What determines the accuracy of CNC machining ?
The proper use of methods and techniques in the preparation of CNC machining programs is of great significance for ensuring and improving the machining accuracy of CNC machine tools. So what determines the accuracy of CNC machining? In the long-term practice, the author has accumulated some programming experience and shared it with everyone.
The machining accuracy of the CNC machine tool is determined by the position detection element, which is composed of the detection element (sensor) and the signal processing device. It is an important part of the closed-loop servo system of the CNC machine tool. Its function is to detect the actual value of the position and speed of the worktable, and send a feedback signal to the numerical control device or servo device, thereby forming a closed-loop control. The detection element generally uses the principle of light or magnetism to complete the detection of position or speed.
The position detection element is divided into direct measurement element and indirect measurement element according to the detection method. When measuring the linear movement of the machine tool, a linear detection element is generally used, called direct measurement, and the position closed-loop control formed is called full-closed control. Its measurement accuracy mainly depends on the accuracy of the measuring element and is not affected by the transmission accuracy of the machine tool.
Because the linear displacement of the machine tool table has a precise proportional relationship with the rotation angle of the drive motor, the method of driving and detecting the rotation angle of the motor or screw can be used to indirectly measure the moving distance of the work table. This method is called indirect measurement. The position closed-loop control is called semi-closed-loop control.
The accuracy of CNC machining measurement depends on the accuracy of the detection components and the feed drive chain of the machine tool. The processing accuracy of closed-loop CNC machine tools is largely determined by the accuracy of the position detection device. CNC machine tools have very strict requirements for position detection elements, and their resolution is usually between 0.001 and 0.01 mm or less.
Compared with the numerical control device, the probability of detecting component failure is relatively high, and the phenomenon of cable damage, component contamination, and collision deformation often occurs. If it is suspected that the detection element is faulty, first check whether the cable is broken, stained, deformed, etc. You can also determine the quality of the detection element by measuring its output. . Let's take an example to illustrate, for example, what should I do when detecting device failure and mechanical oscillation (acceleration/deceleration)?
1) The pulse encoder is malfunctioning. At this time, check whether the voltage of the feedback line terminal on the speed unit drops at a certain point. If there is a drop, it indicates that the pulse encoder is defective, and the encoder should be replaced.
2) The pulse encoder cross coupling may be damaged, causing the shaft speed to be out of sync with the detected speed. The coupling should be replaced.