CNC machine tools in processing, from the calculation of the processing program to control the machine tool for processing are automatically controlled by the CNC system. Because the operator can not intervene, avoid human error, but at the same time there are some errors can not be human compensation. Therefore, the CNC machine tool itself puts forward higher accuracy requirements. On this basis, it is also hoped that the CNC system can give full play to the functions of the CNC machine tool to automatically compensate for some errors.
For open-loop and semi-closed-loop CNC machine tools, its positioning accuracy depends mainly on the accuracy of the feed screw, so CNC machine tools often use high-precision ball screws. But screws always have manufacturing errors, and long-term use will lead to wear and tear. Therefore, one of the common methods to further improve the positioning accuracy of machine tools is to compensate for the pitch error of the ball screw.
1,Hardware method to compensate the principle of ball screw pitch error
In the past in the open-loop CNC machine tools, often use the hardware method to compensate the pitch error, that is, the use of mechanical methods to measure the error distribution curve of a certain axis ball screw in CNC machine tools, the accumulated error value of the pitch reaches a pulse equivalent position to install a block. When the machine table moves, the micro switch installed on the machine tool sends an error compensation signal every time it touches the block, and the stepper motor is instructed to take more or less steps through the control circuit.
This intermittent pulse compensation method adds some mechanical parts (such as compensation lever, block, etc.) and circuitry (such as the circuitry that transmits the compensation signal from the microswitch to the CNC unit) to the table of the CNC machine. In addition, this compensation method is more suitable for stepper motor-driven open-loop CNC machine tools and pulse incremental interpolation methods, but not for DC or AC servo motor-driven semi-closed-loop CNC machine tools. Therefore, most CNC machine tools are currently using software methods to compensate the pitch error of the ball screw.
2,The software method to compensate the ball screw pitch error principle.
Pitch error software compensation method of the basic idea is: in a CNC machine tool on the axis of the ball screw in the entire range of travel, with CNC instructions to position the table in certain positions as the command position, with high-precision position measurement system to measure the actual position of these positions (most of the current use of laser interferometer as a high-precision measuring instrument), the actual position and the command position for comparison, calculate the ball screw The error distribution curve over the entire stroke is calculated. The errors are entered into the CNC in the form of a table. Afterwards, the numerical control automatically takes the error values into account when controlling the axis movement and compensates for them by means of the program prepared.
3,Testing device and result analysis
Take the longitudinal ball screw of CNC vertical milling machine as an example, the effective working length of the directional ball screw is 800mm, and every 15mm of its 300mm length is a command position point. The positioning error of each command position point is measured (its resolution is 0. 01 um) by dual-frequency laser interferometer. The positioning error curve of the longitudinal table before the pitch error compensation is obtained, as shown in the circled curve in the figure. After the pitch error is compensated by the software method, the positioning error curve is shown as the circle curve in the figure. Obviously, the positioning accuracy of CNC machine tool is greatly improved by the compensation of pitch error.
Pitch error compensation is one of the effective ways to improve the positioning accuracy of open-loop and semi-closed-loop CNC machine tools, but it is not a panacea. Repetitive setting errors caused by daily drive system rigidity and manufacturing inaccuracies cannot be compensated. In addition, the pitch error of the screw has a strong relationship with the ambient temperature. Therefore, the only way to further improve the accuracy of CNC machine tools is to use a fully closed-loop CNC system. In the fully closed-loop CNC system, the pitch error and some other errors are in the position closed loop and can be corrected by the closed loop.
