There are two main differences between Cnc Machine tools and ordinary machine tools: one is that the CNC machine tool has a “command system”-a numerical control system; the other is that a CNC machine tool has a drive system that executes motion-a servo system.
The machining errors produced on CNC machine tools have many similarities with the machining errors produced on ordinary machine tools, but they also have unique features, such as the tracking error of the servo feed system and the sampling delay error in the detection system. And so on, these are not available in ordinary machine tool processing. Therefore, in cnc machining, in addition to controlling the type of error sources that often appear in ordinary machine tools, it is also necessary to effectively suppress the error sources that may occur during CNC machining. The influence of these error sources on machining accuracy and the ways to suppress them mainly include the following aspects:
①The influence of machine tool repeat positioning accuracy
The positioning accuracy of a CNC machine tool is the position accuracy of the movement of each coordinate axis of the index-controlled machine tool under the control of the CNC system. The factors that cause the positioning error include the error of the CNC system and the error of the mechanical transmission. The error of the CNC system is related to the interpolation error and tracking error. The repeat positioning accuracy of a machine tool refers to the degree of agreement between the actual position of the coordinate axis and the ideal position during repeated positioning.
②The influence of the detection device
The detection feedback device is also called the feedback element, which is usually installed on the machine tool table or lead screw, which is equivalent to the dial of an ordinary machine tool and human eyes. The detection feedback device converts the displacement of the table into an electrical signal and feeds it back to the numerical control device , If there is an error compared with the command value, the control table moves in the direction of eliminating the error. Numerical control systems can be divided into open loop, closed loop and semi-closed loop systems according to the presence or absence of detection devices. The accuracy of the open-loop system depends on the accuracy of the stepping motor and the lead screw, and the accuracy of the closed-loop system depends on the accuracy of the detection device. The detection device is an important part of high-performance CNC machine tools.
③The influence of tool error
In the machining center, because the tools used have an automatic exchange function, while increasing productivity, it also brings tool exchange errors. When processing a batch of workpieces with the same tool, due to frequent repetitions, the work handle has repeated positioning errors relative to the spindle taper hole, which reduces the processing accuracy.
There are hardware compensation and software compensation to suppress the errors of CNC machine tools. In the past, hardware compensation was generally used. For example, the machining center adopts the function of pitch error compensation. With the development of microelectronics, control, and monitoring technologies, new software compensation technologies have emerged. Its characteristic is to use the compensation control unit of the CNC system communication and the corresponding software to realize the error compensation. Its principle is to use the additional movement of the coordinates to correct the error.
Link to this article：What equipment guarantees the accuracy of CNC machine tools?
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