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Causes for unstable machining of CNC milling machines

As we all know, CNC milling machine is a high precision, high speed and high efficiency processing equipment. Compared with the traditional milling equipment, it is a set of automatic processing and composite centralized processing as one of the processing machine tools. It is a highly mechatronic processing equipment. Its processing characteristics are that the workpiece can be only through one or a small number of repeated clamping, through the replacement of different cutting tools can be concentrated to complete the drilling, milling, boring, hinge, tapping and other processes, including most of the processing content.

Especially, some complex special-shaped workpieces including curved surface or spherical surface, as well as a large number of cross-cutting workpieces can be quickly completed on a small CNC milling machine, and the machining accuracy can be controlled within 0.01 mm or even higher. More importantly, for batch processing can maximize the consistency of processing size. Because of these excellent processing characteristics, it has become the most important processing equipment in the complex die and auto parts industry.

Of course, the CNC milling machine has the above high processing capacity, in addition to its own configuration of advanced electronic control devices, but also equipped with a high positioning accuracy of the feed system. Different from the feed system of traditional milling machine tools, it is no longer composed of ordinary T-type lead screw and gearbox, but composed of servo driver, servo motor, position detection device and high precision ball screw. They are the prerequisites for ensuring high precision and efficient machining of machine tools. Any problems in any part of them will eventually lead to defective workpiece processing. In this paper, a brief introduction to the feed system caused by small NC milling machine NC machining instability reasons and solutions, I hope to give a reference to front-line workers.

At present, the control mode of CNC milling machine is mainly controlled by half closed loop. The control signal is sent to the servo drive device by the CNC numerical control system of the machine tool, then the servo motor is driven to rotate, and then the worktable is driven by the ball screw and the nut pair to realize the machining action. When the relative movement of the workpiece and the tool coincides with the command movement, the workpiece can be processed to meet the standard, but when the relative movement between the workpiece and the tool is not completely in accordance with the command value, the workpiece size will be unstable. This phenomenon is mostly caused by the feed system. Summed up mainly servo control system failure and mechanical failure caused by two aspects.

First, the unstable process caused by the servo control system failure.

Simply speaking, the servo control system of NC milling machine consists of servo controller, servo motor and detection device. When the machine tool is working, the position detection device feeds back the signal of the rotation angle detected by the servo motor to the position control unit, and sends out the signal to control the speed control unit through the position comparison, so as to control the rotation of the servo motor. The main causes of the problems are position detection device, main PCB board, position or speed control unit and servo motor.

1, the main PCB board is bad.

Generally speaking, the power supply of pulse encoder is provided by the main PCB of the system. The power supply of the main PCB board is poor or the connection between the main PCB and other control unit position feedback devices is loose. The distortion of output signal will be caused, which will affect the instability of processing caused by feedback quality.

2, location detection device caused

The position detection device usually uses the circular grating pulse encoder. If the fault occurs, the feedback signal will be distorted or lost. As a result, the rotation angle of servo motor is not accurate, thus affecting the instability of small CNC milling machine. The main reasons for this kind of fault are as follows: first, the output signal distortion is caused by the unqualified power supply voltage of the pulse encoder; second, the output signal distortion is caused by the external signal interference or dirt of the pulse encoder; third, the coupling between the pulse encoder and the motor is damaged or the pulse encoder itself. Damage causes output signal failure.

Two, machining failure caused by mechanical failure.

The mechanical part of CNC milling machine feed system includes two parts: ball screw and guide rail. Their normal operation plays a decisive role in the stability of the whole machine tool processing. The main reasons for instability of machine tool operation include loose connection between servo motor and lead screw, poor lubrication of lead screw and nut pair, high running resistance of worktable on guide rail, excessive clearance of lead screw or improper setting of clearance compensation.

1. The working table has large resistance on the guide rail.

NC milling machine worktable such as running resistance will cause creeping or running jitter, resulting in workpiece size instability. This situation is generally caused by over-tightening of the insert, poor lubrication of the machine tool rail surface or wear of rolling bearings. Generally, it can be restored by adjusting or replacing damaged parts.

2, screw and motor connection loose cause

The ball screw of CNC milling machine is usually connected directly by coupling and servo motor. After long-term use of the machine tool, due to the inertia of the workpiece and the worktable, there is a gap in the coupling, which causes the crawling or shaking of the machine tool. Generally through adjustment or replacement can be resolved.

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