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do you really understand five-axis machining?

Views : 94
Author : jett NG
Update time : 2021-05-19 21:47:46

Have done more than ten years of CNC, do you really understand five-axis machining?

 

5 Axis Machining, as the name suggests, is a mode of CNC machine tool Machining. Using the linear interpolation motion of any 5 coordinates in X, Y, Z, A, B, C, the five-axis machine tool used in five-axis machining is usually called A five-axis machine tool or five-axis machining center. But do you really know about five-axis machining?

 

The development of five-axis technology

 

For decades, it has been widely accepted that five-axis CNC machining technology is the only means for machining continuous, smooth and complex surfaces. Once people in the design, manufacturing complex surface encountered insoluble problems, will seek the five axis machining technology. But...

 

Five-axis NC is the most difficult and widely used NC technology. It integrates computer control, high performance servo drive and precision machining technology, and is applied to the efficient, precise and automatic machining of complex curved surface. The international five axis linkage numerical control technology as a national production equipment automation technology level symbol. Because of its special status, especially for aviation, aerospace, military industry's important influence, as well as the complexity of technology, the western industrial developed countries have been the five axis CNC system as a strategic material to implement export license system, to our country to implement an embargo, restrict the development of our national defense, military industry.

 

Submetalworking small braided hair about "Toshiba machine tool incident" is based on this blockade system!

 

Compared with the three-axis CNC machining, from the point of view of technology and programming, the five-axis CNC machining for complex surfaces has the following advantages:

 

(1) improve the processing quality and efficiency

 

(2) Expand the process range

 

(3) Meet the new direction of complex development

 

But, ha ha, but again... Because of the interference and the pose control of the tool in the machining space, the numerical control program, numerical control system and the structure of the machine tool are much more complicated than the three-axis machine tool. So, the five axes is easy to say, but really hard to achieve! In addition to the use of good operation is really more difficult!

 

Speaking of the five axes, do you really have to say true or false five axes? Xiaobian some time ago released a "false five axis or true five axis? How is it different from the three axes?" The article, in fact, the article mainly tells the difference between true and false 5 axis mainly lies in whether there is RTCP function, for this reason, Xiaobian specifically to find the word!

 

RTCP, Fidia's RTCP is an abbreviation of "Rotating Tool Center Point," which literally translates as "Rotating Tool Center Point," or some literally translates as "Rotating Tool Center Programming." In fact, this is only the result of RTCP. PA's RTCP is the abbreviation of the first few words of "Real-time Tool Center Point Rotation". Hedham calls a similar upgrade technique TCPM, short for "Tool Centre Point Management." Other manufacturers call similar technology TCPC, namely "Tool Center Point Control" abbreviation, Tool Center Point Control.

 

From the literal meaning of FIDIA's RTCP, assuming that the RTCP function is performed manually at a fixed point, the center point of the tool and the actual contact point between the tool and the workpiece surface will remain unchanged. At this time, the center point of the tool falls on the normal line of the actual contact point between the tool and the workpiece surface, and the handle will rotate around the center point of the tool. For ball head tool, The tool center point is the target trajectory point of the numerical control code. To make the handle when performing RTCP function can simply revolve around the target trajectory point (that is, the cutting tool center), the purpose of must be caused by the real-time compensation due to the handle rotation tool center deviation of each linear coordinate so you can keep in center of cutting tools and cutting tools and workpiece surface of actual contact point, the condition of invariable By changing the Angle between the handle and the normal of the actual contact point of the cutter and the workpiece surface, the optimal cutting efficiency of the ball-head cutter can be exerted and the interference can be avoided effectively. Thus, RTCP seems to be more about standing on the tool center point (the target trajectory point of the numerical control code) and dealing with changes in rotation coordinates.

 

Don't have RTCP five-axis machine tools and CNC system must depend on CAM programming and post-processing, planning in advance good dao road, the same parts, machine changed, changed, or the cutting tool must be a new CAM programming and post-processing, and so can only be known as false five axis, the domestic many five-axis nc machine tools and systems are such false five axis. Of course, there's nothing wrong with people trying to call themselves five axes, but this (false) five axis is not that (true) five axis!

 

Xiaobian therefore also consulted the industry experts, in short, the true five axis is five axis five linkage, false five axis may be five axis three linkage, the other two axis only play a positioning function!

 

This is a popular saying, not a standard saying, generally speaking, five axis machine tools are divided into two kinds: one is five axis linkage, that is, five axes can be simultaneously linkage, the other is five axis positioning processing, in fact, is five axis three linkage: That is, the two rotation axis rotation positioning, only three axes can be simultaneously linkage processing, this commonly known as 3+2 mode of the five-axis machine tool, can also be understood as a false five axis.

 

How to? Do you know about the true and false five axes? Have a new view, welcome to leave a message to discuss!

 

This time for the RTCP function is not a detailed description, if you are interested in this aspect, Xiaobian decided to collect some more information in this aspect next time, to give you the answer! If you need to welcome comments!

 

Difficulty and resistance of developing 5-axis NC technology

 

We have already realized the superiority and importance of five-axis numerical control technology. But so far, the use of five-axis CNC technology has been limited to a few well-funded departments, and there are still unsolved problems.

 

The following Xiaobian collected some difficulties and resistance, to see if it corresponds to your situation?

 

1. Five-axis CNC programming is abstract and difficult to operate

 

This is every traditional numerical control programming personnel are deeply headache. Three-axis machine tools have only linear axes, while five-axis CNC machine tools have various forms of structure; The same NC code can obtain the same machining effect on different three-axis CNC machine tools, but the NC code of a certain five-axis machine tool can not be applied to all types of five-axis machine tools. Numerical control programming in addition to linear motion, but also to coordinate the rotation of the related calculation, such as rotation Angle travel test, nonlinear error check, tool rotation calculation, processing information is very large, numerical control 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 complex. Only by repeated practice can programmers and operators master the necessary knowledge and skills. The lack of experienced programming and operators is a major resistance to the popularization of five-axis numerical control technology.

 

Many domestic manufacturers from abroad to buy five-axis CNC machine tools, due to technical training and service is not in place, five-axis CNC machine tools inherent functions are difficult to achieve, the utilization rate of machine tools is very low, many occasions is not as good as the use of three-axis machine tools.

 

2. Very strict requirements for NC interpolation controller and servo drive system

 

The motion of a five-axis machine tool is the synthesis of the motion of five axes. The addition of rotating coordinates not only increases the burden of interpolation operation, but also reduces the machining accuracy greatly due to the small error of rotating coordinates. Therefore, the controller is required to have higher accuracy of operation. The motion characteristics of the five-axis machine tool require the servo drive system to have good dynamic characteristics and large speed range.

 

3. It is particularly important to check the NC program of five-axis CNC

 

In order to improve the machining efficiency, it is urgent to eliminate the traditional "try-cut" checking method. In the five-axis CNC machining, NC program verification work has become very important, because the workpiece is usually processed by five-axis CNC machine tool is very expensive, and the collision is a common problem in five-axis CNC machining: cutting tool into the workpiece; The cutter hits the workpiece with great speed; Collision between cutting tools and machine tools, fixtures and other equipment in the processing range; A moving part on a machine tool collides with a fixed part or work piece. In five-axis numerical control, collision is difficult to predict, so the calibration program must analyze the kinematics and control system of the machine tool.

 

If the CAM system detects an error, the tool path can be processed immediately. However, if NC program errors are found during machining, the tool path cannot be directly modified as in three-axis CNC. in

 

On a three-axis machine tool, the machine operator can directly modify the parameters such as the radius of the tool. In five-axis machining, the situation is not so simple, because the change of tool size and position has a direct impact on the subsequent rotation trajectory.

 

4. Tool radius compensation

 

In the five-axis NC program, the tool length compensation function is still effective, but the tool radius compensation is ineffective. When the cylindrical milling cutter is used for contact forming, different programs need to be worked out for different diameters of the cutter. Current CNC systems are unable to complete tool radius compensation because the ISO file does not provide sufficient data to recalculate the tool position. The user needs to change the tool frequently or adjust the exact size of the tool during NC machining. According to the normal processing procedure, the tool path should be sent back to the CAM system for re-calculation. As a result, the efficiency of the whole process is very low.

 

Norwegian researchers are developing an interim solution to this problem called LCOPS(Low Cost Optimized ProductionStrategy). The data required for tool path correction is transmitted by the CNC application program to the CAM system, and the calculated tool path is sent directly to the controller. LCoPS requires a third party to provide CAM software that connects directly to a CNC machine tool and transmits CAM system files 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 (such as STEP) or CAD system files in a common format.

 

Postprocessor

 

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 transferred from the workpiece coordinate system to the machine coordinate system, and several coordinate transformations are needed in the middle. By using the popular post-processor generator in the market, the post-processor of three-axis CNC machine tool can be generated only by inputting the basic parameters of the machine tool. For 5-axis CNC machine tools, there are only some improved post-processor. The post-processor of the 5-axis CNC machine tool needs to be further developed.

 

In the case of three-axis linkage, the position of the origin of the workpiece on the workbench is not considered in the trajectory of the cutter, and the post-processor can automatically deal with the relationship between the coordinate system of the workpiece and the coordinate system of the machine tool. For five axis linkage, such as in the XYZBC five axis linkage horizontal milling machine processing, the workpiece on the C table position size and B, C table position size between each other, must be taken into account when the tool path. Workers usually spend a lot of time dealing with these position relationships when clamping the workpiece. If the post-processor can handle the data, the installation of the workpiece and the processing of the tool path will be greatly simplified: just clamp the workpiece on the workbench, measure the position and direction of the coordinate system of the workpiece, input the data to the post-processor, post-process the tool path can get the appropriate NC program.

 

6. Nonlinear error and singularity

 

The kinematics of a five-axis CNC machine tool is much more complicated than that of a three-axis machine tool due to the introduction of rotational coordinates. The first problem associated with rotation is the nonlinear error. The nonlinear error belongs to the programming error and can be controlled by reducing the step distance. In the pre-calculation stage, the programmer can not know the magnitude of the nonlinear error, only after the machine tool program is generated by the post-processor, the nonlinear error can be calculated. Linearization of the tool path can solve this problem. Some control systems can linearize the tool path while machining, but usually linearize the tool path in the rear processor.

 

Another problem raised by the axis of rotation is singularity. If the singularity is at the limit of the axis of rotation, a small oscillation near the singularity can cause the axis of rotation to flip 180°, which is quite dangerous.

 

7. Requirements for CAD/ CAM systems

 

For the operation of pentahedron machining, the user must rely on mature CAD/CAM system, and must have experienced programmers to operate the CAD/CAM system.

 

8. Large investment in machine tools

 

In the past, the price difference between five axis machine tools and three axis machine tools was large. Now, a three-axis machine tool with a rotating axis is basically the price of a common three-axis machine tool, which can achieve the functions of a multi-axis machine tool. At the same time, the price of five-axis machine tools is only 30 % ~ 50 % higher than the price of three-axis machine tools.

 

In addition to the investment of machine tool itself, CAD/CAM system software and post-processor must be upgraded to adapt to the requirements of five-axis machining; The calibration program must be upgraded to simulate the entire machine tool.

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