How to achieve machining accuracy?

The machining accuracy of CNC machine tools is ultimately guaranteed by the accuracy of the machine tool itself. The accuracy of CNC machine tools includes geometric accuracy, positioning accuracy, repeat positioning accuracy and cutting accuracy. Geometric accuracy, which is also known as static accuracy, comprehensively reflects the comprehensive geometric shape error of the key parts of CNC machine tools after assembly.

Positioning accuracy: indicating the accuracy that can be achieved by the movement of the measured machine tool under the control of the numerical control device. According to the measured positioning accuracy value, the best workpiece machining accuracy that can be achieved in the automatic machining process of the machine tool can be judged. 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. It is the prerequisite to ensure the accuracy of parts processing.

Repeated positioning accuracy refers to the consistency of the position accuracy obtained by repeatedly running the same program code on the CNC machine tool. It is the consistency of the continuous results obtained by processing a batch of parts under the same conditions (on the same CNC machine tool, with different operating methods, using 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.

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

I. Principle introduction

The motion chain of CNC machine tools 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. Fully closed loop control feed servo system

Install the position detection device (such as grating ruler, linear induction synchronizer, etc.) on the moving parts of the machine tool (such as the worktable), and provide real-time feedback to the position of the moving parts, and the state of the machine tool will be notified to the servo motor after processing by the CNC system. The motor automatically compensates for motion error through system commands. But because it puts the large inertia links of the lead screw, nut pair and machine tool table in a closed loop, it is more troublesome to debug the system in a stable state during debugging. In addition, measuring devices such as grating rulers and linear induction synchronizers are expensive and complicated to install, which may cause oscillations. Therefore, 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 mechanical manufacturing level 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 fairly high feed accuracy. Most machine tool manufacturers have widely adopted semi-closed-loop CNC systems.

II. Practical application

1. Fully closed loop control system

Position detection devices (such as grating rulers, linear induction synchronizers, etc.) have different accuracy levels (±0.01mm, ±0.005mm, ±0.003mm, ±0.02mm), so the full closed loop control will also have errors, and the positioning accuracy is affected by the accuracy Level influence.

The thermal performance (thermal deformation) of the position detection device. The measuring device is generally non-metallic. The coefficient of thermal expansion is inconsistent with the various parts of the machine tool. It is a key link in the accuracy of the machine tool. Therefore, it is necessary to solve the heat generation problem during the machine tool processing to overcome the Thermal deformation caused by temperature. High-end machine tools will adopt various methods, such as screw hollow cooling, guide rail lubrication, cutting fluid constant temperature cooling, etc., to reduce thermal deformation during the machining process of the machine tool.

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 to install it near the side of the lead screw, the other is to install it on the outside of the guide rail. It is recommended to choose the first installation method as much as possible, but it is inconvenient for inspection and maintenance. On the contrary, a high-precision grating ruler was selected, but it did not actually achieve the accuracy required by the CNC machine tool. Even in the first case, the installation position of the grating ruler is relatively close to the driving axis, but after all, the installation position is at a certain distance from the driving axis. The combination of this distance and the swing of the object during driving brings great control to the detection and control of the grating ruler. Big trouble. When the driving object swings to the mounting side of the grating ruler, the grating ruler mistakenly believes that the moving speed is insufficient during detection, and the system gives an acceleration signal, and the driving object immediately swings to the other side, and the grating ruler mistakenly believes that the moving speed is too high during detection. If it is fast, the system will give a deceleration signal, so it runs repeatedly, and it does not improve the control of each linear coordinate axis of the CNC machine tool. On the contrary, it aggravates the vibration of the driving object, resulting in a peculiar phenomenon that the fully closed loop is not as good as the semi-closed loop.

Production environment impact: Generally, the environment of machining factories is harsh. Dust and vibration are common phenomena. However, grating scales and linear induction synchronizers are precision components. The working principle is to measure the relative moving position by the reflection of light. Dust and vibration are exactly what The biggest factor affecting measurement accuracy. In addition, the cutting oil mist and water mist are more serious during the machining of the machine tool, which has a great impact on the grating ruler and linear induction synchronizer. Therefore, to use a fully closed-loop control system, in addition to installing and sealing well, the production environment must be improved. Otherwise, this phenomenon will occur. The accuracy of the new machine tool that has just arrived is good, but after less than a year, not only the accuracy decreases, but the machine often alarms.

2. Semi-closed loop control system

Since the measuring device is installed on the top of the motor or the lead screw, it is easier to seal, so there is no requirement for the environment. The accuracy error of the semi-closed loop control system mainly depends on the forward and reverse clearance of the lead screw. With the improvement of mechanical processing technology, the current manufacturing technology level of imported lead screws is relatively high, and the high-precision lead screw pair basically eliminates the forward and reverse gaps. In addition, in the assembly process, the screw pair adopts a double-row reverse ball screw pair, which can completely eliminate the forward and reverse gaps. In addition, many machine tool factories use the pre-stretching method of the lead screw when assembling the machine tool, which eliminates the influence of the thermal deformation of the machine tool on the accuracy of the lead screw transmission. Therefore, the current semi-closed loop control system has been able to ensure that the machine tool achieves high accuracy.

III. Conclusion

In summary, it can be seen that, in theory, if external factors are not taken into consideration, the full-closed-loop control may improve the basic positioning accuracy than the semi-closed-loop control. However, if the machine tool heating, environmental pollution, temperature rise, vibration, installation and other factors cannot be solved well, the phenomenon that the fully closed loop is not as good as the semi-closed loop will appear. It may be effective 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 thus lose its effect. At the same time, when there is a problem with the grating ruler, an alarm will be generated, causing the machine tool to fail to work.

For low- and mid-end machine tools, due to the consideration of production costs and competitiveness, the full-closed-loop control has been simplified. For example, sealing and temperature rise control are not well guaranteed. Under this condition, the cost is relatively high, and simply configuring the grating ruler cannot improve the accuracy of the machine tool.