in real time the offset of each linear coordinate of the tool center point caused by the rotation of the tool handle. It is possible to change the included angle between the tool holder and the normal at the actual contact point of the tool and the workpiece surface while keeping the center point of the tool and the actual contact point between the tool and the surface of the workpiece unchanged, so as to achieve the best cutting effect of the ball-end cutter Efficiency, and effectively avoid interference and other effects. Therefore, RTCP seems to stand more on the center point of the tool (that is, the target trajectory point of the NC code) to deal with the change of the rotation coordinates.
Five-axis machine tools and CNC systems without 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, so only It is called false five-axis, and many domestic five-axis CNC machine tools and systems belong to this type of false five-axis. Of course, there is nothing wrong with people insisting on calling themselves five-axis linkage, but this (false) five-axis is not the other (true) five-axis!
Therefore, the editor also consulted experts in the industry. In short, the real five-axis is five-axis five-linkage, and the fake five-axis may be five-axis three-linkage, and the other two axes only play a positioning function!
This is a popular saying, not a standard one. Generally speaking, there are two types of five-axis machine tools: one is five-axis linkage, that is, five axes can be linked at the same time, and the other is five-axis positioning processing. It is a five-axis three-linkage: that is, two rotating axes are rotated and positioned, and only three axes can be processed simultaneously. This kind of five-axis machine tool, commonly known as the 3+2 mode, can also be understood as a false five-axis.
The current form of five-axis CNC machine tools
In the mechanical design of 5-axis machining centers, machine tool builders have been working unremittingly to develop new motion modes to meet various requirements. Based on the various types of five-axis machine tools currently on the market, although there are various types of mechanical structures, there are mainly the following types:
- Two rotation 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 (protruding type swing head type)
- Two rotation coordinates directly control the rotation of the space (double turntable form)
- Two coordinate axes are on the table, but the rotation axis is not perpendicular to the linear axis (plumbed table type)
- One of the two rotation coordinates acts on the tool, and the other acts on the workpiece (one pendulum and one revolution form)
- After seeing the five-axis machine tools with these structures, I believe we should understand what and how the five-axis machine tools are moving.
Five-axis CNC programming is abstract and difficult to operate
This is a headache for every traditional NC programmer. Three-axis machine tools only have linear coordinate axes, while five-axis CNC machine tools have various structures; the same NC code can obtain the same processing effect on different three-axis CNC machine tools, but the NC code of a certain five-axis machine tool cannot be applied to All types of five-axis machines. In addition to linear motion, NC programming also needs to coordinate calculations related to rotary motion, such as rotation angle stroke inspection, nonlinear error check, tool rotation motion calculation, etc. The amount of information to be processed is very large, and NC programming is extremely abstract.
The operation and programming skills of five-axis CNC machining are closely related. If the user adds special functions to the machine tool, the programming and operation will be more complicated. Only by repeated practice can programming and operators 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 insufficient technical training and services, the inherent functions of five-axis CNC machine tools are difficult to realize, and the utilization rate of machine tools is very low. In many cases, it is better to use three-axis machine tools.
Very strict requirements on NC interpolation controller and servo drive system
The movement of the five-axis machine tool is the synthesis of the movements of the five coordinate axes. The addition of rotating coordinates not only increases the burden of interpolation calculations, but also the small errors of rotating coordinates will greatly reduce the machining accuracy. Therefore, the controller is required to have higher operation precision.
The kinematic 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
In order to improve the efficiency of machining, it is urgent to eliminate the traditional “trial cutting method” calibration method. In 5 axis CNC machining aluminum,copper,titanium etc, the verification of NC programs has also become very important, because the workpieces usually processed by five-axis CNC machine tools are very expensive, and collision is a common problem in five-axis CNC machining: the tool cuts into the workpiece; Collision to the workpiece at a very high speed; collision between the tool and the machine tool, fixture and other equipment in the processing range; collision between the moving part on the machine tool and the fixed part or the workpiece. In five-axis CNC, the collision is difficult to predict, and the calibration program must conduct a comprehensive analysis of the kinematics of the machine tool and the 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 machining, the tool path cannot be directly modified like in three-axis CNC. On a three-axis machine tool, the machine operator can directly modify parameters such as tool radius. In five-axis machining, the situation is not so simple, because changes in tool size and position have a direct impact on the subsequent rotational motion trajectory.
Tool Radius Compensation
In the five-axis linkage NC program, the tool length compensation function is still valid, but the tool radius compensation is invalid. When contact forming milling is performed with a cylindrical milling cutter, different programs need to be compiled for different diameter cutters. None of the current popular CNC systems can complete 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 during 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 toolpath correction is transferred from the CNC application to the CAM system, and the calculated toolpath is sent directly to the controller. LCOPS requires a third party to provide CAM software that can be connected directly to the CNC machine, where CAM system files are transferred instead of ISO codes. The ultimate solution to this problem depends on the introduction of a new generation of CNC control systems that 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 has two rotating coordinates. The tool position is converted 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, only the basic parameters of the machine tool can be input 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 has yet to be further developed.
When the three axes are linked, the position of the origin of the workpiece on the machine table does not need to be considered in the tool trajectory, and the post-processor can automatically handle the relationship between the workpiece coordinate system and the machine tool 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 size of the workpiece on the C turntable and the position dimensions between the B and C turntables, when generating the tool path 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 table, measure the position and orientation 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 nonlinear 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 nonlinear error, and the nonlinear error can be calculated only after the machine tool program is generated by the post-processor. Tool path linearization can solve this problem. Some control systems are capable of linearizing the toolpath while machining, but usually this is done in a post-processor.
Another problem caused by the axis of rotation is singularity. If the singularity is at the extreme position of the rotation axis, a small oscillation near the singularity will result in a 180° flip of the rotation axis, which is quite dangerous.
Requirements for CAD/CAM systems
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.
There used to be a huge price gap between five-axis machines and three-axis machines. Now, adding a rotary 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 be upgraded to meet the requirements of five-axis machining; the calibration program must be upgraded so that it can simulate the entire machine tool .
Future intelligent trend of five-axis processing machine tools
The control mode and man-machine interface of smart equipment will undergo great changes. With the improvement of network performance such as WiFi broadband and Bluetooth short-distance communication, network-based mobile control methods based on tablets, mobile phones and wearable devices will become more and more popular. . The touch screen and multi-touch graphical human-machine interface that keep pace with the times will gradually replace buttons, switches, mice and keyboards. People, especially young people, have become accustomed to the operation of smart electronic consumer products, and can respond quickly, switch screens, upload or download data, which greatly enriches the content of human-computer interaction and significantly reduces the rate of misuse.