The machining accuracy of CNC machine tools ultimately depends on the accuracy of the machine itself. The accuracy of CNC machine tools includes geometric accuracy, positioning accuracy, repetitive positioning accuracy and cutting accuracy.
Geometric accuracy: also known as static accuracy, is a comprehensive reflection of the key parts of CNC machine tools after assembly of the comprehensive geometric errors.
Location accuracy: It shows that the measured machine tool motion parts under the control of numerical control device can achieve the accuracy of movement. According to the measured positioning accuracy values, the best workpiece processing accuracy can be judged in the process of automatic machining of machine tools. It refers to the difference between the actual position of parts or cutters and the standard position (theoretical position, ideal position). The smaller the gap, the higher the accuracy. It is the precondition of ensuring the machining accuracy of parts.
Repeated positioning accuracy: refers to the degree of consistency of positioning accuracy obtained by repeatedly running the same program code on CNC machine tools. It is the consistency of continuous results obtained by processing a batch of parts under the same conditions (on the same NC machine tool, different operation methods and the same part program).
Cutting accuracy: It is a comprehensive inspection of the geometric accuracy and positioning accuracy of machine tools under cutting conditions.
From the above, it can be seen that the accuracy of CNC machine tools is high or low in mechanical and electrical aspects, such as spindle accuracy, such as jumping, bus, screw accuracy, fixture accuracy in processing, rigidity of machine tools and so on. In electrical aspect, the main control modes are semi-closed loop, full-closed loop, feedback and compensation mode, interpolation accuracy in processing, etc. So the accuracy of machine tool does not depend on whether the machine tool is fully closed-loop.
Introduction to Principles
The motion chain of NC machine tool includes NC device servo encoder servo driver motor screw moving parts. According to the installation position of 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
Position detection device (such as grating ruler, linear induction synchronizer, etc.) is installed on the moving parts (such as worktable) of machine tool, and the position of moving parts is feedback in real time. After processing by numerical control system, the machine tool state is informed to the servo motor, and the servo motor can compensate the motion error automatically by system instructions. But because it puts the big inertia links such as screw, nut pair and machine tool worktable in the closed loop, debugging of the system stability state is more troublesome. In addition, measuring devices such as grating ruler and linear induction synchronizer are expensive and complicated to install, which may cause oscillation. Therefore, closed-loop control is not used in general machine tools.
2. Semi-closed-loop control feed servo system
The position detection device is installed at the end of the driving motor or the end of the screw, which is used to detect the rotation angle of the screw or the servo motor. The actual position of the moving parts of the machine tool is measured indirectly and sent back to the control system through feedback. Due to the improvement of mechanical manufacturing level and the accuracy of speed detection components and screw pitch, semi-closed-loop CNC machine tools have achieved considerable feed accuracy. Semi-closed-loop CNC system is widely used by most machine tool manufacturers.
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 will be affected by the accuracy level.
The thermal performance (thermal deformation) of position detection device is generally non-metallic material. The thermal expansion coefficient is not consistent with the parts of machine tool. It is the key link of the accuracy of machine tool. Therefore, it is necessary to solve the heating problem in the process of machine tool processing in order to overcome the thermal deformation caused by temperature. High-end machine tools will adopt various ways, such as screw hollow cooling, guide rail lubrication, cutting fluid constant temperature cooling, to reduce the thermal deformation of machine tools in the process of processing.
Position detection device installation is also very important. In theory, the closer to the driving axis (screw pair), the more accurate the measurement. Due to the structural space limitation, there are only two ways to install the grating ruler, one is to install near the screw pair side, the other is to install outside the guide rail. It is recommended to choose the first installation method as far as possible, but it is not convenient for maintenance and repair. On the contrary, the grating ruler with high precision is chosen, but it does not meet the precision required by NC machine tools. Even in the first case, the installation position of the grating ruler is closer to the driving axis, but the installation position has a certain distance from the driving axis after all. This distance combined with the swing of the object when driving brings great trouble to the detection and control of the grating ruler. When the driving object swings to the mounting side of the grating ruler, the grating ruler mistakenly considers that the moving speed is insufficient, and the system gives the acceleration signal, while the driving object swings to the other side immediately. The grating ruler mistakenly considers that the moving speed is too fast in the detection, and the system gives the deceleration signal, so the system runs repeatedly and unexpectedly remains unchanged. Good control of linear coordinate axes of NC machine tools, on the contrary, exacerbates the vibration of the driving object, resulting in a strange phenomenon that the full closed loop is inferior to the half closed loop.
Environmental impact of production: In general, the environment of mechanical processing plants is bad, dust and vibration are common phenomena, but grating ruler and linear induction synchronizer are precise components. Their working principle is to measure relative moving position by reflection of light. Dust and vibration are the most important factors affecting measurement accuracy. In addition, the machine tool in processing time 。
