The feed mechanism of CNC machine tools uses servo motors or stepping motors to connect with ball screws. They are usually connected by couplings, toothed timing belts, or gears. In many cases, due to structural limitations, especially after the use of a servo motor or a hybrid stepper motor, the direct connection of the coupling becomes the most common connection method for the motor and the ball screw.
Due to the high feed rate of CNC machine tools, such as rapid forward and retreat speeds of up to 20 m/min or more, positive and negative conversions occur frequently during the entire machining process. The momentary shock that the coupling is subjected to is large, which can easily cause the coupling to loosen and twist. As the use time increases, the loosening and twisting of the coupling increase. In the actual processing, the main manifestations are normal motion in all directions, encoder feedback is normal, and the system has no alarm. However, the motion value is always unable to conform to the command value, the processing error value becomes 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. They can be handled separately according to their structure.
Rigid coupling
Rigid couplings currently use coupling methods with coupling sleeves and taper pins, and most feed motors have shafts with flat keys. This connection, after a period of use, the tapered pin began to loosen, the gap in the side of the keyway gradually increased, and sometimes even caused the taper pin off, resulting in dimensional instability of the part. There are two ways to solve:
(1) Adopt a special small-threaded taper pin and tighten it with nuts and elastic washers to prevent the taper pin from loosening due to rapid changeover. The method can well solve the problem of loosening of the conical pin, and at the same time, reduces the torque that the flat key bears. Of course, this method has a small nut on the tapered pin and must ensure that the coupling has a certain amount of rotation space.
(2) Use two large and small elastic pins to replace the conical pin connection. Although this method does not have a high accuracy of the conical pin connection method, it can solve the loosening problem of the conical pin well. The elastic pin has a certain elasticity. Decompose part of the flat key to withstand the torque, and the compact structure, the assembly is also very convenient. After the application in the maintenance, the effect is very good. However, when assembling, it should be noted that the large and small elastic pins are required to be assembled with each other by 180°, otherwise the accuracy of the parts machining will be affected.
2. Flexible coupling
The flexible coupling is a widely used coupling of numerically-controlled machine tools. It can compensate for "interference" caused by coaxiality and verticality errors. Under the conditions allowed by the structure, the servo feed system of most CNC machine tools adopts a flexible coupling structure. However, when the flexible coupling is assembled, it is difficult to determine whether the taper sleeve is locked. If the friction force is insufficient after the taper sleeve expands, a relative slip and torsion occurs between the shaft end of the screw shaft and the shaft of the motor, resulting in the operation of the CNC machine tool. Among them, the size of the parts being machined has a regular gradual change (from small to large or large), and the value of each change is basically constant. If you adjust the rapid feed rate of the machine tool, this change will also change. At this time, the CNC system does not alarm, because the motor rotation is normal, and the encoder feedback is also normal. Once this happens on a machine tool, the simple method of tightening the screws at both ends may not work. The solution is to try to lock the coupling's elastic taper sleeve. If the taper sleeve is too loose, the taper sleeve can be cut axially, and the screws at both ends can be completely eliminated.