The screw is an important transmission component for ensuring the accuracy of machine tools, and the reverse clearance of the screw directly affects the accuracy of the machine tool and the stability of the machining accuracy!
Screw reverse clearance, also known as screw backlash, screw clearance, or screw loss of momentum
The reverse clearance of the screw (lead screw) is also known as the back clearance of the screw, the clearance of the screw, or the loss of momentum of the screw
In the feed transmission chain of CNC machine tools, gear transmission, ball screws, nut pairs, etc. all have reverse clearances, which can cause the servo motor to idle while the worktable does not actually move when the worktable moves in the opposite direction. For CNC machine balls using a semi closed loop servo system, the presence of reverse clearance can affect the positioning accuracy and repeat positioning accuracy of the machine tool, thereby affecting the machining accuracy of the product. Therefore, the data filing system needs to provide reverse clearance compensation function to automatically compensate for some regular errors during the machining process and improve the accuracy of the machined parts. And as the use of CNC machine tools increases, the reverse clearance will gradually increase due to the increase of motion pair clearance caused by wear. Therefore, it is necessary to regularly measure and compensate for the reverse clearance of each coordinate axis of the CNC machine tool.
1. Reverse gap compensation process
Under the condition of no compensation in the CNC system, multiple measurements are taken within the measuring range of the machine tool at three positions near the midpoint and both ends of the stroke. The average reverse clearance B. of the m daily mark position P is measured using a dial gauge or dial gauge, and the maximum value obtained from the average is taken as the reverse clearance value B. This value is then input into the CNC system's reverse clearance compensation parameters.
When the CNC system controls the coordinate axis to move in the opposite direction, it automatically causes the axis to move in the opposite direction first, and then moves according to the instructions. That is, the CNC system controls the servo motor to travel an additional distance, which is equal to the reverse clearance value B., in order to compensate for the reverse clearance.
It should be pointed out that this method is only suitable for semi closed loop CNC systems. For fully closed loop CNC systems, the above compensation methods cannot be adopted.
2. Reverse gap compensation method
Laser interferometer and dial gauge/micrometer can be used
Method of dial gauge/micrometer:
Use a hand pulse generator to move the relevant axis (set the hand pulse magnification to a gear of 1 × 100, that is, for each step of change, the motor feed is 0.1mm), and use a dial gauge to observe the movement of the relevant axis. After maintaining normal unidirectional motion accuracy, the forward motion is taken as the starting point. For each step of the hand pulse change, the actual distance of the machine tool's axis movement is d=d1=d2=d3...=0.1mm, indicating that the motor is running well and the positioning accuracy is good. In terms of the actual movement displacement of the machine tool, it can be divided into four stages: ① Machine tool movement distance d1>d=0.1mm (slope greater than 1); ② Manifesting as d=0.1mm>d2>d3 (slope less than 1); ③ The machine tool mechanism has not actually moved, showing the most standard reverse clearance; ④ The movement distance of the machine tool is equal to the given value of the hand pulse (with a slope of 1), and it returns to normal movement of the machine tool.
Note:
The reverse clearance measured at different speeds varies. Generally, the reverse clearance value at low speeds is larger than that at high speeds, especially when the machine tool axis load is large and the motion resistance is high.
So some CNC systems provide two compensation values, high-speed G00 and low-speed G01.
Gap measurement and compensation of CNC machine tools
Gap measurement and compensation of CNC machine tools
There is always a gap in the feed transmission of machine tools, and if there is a gap without compensation, it will directly affect the servo accuracy of the feed.
In the feed transmission of this machine tool, the difference between the NC command movement value and the actual movement value of the moving parts, that is, the existence of clearance, is generally caused by the following reasons:
bearing clearance
The gap between the rolling screw pairs and the bending vibration of the screw
Before leaving the factory, we carefully measured the clearance value of the feed system and compensated it. However, after long-term use, the compensation amount is not appropriate due to wear and tear. When it affects the machining accuracy, users need to reset the gap compensation amount themselves.
The gap compensation amount can be reset based on the parameters recorded in the CNC device. For detailed instructions on changing parameters, please refer to the user manual of the CNC system.
Method for gap measurement:
1) Quickly move the moving parts 50mm from their resting position towards the negative direction.
2) Align the contacts of the dial gauge with the positive side of the moving part and make the gauge needle zero.
3) Quickly move the moving parts 50mm from their resting position towards the negative direction.
4) Move the moving parts quickly 50mm in the positive direction from their new resting position.
5) Read the value on the dial gauge at this time, which is called reverse deviation and includes the total clearance in the transmission chain, reflecting the accuracy of its transmission system.
Note:
1) The above actions can be performed by programming a simple program. For the convenience of reading, the program should delay for 3-5 seconds at the stopping point when performing item 4).
2) The above actions should be repeated 5 times, and the arithmetic mean should be taken as the gap compensation value.
3) According to the measured reverse deviation values of X, Y, and Z, compensate them to their corresponding parameter numbers.
