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, repeated positioning accuracy and cutting accuracy.
Geometric accuracy: also known as static accuracy, is a comprehensive reflection of the key parts of CNC machine tool after assembly of the comprehensive geometric error.
Positioning accuracy: it indicates the accuracy that can be achieved under the control of the numerical control device. According to the measured positioning accuracy value, the best workpiece processing accuracy can be determined in the automatic processing process of the machine tool. It refers to the difference between the actual position of the part or tool and the standard position (theoretical position and ideal position). The smaller the difference, the higher the accuracy. It is the premise to ensure the machining accuracy of parts.
Repeat positioning accuracy: refers to the consistency of position accuracy obtained by repeatedly running the same program code on the CNC machine. It is the consistent degree of continuous results obtained by processing a batch of parts under the same conditions (the same CNC 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 the machine tool under cutting conditions.
From the above, it can be seen that the accuracy of CNC machine tools can be divided into mechanical and electrical aspects, such as spindle accuracy, such as runout, bus, etc; Precision of lead screw; The accuracy of the fixture, the rigidity of the machine tool and so on. In the electrical aspect, there are mainly control methods, such as semi closed loop, full closed loop, feedback and compensation methods, interpolation accuracy in processing, etc. Therefore, the accuracy of the machine tool does not depend on whether the machine tool is fully closed-loop or not.
1、 Principle introduction
The motion chain of CNC machine tool includes CNC device → servo encoder → servo driver → motor → screw → moving parts. According to the installation position of position detection device, it can be divided into full closed-loop control, semi closed-loop control and open-loop control.
- 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 workbench), and the position of the moving parts is fed back in real time. After processing by the numerical control system, the status of the machine tool is informed to the servo motor, and the servo motor automatically compensates the motion error through the system instruction. But because it puts the large inertia links such as lead screw, nut pair and machine tool worktable in the closed loop, it is difficult to debug the stable state of the system. In addition, measuring devices such as grating ruler and linear inductosyn are expensive and complex to install, which may cause oscillation. Therefore, general machine tools do not use full closed-loop control.
- 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 rod, which is used 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 feed back to the control system. Due to the improvement of the level of mechanical manufacturing and the accuracy of speed detection elements and screw pitch, the semi closed loop CNC machine tool has achieved a fairly high feed accuracy. Most machine tool manufacturers widely use the semi closed loop CNC system.
2、 Practical application
- Full closed loop control system
Position detection devices (such as grating ruler, linear inductosyn, etc.) have different accuracy levels（ ± 0.01mm、 ± 0.005mm、 ± 0.003mm、 ± 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 (thermal deformation) of position detection device is generally made of non-metallic materials. The coefficient of thermal expansion is not consistent with the parts of the machine tool. It is the key link of the working accuracy of the machine tool. Therefore, it is necessary to solve the heating problem in the machining process of the machine tool in order to overcome the thermal deformation caused by temperature. High end machine tools will adopt various methods, such as hollow cooling of lead screw, lubrication of guide rail, constant temperature cooling of cutting fluid, etc. to reduce the thermal deformation in the process of machining.
In theory, the closer to the driving axis (screw pair), the more accurate the measurement. Due to the limitation of structure space, there are only two ways to install the grating ruler, one is installed near the screw pair, the other is installed outside the guide rail. It is recommended to select the first installation method as far as possible, but it is inconvenient to repair and maintain. On the contrary, the grating ruler with high accuracy is selected, but the accuracy required by CNC machine tool is not achieved. Even in the first case, the installation position of the grating ruler is close to the driving axis, but there is a certain distance between the installation position and the driving axis after all. The combination of this distance and 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 installation side of the grating ruler, the grating ruler mistakenly thinks that the moving speed is insufficient during the detection, and the system gives the acceleration signal. When the driving object swings to the other side immediately, the grating ruler mistakenly thinks that the moving speed is too fast during the detection, and the system gives the deceleration signal, On the contrary, it aggravates the vibration of the driving object, leading to the strange phenomenon that the full closed loop is not as good as the half closed loop.
Impact of production environment: generally, the environment of machining factory is relatively harsh, and dust and vibration are common phenomena. However, grating ruler and linear induction synchronizer are precision components, and their working principle is to measure the relative moving position by light reflection. Dust and vibration are the biggest factors affecting the measurement accuracy. In addition, when the machine tool is working, the cutting oil mist and water mist are serious, which has a great impact on the grating ruler and linear induction synchronizer. Therefore, to use the full closed-loop control system, in addition to doing a good job in the installation and sealing, we must improve the production environment. Otherwise, this phenomenon will appear. The new machine tool has good accuracy, but it has been used for less than a year. Not only has the accuracy decreased, but the machine tool often alarms.
- Semi closed loop control system
Because the measuring device is installed on the top of the motor or lead screw, it is easy to seal, so there is no requirement for the environment. The precision error of the semi closed loop control system mainly depends on the forward and reverse clearance of the lead screw. With the improvement of machining technology, the manufacturing technology level of imported lead screw is higher, and the high-precision lead screw pair basically eliminates the forward and reverse clearance. In addition, in the assembly process, the double row reverse ball screw pair is used, which can completely eliminate the forward and reverse clearance. In addition, many machine tool plants use pre stretching method to eliminate the influence of thermal deformation of machine tool on the transmission accuracy of lead screw. So at present, the semi closed loop control system has been able to ensure the machine tool to achieve high accuracy.
To sum up, it can be seen that in theory, if the external factors are not considered, the full closed-loop control may improve the positioning accuracy of the foundation than the semi closed-loop control. But if we can’t solve the problems of machine heating, environmental pollution, temperature rise, vibration, installation and other factors, the phenomenon of full closed loop is worse than half closed loop. It may be effective in a short time, but as time goes on, the influence of dust and temperature changes on the grating ruler will seriously affect the measurement feedback data, thus losing its function. At the same time, when there is a problem with the grating ruler, an alarm will be generated, resulting in the machine can not work.
Due to the consideration of production cost and competitiveness, the middle and low end machine tools are simplified in the full closed-loop control, such as sealing, temperature rise control and so on. Under this condition, the accuracy of the machine tool can not be improved by simply configuring the grating ruler at high cost.