What determines the accuracy of CNC machining？

The machining accuracy of CNC machine tools is determined by the position detection element, which is composed of detection element (sensor) and signal processing device, and is an important part of the closed-loop servo system of CNC machine tools. Its function is to detect the actual value of the position and speed of the worktable, and send feedback signal to the numerical control device or servo device, so as to form a closed-loop control. Generally, the detection element uses the principle of light or magnetic to complete the detection of position or speed.

According to the detection mode, position detection elements are divided into direct measurement elements and indirect measurement elements. When measuring the linear movement of the machine tool, the linear detector is usually used, which is called direct measurement, and the position closed-loop control is called full closed-loop control. The measurement accuracy mainly depends on the accuracy of the measuring elements, and is not affected by the transmission accuracy of the machine tool.

Because the linear displacement of the machine tool worktable has an accurate proportional relationship with the rotation angle of the driving motor, the method of driving and detecting the rotation angle of the motor or lead screw can be used to indirectly measure the moving distance of the worktable. This method is called indirect measurement, and the position closed-loop control is called semi closed-loop control.

The measurement accuracy of NC machining depends on the accuracy of the detection element and the feed transmission chain of the machine tool. The machining 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 on the position detection components, and their resolution is usually between 0.001-0.01mm or less.

Compared with the numerical control device, the failure probability of detection element is relatively high, which often leads to cable damage, component fouling, collision and deformation. If it is suspected that it is the fault of the detection element, we should first check whether the wireless cable is broken, defaced, deformed, and so on. We can also determine the quality of the detection element by measuring its output, which requires us to master the working principle and output signal of the detection element. Now let's give an example, for example, when the detection device fails and the mechanical oscillation (acceleration or deceleration) occurs, what should be done?

(1) The pulse encoder is faulty. 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 bad, and the encoder should be replaced.

(2) The cross coupling of pulse encoder may be damaged, which may cause the shaft speed to be out of sync with the detected speed, so the coupling should be replaced.

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