Five Axis Machining, as the name suggests, is a mode of CNC machining. The machine tool used for five axis machining, which uses linear interpolation motion in any five coordinates of X, Y, Z, A, B, and C, is usually referred to as 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
Five axis linkage CNC is the most difficult and widely used technology in CNC technology. It integrates computer control, high-performance servo drive, and precision machining technology, and is applied to efficient, precise, and automated machining of complex surfaces. Internationally, the five axis linkage CNC technology is regarded as a symbol of a country's level of production equipment automation technology. Due to its special status, especially its significant impact on aviation, aerospace, and military industries, as well as its technological complexity, developed Western industrial countries have always implemented an export license system for five axis CNC systems as strategic materials. Compared with three-axis linkage CNC machining, from the perspective of technology and programming, using 5-axis CNC machining for complex surfaces has the following advantages: 1 Improve processing quality and efficiency. 2. Expand the scope of the process. 3. Meet the new direction of composite development. However, due to interference and tool position control in the machining space, the CNC programming, CNC system, and machine structure of five axis CNC machining are much more complex than those of three-axis machine tools. So, the five axis is easy to say, but it's really difficult to achieve! Moreover, it is even more difficult to operate and apply effectively!
Speaking of the Five Axes, I have to say whether it is true or false? The main difference between true and false 5-axis is whether it has RTCP function. Therefore, the editor specifically searched for this word! RTCP, please explain that Fidia's RTCP stands for "Rotating Tool Center Point", which literally means "rotating tool center". In the industry, it is often slightly translated as "rotating around tool center", and some people directly translate it as "programming around tool center". In fact, this is only the result of RTCP. The RTCP of PA is an abbreviation for the first few words of "Real time Tool Center Point rotation". Heidelberg referred to a similar upgrade technology as TCPM, which stands for Tool Centre Point Management. Some manufacturers refer to similar technologies as TCPC, which stands for "Tool Center Point Control" or 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. At this point, the tool center point falls on the normal at the actual contact point between the tool and the workpiece surface, and the tool holder will rotate around the tool center point. For ball end cutters, the tool center point is the target trajectory point of the CNC code. In order to achieve the goal of simply rotating the tool holder around the target trajectory point (i.e. tool center point) during RTCP function, it is necessary to compensate in real time for the offset of the coordinates of the tool center point caused by the rotation of the tool holder. Only in this way can the angle between the tool holder and the normal at the actual contact point between the tool and the workpiece surface be changed while keeping the tool center point and the actual contact point between the tool and the workpiece surface unchanged, thereby maximizing the cutting efficiency of the ball end tool and effectively avoiding interference. Therefore, RTCP seems to be more focused on standing at the center point of the tool (i.e. the target trajectory point of the CNC code), handling changes in rotational coordinates.
Five axis machine tools and CNC systems without RTCP must rely on CAM programming and post-processing, planning the tool path in advance. For the same part, if the machine tool or tool is changed, CAM programming and post-processing must be performed again, so it can only be called a fake five axis. Many domestic five axis CNC machine tools and systems belong to this type of fake five axis. Of course, it's understandable for others to insist on calling themselves a five axis linkage, but this fake five axis is not a real five axis! Therefore, the editor also consulted industry experts. In short, the true five axis refers to five axis five linkage, while the false five axis may be five axis three linkage, and the other two axes only serve the positioning function!
This is a common saying, not a standardized one. Generally speaking, there are two types of five axis machine tools: one is five axis linkage, which means all five axes can be linked at the same time, and the other is five axis positioning machining. In fact, it is five axis three linkage, which means two rotating axes can rotate and position, and only three axes can be linked at the same time. This type 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 manufacturers have always been committed to developing new motion modes to meet various requirements. Considering the various types of five axis machine tools currently available in the market, although their mechanical structures are diverse, there are mainly several forms: two rotating coordinates directly control the direction of the tool axis (double pendulum form), two coordinate axes are at the top of the tool but not perpendicular to the linear axis (vertical pendulum form), two rotating coordinates directly control the rotation of space (double rotary table form), two coordinate axes are on the workbench but not perpendicular to the linear axis (vertical workbench form), two rotating coordinates act on the tool, one acts on the workpiece (one pendulum and one rotation form), and so on.
The difficulties and obstacles 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 financially strong departments, and there are still unresolved problems.
Abstract and difficult operation of five axis CNC programming
This is a headache for every traditional CNC programmer. Three axis machine tools only have linear coordinate axes, while five axis CNC machine tools have various structural forms; The same NC code can achieve the same machining effect on different three-axis CNC machine tools, but the NC code of a certain type of five axis machine tool cannot be applied to all types of five axis machine tools. In addition to linear motion, CNC programming also needs to coordinate the relevant calculations of rotational motion, such as rotation angle stroke inspection, nonlinear error verification, tool rotation motion calculation, etc. The amount of information processed is large, and CNC programming is extremely abstract.
The operation and programming skills of five axis CNC machining are closely related. If users add special functions to the machine tool, programming and operation will be more complex. Only through repeated practice can programming and operators master the necessary knowledge and skills. The lack of experienced programmers 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, the inherent functions of five axis CNC machine tools are difficult to achieve, and the utilization rate of machine tools is very low. In many situations, it is better to use three-axis machine tools. The requirements for NC interpolation controllers and servo drive systems are very strict; The motion of a five axis machine tool is a combination of five coordinate axis movements. The addition of rotating coordinates not only increases the burden of interpolation operations, but also significantly reduces machining accuracy due to small errors in rotating coordinates. Therefore, it is required that the controller has higher operational accuracy. The motion characteristics of a five axis machine tool require the servo drive system to have good dynamic characteristics and a large speed range.
The 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 the NC program has also become very important, because the workpiece usually processed by five axis CNC machine tools is very expensive, and collision is a common problem in five axis CNC machining: the tool cuts into the workpiece; The tool collides with the workpiece at a very high speed; Collision between cutting tools, machine tools, fixtures, and other equipment within the processing range; Collision between moving parts and fixed parts or workpieces on the machine tool. In five axis CNC, collision prediction is difficult, and the verification program must comprehensively analyze the kinematics and control system of the machine tool.
If the CAM system detects an error, it can immediately process the tool path; But if NC program errors are found during the machining process, the tool path cannot be directly modified like in three-axis CNC. On a three-axis machine tool, the operator can directly modify parameters such as tool radius. In five axis machining, the situation is not so simple because the 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 effective, but the tool radius compensation is ineffective. When using cylindrical milling cutters for contact forming milling, different programs need to be developed for tools with different diameters. The currently popular CNC systems are unable to complete tool radius compensation because the ISO file does not provide sufficient data to recalculate the tool position. Users need to frequently change tools or adjust the exact size of the tools during CNC machining. According to normal processing procedures, the tool trajectory should be sent back to the CAM system for recalculation. As a result, the efficiency of the entire processing process is very low.
Norwegian researchers are developing a temporary solution to this problem called LCOPS (Low Cost Optimized Production Strategy). The data required for tool trajectory correction is transmitted from the CNC application to the CAM system, and the calculated tool trajectory is directly sent to the controller. LCOPS requires third-party CAM software that can be directly connected to CNC machine tools, transmitting CAM system files instead of ISO codes. The ultimate solution to this problem depends on the introduction of a new generation CNC control system, which can recognize workpiece model files in common formats (such as STEP) or CAD system files.
Post Processors
The difference between a five axis machine tool and a three-axis machine tool is that it also has two rotational coordinates. The tool position is converted from the workpiece coordinate system to the machine coordinate system, and several coordinate transformations are required in between. By utilizing the popular post processor generator in the market, it is possible to generate a post processor for a three-axis CNC machine tool by simply inputting the basic parameters of the 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 still needs further development.
When the three-axis linkage is used, the position of the workpiece origin on the machine tool workbench does not need to be considered in the tool trajectory. The post processor can automatically handle the relationship between the workpiece coordinate system and the machine tool coordinate system. For five axis linkage, such as machining on a horizontal milling machine with X, Y, Z, B, and C five axis linkage, the position dimensions of the workpiece on the C turntable and the position dimensions between the B and C turntables must be considered when generating tool paths. Workers usually spend a lot of time dealing with these positional relationships when clamping workpieces. If the post processor can process this data, the installation of workpieces and the processing of tool trajectories will be greatly simplified; Simply clamp the workpiece onto the workbench, measure the position and direction of the workpiece coordinate system, input this data into the post processor, and perform post processing on the tool path to obtain the appropriate NC program.
Nonlinear error and singularity problems
Due to the introduction of rotational coordinates, the kinematics of a five axis CNC machine tool is much more complex than that of a three-axis machine tool. The first issue related to rotation is non-linear error. Nonlinear errors should be attributed to programming errors and can be controlled by reducing the step distance. In the pre calculation stage, programmers cannot determine the magnitude of nonlinear errors. Only after generating the machine program through the post processor can nonlinear errors be calculated. Linearization of tool trajectory can solve this problem. Some control systems can linearize the tool path while machining, but it is usually done in the post processor.
Another issue caused by the rotation axis is singularity. If the singularity is at the limit position of the rotation axis, any small oscillation near the singularity can cause a 180 ° flip of the rotation axis, which is quite dangerous.
A significant investment in purchasing machine tools
Previously, there was a significant price difference between five axis machine tools and three axis machine tools. Nowadays, adding a rotating axis to a three-axis machine tool is basically the price of a regular three-axis machine tool, which can achieve the functions of a multi axis machine tool. Meanwhile, the price of a five axis machine tool is only 30% to 50% higher than that of a three axis machine tool. In addition to the investment in the machine tool itself, it is also necessary to upgrade the CAD/CAM system software and post processor to meet the requirements of five axis machining; The verification program must be upgraded to enable simulation processing of the entire machine tool.
Future Development Trends of Five Axis Machining Machine Tools
Five axis machining center is a high-precision and high-efficiency CNC machining equipment with the ability to perform machining operations in multiple directions, widely used in aerospace, automotive manufacturing, mold processing and other fields. With the continuous development of manufacturing and technological progress, five axis machining centers are also constantly evolving and innovating, showing some obvious development trends and innovative directions.
Firstly, one of the development trends is intelligence and automation. With the continuous development of technologies such as artificial intelligence, big data, and cloud computing, machining centers are also moving towards intelligence and automation. By introducing intelligent control systems and automation devices, automatic adjustment, monitoring, and maintenance of equipment can be achieved, improving production efficiency and processing accuracy, reducing human intervention, lowering labor costs, and achieving intelligent production.
Secondly, the second development trend is towards multifunctionality and flexibility. With the diversification and personalization of market demand, the manufacturing industry has increasingly high requirements for equipment. The machining center not only needs to have high-precision and high-efficiency machining capabilities, but also needs to have multifunctionality and flexibility to adapt to the machining needs of different shapes and materials. Therefore, the future will develop towards multifunctionality and flexibility, which can achieve various processing operations