The feed mechanism of the CNC machine tool is connected to the ball screw by a servo motor or a stepping motor. Generally, it is directly connected by a coupling, connected by a toothed synchronous belt or connected by gears. In many occasions, due to structural limitations, especially after the use of a servo motor or a hybrid stepper motor, the direct coupling of the coupling becomes the most common method of connection between the motor and the ball screw.

Since the feed speed of CNC machine tools is relatively fast, such as fast forward and fast reverse, the speed is sometimes as high as 20m / min or more, and the positive and negative conversions are frequent during the entire processing process. The instantaneous impact of the coupling is relatively large, and it is easy to cause the coupling to loosen and twist. With the increase of use time, the situation of loosening and twisting intensifies. In actual processing, it mainly shows that the movement in all directions is normal, the encoder feedback is normal, and the system has no alarm. However, the movement value cannot always match the command value. The processing error value is getting larger and larger, and even the processed parts are scrapped. When this happens, it is recommended to check the coupling.

Couplings can be divided into rigid couplings and flexible couplings according to the structure, which can be treated separately according to their structure.

1. Rigid coupling

Rigid couplings currently mainly use the coupling method of coupling sleeve and taper pin, and most of the feed motor shafts are equipped with flat keys. This connection, after a period of use, the conical pin begins to loosen, and the side clearance of the keyway gradually increases, sometimes even causing the taper pin to fall off, resulting in unstable parts processing dimensions. There are two solutions:

(1) A specially made taper pin with a small head and a thread is used to lock it with a nut and an elastic washer to prevent the taper pin from loosening due to rapid conversion. This method can well solve the problem of loosening of the conical pin, and at the same time reduce the torque that the flat key bears. Of course, in this method, because the small end of the conical pin has a nut, it must ensure that the coupling has a certain rotation space.

(2) Two large and small elastic pins are used to replace the conical pin connection. Although this method does not have the high precision of the conical pin connection method, it can solve the problem of loosening of the conical pin. The elastic pin has a certain elasticity and can Decompose the torque that some flat keys bear, and the structure is compact and the assembly is also very convenient. After application in maintenance, the effect is very good. However, when assembling, it should be noted that the size of the elastic pins must be assembled with each other at 180 °, otherwise it will affect the accuracy of parts processing.

2. Flexible coupling

Flexible coupling is a coupling widely used in CNC machine tools. It can compensate for the "interference" phenomenon caused by the coaxiality and verticality errors. Under the condition that the structure permits, the servo feed system of most CNC machine tools adopts flexible coupling structure. However, it is difficult to grasp whether the tapered sleeve is locked when the flexible coupling is assembled. If the frictional force is insufficient after the tapered sleeve is expanded, it will cause a relative slip twist between the screw shaft head and the motor shaft head, causing the CNC machine to work In the process, the size of the parts to be processed shows regular and gradual changes (from small to large or large to small), and the value of each change is basically constant. If the machine tool rapid feed speed is adjusted, the amount of change will also change. At this time, the CNC system does not alarm because the motor rotation is normal and the encoder feedback is normal. Once this happens to the machine tool, simply tightening the screws at both ends may not work. The solution is to try to lock the elastic tapered sleeve of the coupling. If the tapered sleeve is too loose, a slit can be cut in the axial direction of the tapered sleeve. After tightening the screws at both ends, the failure can be completely eliminated.

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