In the dry machining industry, machining accuracy is a mantra that is often mentioned several times a day. When meeting and talking to people in the industry, machining accuracy should be mentioned in less than three sentences. So what is the guarantee of machining precision of machine tools? This video clearly explains the precision electrical control of machine tools, let’s take a look.
The machining accuracy of CNC machine tools is ultimately guaranteed by the accuracy of the machine tool itself.CNC machine tool accuracy includes geometric accuracy, positioning accuracy, repeated positioning accuracy and cutting accuracy。
Geometric precision:
Also known as static accuracy, it comprehensively reflects the overall geometric shape error of key components of CNC machine tools after assembly.
Positioning accuracy:
It shows the precision that can be achieved by measuring the movement of each moving part of the machine tool under the control of the CNC device. According to the measured positioning accuracy value, the best processing accuracy of the workpiece that can be achieved during automatic processing of the machine tool can be judged. It refers to the difference between the actual position of the part or tool and the standard position (theoretical position, ideal position). The smaller the deviation, the higher the accuracy. This is the prerequisite for ensuring the precision of workpiece processing.
Repeat positioning accuracy:
It refers to the consistency of positional accuracy achieved 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 (same CNC machine tool, different operating modes and same part program).
Cutting precision:
This is a comprehensive inspection of the geometric accuracy and positioning accuracy of the machine tool under cutting processing conditions.
From the above, it can be seen that the precision of CNC machine tools is divided into mechanical and electrical aspects. Mechanical aspects include spindle accuracy, such as runout accuracy, lead screw mounting accuracy during processing, machine tool rigidity; , etc. The electrical aspect mainly includes control methods such as semi-closed loop, fully closed loop, etc., as well as feedback and compensation methods, interpolation accuracy during processing, etc. Therefore, machine tool accuracy does not depend on whether the machine tool is fully closed loop.
1. Introduction to the principle
The drive train of CNC machine tool includes CNC device → servo encoder → servo motor → motor → lead 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 power servo system
The position detection device (such as a grating ruler, linear induction synchronizer, etc.) is installed on the moving parts of the machine tool (such as the workbench) and provides feedback by real time on the position of moving parts after processing. Through the CNC system, the status of the machine tool is notified to the servo motor, servo motor automatically compensates for movement errors through the system instructions. However, because it places large inertial links such as the screw, nut pair and worktable of the machine tool in a closed loop, steady-state debugging of the system is more troublesome during debugging . Additionally, measuring devices such as grating rulers and linear induction synchronizers are relatively expensive, complex to install, and can cause oscillation, so machine tools do not typically use fully closed-loop control .
2. Semi-closed loop control power supply servo system
The position detection device is installed at the end of the drive motor or the end of the screw to detect the rotation angle of the screw or servo motor, indirectly measuring the actual position of the moving parts of the machine -tool and sending it back to the control system. through comments. With the improvement of the mechanical manufacturing level and the improvement of speed sensing components and screw pitch accuracy, semi-closed loop CNC machine tools have been able to achieve very high feed accuracy . Most machine tool manufacturers widely adopt semi-closed loop CNC systems.
2. Practical application
1. Fully closed-loop control system
Position sensing devices (such as grating rulers, linear induction synchronizers, etc.) have different levels of accuracy (±0.01mm, ±0.005mm, ±0.003mm, ±0.02mm), so fully closed-loop control will also have errors, and the positioning accuracy is affected by the influence of level.
Thermal performance (thermal deformation) of the position detection device. The measuring device is usually made of non-metallic materials. The thermal expansion coefficient is inconsistent with different components of the machine tool. work of the tool. Therefore, the heating problem during machine tool processing needs to be solved to overcome the problem due to thermal deformation caused by temperature. High-end machine tools will use various methods, such as hollow screw cooling, guide rail lubrication, and constant temperature cooling of cutting fluid, to reduce thermal deformation during machine tool processing.
The installation of the position detection device is also very important. In theory, the more precise the measurement. Due to the limitation of structural space, there are only two ways to install the grid ruler. One is to install it on the secondary side near the screw and the other is to install it outside the guide rail. It is recommended to choose the first installation method if possible, but it is inconvenient for inspection and maintenance. On the contrary, a high-precision grid ruler was selected, but it actually did not achieve the precision required by CNC machine tools. Even in the first case, the installation position of the grid ruler is relatively close to the driving axis, but there is a certain distance between the installation position and the driving axis. This distance combined with the swing of the object during training adds a lot. of problems in the detection and control of the network rule. Big problem. When the motor object switches to the side where the network rule is installed, the network rule wrongly thinks that the moving speed is insufficient during detection, and the system gives an acceleration signal, and the motor object switches immediately on the other side, and the network rule mistakenly thinks that the movement speed is too fast when detecting. Fast, the system will give a deceleration signal. This repeated operation does not improve the control of the linear coordinate axes of the CNC machine tool. the driven object, resulting in the strange phenomenon that a fully closed loop is not as good as a half-closed loop.
Impact on production environment: Generally, the environment of mechanical processing plants is relatively harsh, and dust and vibration are common phenomena. However, grating rulers and linear induction synchronizers are precision components. Their operating principle is to measure relative movement positions by reflection. the light. Dust and vibration are exactly the most important factor affecting measurement accuracy. In addition, when the machine tool is being processed, the cutting oil mist and water mist are relatively large, which has a great impact on the network rule and linear induction synchronizer. Therefore, to use a fully closed-loop control system, in addition to installation and sealing, the production environment needs to be improved. Otherwise, this phenomenon will occur. The accuracy of the new machine tool is good, but after less than a year of use, not only does the accuracy decrease, but the machine tool also often alarms.
2. Semi-closed loop control system
Since the measuring device is installed on top of the motor or screw, it is easier to seal and therefore there are no environmental requirements. The precision error of the semi-closed loop control system mainly depends on the forward and reverse clearance of the screw. With the improvement of machining technology, the current manufacturing technology level of imported screws is relatively high, and the coordination of high-precision screw pairs basically eliminates the front and back play. In addition, in the assembly process, the pair of screws adopts a pair of double-row inverted ball screws, which can completely eliminate the front and rear gaps. In addition, many machine tool factories pre-stretch the screw during machine tool assembly to eliminate the impact of thermal deformation of the machine tool on the precision of screw transmission. Therefore, the current semi-closed loop control system can ensure that machine tools achieve high precision.
3. Conclusion
From the above, it can be seen that in theory, if external factors are not considered, fully closed-loop control can improve the basic positioning accuracy compared with semi-closed-loop control. However, if factors such as machine tool heating, environmental pollution, temperature rise, vibration and installation cannot be properly resolved, a fully closed loop will be less than one semi-closed loop. It can be effective in a short time, but over time, the impact of dust and temperature changes on the network ruler will seriously affect the measurement feedback data and lose its effect. At the same time, if there is a problem with the network rule, an alarm will be generated, preventing the machine tool from working.
Due to production cost and competitiveness considerations, mid-to-low-end machine tools have simplified the supporting facilities of complete closed-loop control. For example, waterproofing, temperature rise control, etc. are not well guaranteed. Under such conditions, simply configuring a network ruler cannot improve the accuracy of the machine tool at high cost.
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