Trouble Shooting for Loose Coupling of CNC Machine Tool

The feed mechanism of the cnc machine tool is connected with 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 a gear. In many occasions, due to structural limitations, especially after the use of a servo motor or a hybrid stepper motor, direct coupling has become the most common connection method between the motor and the ball screw.

Due to the relatively fast feed speed of cnc machine tools, such as fast forward and fast reverse speeds sometimes as high as 20m/min or more, the positive and negative conversions are frequent during the entire processing. The momentary impact that the coupling bears is relatively large, which can easily cause the coupling to loosen and twist. As the use time increases, the loosening and twisting situation intensifies. In actual processing, it is mainly manifested as normal motion in all directions, normal encoder feedback, and no alarm in the system, but the motion value has always failed to 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 their structure, which can be handled separately according to their structures.

1. Rigid coupling

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

(1) Use a special small head threaded taper pin, which is locked 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 the looseness of the tapered pin, and at the same time reduce the torque borne by the flat key. Of course, this method has a nut on the small head of the tapered pin, so it is necessary to ensure that the coupling has a certain rotation space.

(2) Two elastic pins, one large and one small, are used to replace the conical pin connection. Although this method is not as accurate as the conical pin connection method, it can solve the problem of looseness of the conical pin. The elastic pin has a certain degree of elasticity and can be used. The torque that the flat key bears is broken down, and the structure is compact, and the assembly is also very convenient. After being used in maintenance, the effect is very good. But pay attention to when assembling, the large and small elastic pins must be assembled 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 coaxiality and perpendicularity errors. Under the condition that the structure permits, most of the servo feed systems of CNC machine tools adopt a flexible coupling structure. However, it is difficult to know whether the taper sleeve is locked when the flexible coupling is assembled. If the friction force is insufficient after the taper sleeve is expanded, the screw shaft head and the motor shaft head will have relative slip and torsion, which will cause the CNC machine tool to operate. In the process, the size of the processed part presents a regular and gradual change (from small to large or large to small), and the value of each change is basically constant. If the rapid feed rate of the machine tool is adjusted, this amount of change will also change. At this time, the CNC system does not give an alarm. Since the motor rotation is normal, the encoder feedback is also normal. Once this happens on the machine tool, the method of simply tightening the screws at both ends may not be effective. The solution is to lock the elastic tapered sleeve of the coupling. If the tapered sleeve is too loose, the tapered sleeve can be slit axially. After tightening two screws, the fault can be completely eliminated.

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