What guarantees the machining accuracy of machine tools?

Dry machining industry, machining accuracy is hanging in the mouth of the mantra, every day to recite several times, see the industry chat, not out of three sentences must also mention machining accuracy. Then the machine tool machining accuracy in the end is to rely on what to guarantee it?

CNC machine tool processing accuracy ultimately depends on the accuracy of the machine itself to ensure that the CNC machine tool accuracy, including geometric accuracy, positioning accuracy, repeat 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 integrated geometric shape error.

Positioning accuracy: is to show that the measured moving parts of the machine tool under the control of the CNC device, the accuracy of the movement can be achieved. According to the actual measured positioning accuracy values, you can determine the best workpiece processing accuracy that can be achieved during the automatic processing of machine tools. Is the actual position of the part or tool, etc. and the gap between the standard position (theoretical position, ideal position), the smaller the gap, the higher the accuracy. It is the premise that the machining accuracy of the part can be guaranteed.

Repeat positioning accuracy: refers to the consistent degree of position accuracy obtained by repeatedly running the same program code on the CNC machine tool. Is in the same conditions (the same CNC machine tool, different methods of operation, the application of the same parts program) processing a number of parts obtained by the consistent degree of continuous results.

Cutting accuracy: is the geometric accuracy of the machine tool and positioning accuracy in cutting processing conditions of a comprehensive check.

As seen above, the accuracy of CNC machine tools are divided into mechanical and electrical two aspects, mechanical aspects such as spindle accuracy, such as runout, busbar, etc.; screw accuracy; processing accuracy of the fixture, the rigidity of the machine, etc.. Electrical aspects are mainly control methods such as semi-closed loop, fully closed loop, etc., and feedback and compensation methods, interpolation accuracy during processing. So the machine accuracy does not depend on whether the machine is fully closed-loop.

First, the principle of introduction

CNC machine tool motion chain includes CNC device → servo encoder → servo drive → motor → screw → moving parts, according to the position detection device installation position is different, divided into fully closed-loop control, semi-closed-loop control, open-loop control.

1、Fully closed-loop control feed servo system

Position detection device (such as scale, linear induction synchronizer, etc.) installed in the machine tool moving parts (such as the table), and real-time feedback on the position of the moving parts, through the CNC system processing will be informed of the machine state servo motor, servo motor through the system instructions to automatically compensate for motion errors. But because it will be the screw, nut vice and machine table these large inertia links in the closed loop, debugging, its system stability state debugging more trouble. In addition, like the scale, linear induction synchronizer such measuring devices are more expensive, complex installation, may cause oscillations, so the general machine tool does not use full closed-loop control.

2, semi-closed-loop control feed servo system

Position detection device installed in the end of the drive motor or the end of the screw, used to detect the screw or servo motor rotation angle, indirectly measured the actual position of the machine tool moving parts, sent back to the control system by feedback. Due to the improvement of machinery manufacturing level and speed detection components and screw pitch accuracy, semi-closed-loop CNC machine tools have been able to achieve a fairly high feed accuracy. Most of the machine tool manufacturers widely used semi-closed-loop CNC system.

Second, the actual application

1, full closed-loop control system

Position detection devices (such as scale, linear induction synchronizer, etc.) have different accuracy levels (± 0.01mm, ± 0.005mm, ± 0.003mm, ± 0.02mm), so the full closed-loop control will also have errors, positioning accuracy is affected by the level of accuracy.

Position detection device thermal performance (thermal deformation), measuring devices are generally non-metallic materials, the coefficient of thermal expansion and the machine tool components are not consistent, it is the key link in the work accuracy of the machine tool, so it is necessary to solve the problem of heat during the processing of the machine tool to overcome the thermal deformation caused by temperature. High-end machine tools will use a variety of ways, such as screw hollow cooling, guide lubrication, cutting fluid constant temperature cooling, etc. to reduce the thermal deformation of the machine tool machining process.

Position detection device installation is also very important, theoretically, the closer the drive axis (screw sub), the more accurate the measurement. Due to the structure of space constraints, the scale is installed in only two ways, one is installed near the side of the screw sub, the other is installed in the outer side of the guide. Recommended to choose the first installation method as far as possible, but the maintenance and repair is not convenient. Conversely, a high precision scale is selected, and the actual accuracy required by the CNC machine tool is not achieved. Even if the first case, the installation position of the scale is relatively close to the drive axis, but the installation position is, after all, a certain distance from the drive axis, which is a little distance and the combination of the swing of the object when driving, the detection control of the scale brings a lot of trouble. When the drive object to the scale installation side swing, the scale in the detection of the moving speed is wrongly considered insufficient, the system is given to accelerate the signal, and the drive object immediately swing to the other side, the scale in the detection of the moving speed is wrongly considered too fast, the system is given to decelerate the signal, so repeatedly run, actually did not improve the CNC machine control of each linear axis, but intensified the vibration of the drive object, resulting in Fully closed-loop is not as peculiar as semi-closed-loop phenomenon.

Production environment impact: general machining factory environment is relatively harsh, dust, vibration is a common phenomenon, but the scale, linear induction synchronizer belongs to the precision components, the working principle is to rely on the reflection of light to measure the relative movement of position, dust, vibration is precisely the biggest factor affecting the measurement accuracy. In addition, the machine tool in processing, cutting oil mist, water mist is more serious, the scale, linear induction synchronizer impact is very big. So to use a fully closed-loop control system, in addition to a good installation seal, must improve the production environment. Otherwise, this phenomenon will occur, just come to the new machine precision is good, but with less than a year, not only the accuracy of the decline, the machine often alarm.

2, semi-closed-loop control system

As the measurement device is mounted on the motor or screw top, it is easier to seal, so there is no requirement for the environment. Semi-closed-loop control system accuracy error depends mainly on the forward and reverse clearance of the screw. With the improvement of machining process, the manufacturing process level of imported screw is now higher, and the high precision screw sub fit basically eliminates the forward and reverse clearance. In addition, in the assembly link, the screw vice adopts double row reverse ball screw vice, which can completely eliminate the forward and reverse clearance. In addition, many machine tool factories, in the machine tool assembly, the screw will be pre-stretched way, eliminating the machine tool thermal deformation of the screw drive accuracy. So the current semi-closed-loop control system has been able to ensure that the machine tool to achieve a high degree of accuracy.

III. Conclusion

In summary, it can be seen that, in theory, if external factors are not taken into account, fully closed-loop control than semi-closed-loop control may improve the positioning accuracy of the base. However, if the machine tool heat, environmental pollution, temperature rise, vibration, installation and other factors can not be well resolved, there will be a fully closed-loop is not as good as semi-closed-loop phenomenon. Short time may have effect, but a long time, dust, temperature changes on the scale, will seriously affect the measurement of feedback data, thus losing its role. At the same time the scale problems, will produce an alarm, resulting in machine tools can not work.

Mid- and low-end machine tools, due to consideration of production costs and competitiveness, in the full closed-loop control package are simplified, such as sealing, temperature control, etc. does not have a good guarantee. In this condition, spend a larger cost, simply configure the scale does not improve the accuracy of the machine tool.