The existence of backlash will affect the positioning accuracy and repeat positioning accuracy of the CNC machining center, thereby affecting the processing accuracy of the product. This requires the digital frustration system to provide a backlash compensation function to automatically compensate for some problems during processing. Regular errors can improve the accuracy of processed parts. And with the increase of the NC machining center when it is used, the backlash will gradually increase due to the increase of the movement pair gap caused by wear. Therefore, it is necessary to periodically measure and compensate the backlash of each coordinate axis of the NC machining center. .
1. Principle of backlash compensation
Under the condition of no compensation in the numerical control system, within the range of the machine tool’s measuring stroke, multiple measurements are made at three positions close to the midpoint and both ends of the stroke, and the position of each target point P is measured with a dial indicator or a dial indicator. The average backlash B. Take the maximum value of the average value obtained as the backlash value B, and input it into the backlash compensation parameter of the numerical control system.
When the CNC system controls the coordinate axis to move in the reverse direction, it will automatically make the coordinate axis move in the reverse direction first, and then move according to the instruction. That is, the numerical control system will control the servo motor to travel a certain distance, which is equal to the backlash value In order to compensate for the backlash.
It needs to be pointed out that this method is only suitable for semi-closed-loop CNC systems. The above compensation method cannot be adopted for a fully closed-loop numerical control system.
2. Backlash compensation method
Can use laser interferometer and dial indicator/dial indicator
Dial indicator/Dial indicator method:
Move the relevant axis with the MPG generator (set the MPG ratio to 1×100 gear, that is, the motor feeds 0.1mm for each step), and observe the movement of the relevant axis with the dial indicator. After the one-way movement accuracy remains normal, it is the positive movement as the starting point. Each time the MPG changes one step, the actual distance d=d1=d2=d3…=0.1mm of the machine axis movement indicates that the motor is running well and the positioning accuracy is good. The return to the actual movement displacement of the machine tool can be divided into four stages: ①The movement distance of the machine tool d1＞d=0.1mm (the slope is greater than 1); ②It is shown as d=0.1mm＞d2＞d3 (the slope is less than 1) ③The machine tool mechanism does not actually move, showing the *standard backlash; ④The machine tool movement distance is equal to the MPG given value (the slope is equal to 1), and the machine returns to the normal movement of the machine tool.
Note: The backlash measured at different speeds is different. Generally, the backlash value at low speed is larger than that at high speed, especially when the machine tool shaft load is large and the movement resistance is large. Therefore, some CNC systems provide two compensation values, high-speed G00 and low-speed G01.
Before the machine tool leaves the factory, we have carefully measured the gap value of the feed system and compensated. However, after long-term use of the machine tool, due to wear and other reasons, the compensation amount is not appropriate. When it affects the machining accuracy, the user needs to re-set the clearance compensation amount. The amount of backlash compensation can be reset according to the parameters recorded in the numerical control device. For the detailed description of the variable* parameters, please refer to the operating instructions of the numerical control system.
3. Method of backlash measurement
1) Quickly move the moving parts 50mm in the negative direction from the resting position.
2) Align the dial indicator contact with the positive side of the moving part, and make the indicator point to zero.
3) Move the moving parts from the resting position to the negative direction quickly by 50mm.
4) Quickly move the moving parts 50mm in the positive direction from the new stop position.
5) Read the value of the dial indicator at this time. This value is called the reverse deviation, which includes the total gap in the transmission chain and reflects the accuracy of the transmission system.
Note:
1) The above actions can be performed by editing a simple program. For the convenience of reading when proceeding to item 4), the program should be delayed for 3 to 5 seconds at the stop point.
2) The above action should be repeated 5 times, and the arithmetic mean value shall be taken as the backlash compensation value.
3) According to the measured reverse deviation values of X, Y, Z, respectively compensate to their corresponding parameter numbers.