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What is the guarantee of machine tool machining accuracy

2021-04-28 13:40

The machining accuracy of CNC machine tools is ultimately guaranteed by the accuracy of the machine itself. The accuracy of CNC machine tools includes geometric accuracy, positioning accuracy, repeated positioning accuracy and cutting accuracy.  

Geometric accuracy:Also known as static accuracy, is a comprehensive reflection of the key components of CNC machine tools after assembly of the comprehensive geometric error.
Positioning accuracy:It is to show that the measured machine tool parts of the movement under the control of the numerical control device, the movement can achieve accuracy. According to the measured positioning accuracy value, it can be judged that the best workpiece machining accuracy can be achieved in the automatic machining process of the machine tool. It refers to the gap between the actual position of the part or tool and the standard position (theoretical position, ideal position). The smaller the gap, the higher the accuracy. Is the premise of parts machining accuracy can be guaranteed.
Repeated positioning accuracy:Refers to the degree of consistency of the position accuracy obtained by repeatedly running the same program code on a CNC machine tool. It is the consistency of continuous results obtained by processing a batch of parts under the same conditions (on the same CNC machine tool, with different operation methods and the same part program).
Cutting accuracy:It is a comprehensive inspection of the geometric accuracy and positioning accuracy of the machine tool under cutting conditions.

From the above, it can be seen that the accuracy of CNC machine tools is divided into two aspects: mechanical and electrical. Mechanical aspects such as spindle accuracy, such as beating, bus, etc.; The accuracy of the lead screw; The accuracy of the fixture during processing, the rigidity of the machine tool, and so on. Electrical aspects are mainly control methods such as semi-closed loop, full closed loop, etc., as well as feedback and compensation methods, interpolation accuracy during processing, etc. Therefore, the accuracy of the machine tool does not depend on whether the machine tool is fully closed-loop.

Introduction to 1. Principle   
The CNC machine tool motion chain includes CNC device → servo encoder → servo drive → motor → screw → moving parts. According to the different installation positions of the position detection device, it is divided into full closed-loop control, semi-closed-loop control and open-loop control.
1, Full closed-loop control feed servo system
The position detection device (such as grating ruler, linear induction synchronizer, etc.) is installed on the moving parts of the machine tool (such as the worktable), and the position of the moving parts is fed back in real time. After being processed by the numerical control system, the state of the machine tool is informed to the servo motor, and the servo motor automatically compensates the motion error through the system instruction. However, because it puts the lead screw, nut pair and machine tool worktable in the closed loop, the system stable state debugging is more troublesome. In addition, measuring devices such as grating ruler and linear inductosyn are expensive and complicated to install, which may cause oscillation, so general machine tools do not use full closed-loop control.
2. Semi-closed loop control feed servo system

The position detection device is installed at the end of the drive motor or the end of the screw rod to detect the rotation angle of the screw or servo motor, indirectly measure the actual position of the moving parts of the machine tool, and send it back to the control system through feedback. Due to the improvement of the level of mechanical manufacturing and the improvement of the accuracy of the speed detection element and the screw pitch, the semi-closed loop CNC machine tool has been able to achieve a very high feed accuracy. Most of the machine tool manufacturers widely used the semi-closed loop CNC system.

2. practical application
1. Full closed-loop control system    
Position detection devices (such as grating ruler, linear induction synchronizer, etc.) have different accuracy levels (± 0.01mm, ± 0.005mm, ± 0.003mm, ± 0.02mm), so there will be errors in the full closed-loop control, and the positioning accuracy is affected by the accuracy level.
The thermal performance of the position detection device (thermal deformation), the measuring device is generally non-metallic materials, and the thermal expansion coefficient is inconsistent with the various parts of the machine tool. It is a key link in the accuracy of the machine tool. High-end machine tools will adopt various methods, such as hollow cooling of the lead screw, lubrication of the guide rail, constant temperature cooling of the cutting fluid, etc., to reduce the thermal deformation during machine tool processing.
The installation of the position detection device is also very important. In theory, the closer to the drive axis (screw pair), the more accurate the measurement. Due to the limitation of structural space, there are only two ways to install the grating ruler, one is installed near the side of the screw, and the other is installed on the outside of the guide rail. It is recommended to select the first installation method as much as possible, but it is inconvenient to repair and maintain. On the contrary, the choice of high-precision grating ruler, and the actual accuracy of CNC machine tools did not meet the requirements. Even in the first case, the installation position of the grating ruler is relatively close to the drive axis, but the installation position has a certain distance from the drive axis after all. After this distance is combined with the swing of the object during driving, the detection and control of the grating ruler brings a lot of trouble. When the driving object swings to the installation side of the grating ruler, the grating ruler mistakenly thinks that the moving speed is insufficient during detection, and the system gives an acceleration signal, while the driving object swings to the other side immediately, the grating ruler mistakenly thinks that the moving speed is too fast during detection, and the system gives a deceleration signal. This repeated operation does not improve the control of each linear coordinate axis of the numerical control machine tool, but aggravates the vibration of the driving object, this leads to the peculiar phenomenon that the full closed loop is not as good as the half closed loop.
Production environment impact:General machining factory environment is relatively harsh, dust, vibration is a common phenomenon, but the grating ruler, linear inductosyn are precision components, the working principle is to rely on the reflection of light to measure the relative moving position, dust, vibration is precisely the biggest factor affecting the measurement accuracy. In addition, when the machine tool is processed, the cutting oil mist and water mist are more serious, which has a great influence on the grating ruler and the linear induction synchronizer. Therefore, to use a full closed-loop control system, in addition to the installation of seals, we must improve the production environment. Otherwise, this phenomenon will occur. The accuracy of the new machine tool is good, but after less than a year, not only the accuracy decreases, but also the machine tool often alarms.
2. Semi-closed loop control system

Since the measuring device is installed at the top of the motor or screw, it is easier to seal, so there are no environmental requirements. The accuracy error of the semi-closed loop control system mainly depends on the positive and negative clearance of the lead screw. With the improvement of machining technology, the current level of manufacturing technology of imported screw is higher, and the high-precision screw pair basically eliminates the positive and negative clearance. In addition, in the assembly process, the screw pair adopts a double-row reverse ball screw pair, which can completely eliminate the positive and negative clearance. In addition, many machine tool factories, when the machine tool is assembled, the screw is pre-stretched, which eliminates the influence of thermal deformation of the machine tool on the accuracy of the screw drive. Therefore, the current semi-closed loop control system has been able to ensure that the machine tool to achieve high accuracy.

3. Conclusion
In summary, it can be seen that in theory, if external factors are not considered, full closed-loop control may improve the positioning accuracy of the foundation than semi-closed-loop control. However, if the heat, environmental pollution, temperature rise, vibration, installation and other factors cannot be solved well, the phenomenon that the full closed loop is not as good as the half closed loop will appear. There may be an effect in a short time, but over time, the impact of dust and temperature changes on the grating ruler will seriously affect the measurement feedback data and lose its effect. At the same time, when the grating ruler has problems, an alarm will be generated, causing the machine tool to fail to work.

In the low-end machine tools, due to the consideration of production costs and competitiveness, the whole closed-loop control supporting have been simplified, such as sealing, temperature rise control and so on are not well guaranteed. Under such conditions, it costs a lot of money, and the simple configuration of the grating ruler cannot improve the accuracy of the machine tool.