The servo system is the link between the numerical control device and the machine tool host, which is used to receive the feed pulse or feed displacement information issued by the interpolator of the numerical control device. After a certain signal conversion and voltage and power amplification, the servo motor drives the transmission mechanism and finally converts it into the linear displacement or rotary displacement of the machine tool table relative to the tool.

In order to improve the performance of CNC machine tools, high requirements are put forward for the feed servo system used in machine tools. Due to the different machining tasks completed by various CNC machine tools, the requirements for the feed servo system are not the same. The following are common faults of the servo system.

1.Overdrive
When the feed movement exceeds the soft limit set by the software or the hard limit set by the limit switch, an overrange alarm will occur, and the alarm content will generally be displayed on the CRT, according to the numerical control system manual, the fault can be eliminated and the alarm is lifted.

2. Overload
When the load of the feed movement is too large, frequent positive and reverse movement and poor lubrication of the transmission chain, it will cause overload alarm. Generally, alarm information such as overload, overheating or overcurrent of the servo motor is displayed on the CRT. At the same time, the feed drive unit, indicator light or digital tube in the strong current cabinet will prompt the overload and overcurrent information of the drive unit.

3. Move
There is a flickering phenomenon in the feed:
① The speed measurement signal is unstable, such as the fault of the speed measurement device, the interference of the speed measurement feedback signal, etc.
The speed control signal is unstable or interfered with;
③ The terminal contact is not good, such as loose screws.
When the movement occurs at the moment of commutation from the positive and reverse motion, it is generally due to the reverse gap of the feed transmission chain or the gain of the servo system is too large.

4. Crawling
Occurs in the starting acceleration section or low speed feed, generally due to the poor lubrication of the feed transmission chain, low gain of the servo system and excessive external load and other factors. In particular, it should be noted that the coupling used for the connection of the servo motor and the ball screw is not synchronized with the rotation of the ball screw due to the loose connection or the defects of the coupling itself, such as cracks, so that the feed movement is fast and slow, resulting in crawling.

5, the machine vibration
When the machine tool is running at high speed, vibration may occur, and overcurrent alarm will occur. The machine tool vibration problem generally belongs to the speed problem, so we should find the speed ring; The whole adjustment process of the speed of the machine tool is completed by the speed regulator, that is, any problem related to speed should be found the speed regulator, so the vibration problem should be found the speed regulator. Mainly from the given signal, feedback signal and speed regulator itself three aspects to find the fault.

6, the servo motor does not turn
In addition to the speed control signal, the drive unit of the CNC system to feed has an enable control signal, which is generally the DC 24V relay coil voltage. Servo motor does not turn, the common diagnostic methods are:
① Check whether the CNC system has a speed control signal output;
② Check whether the enable signal is connected. Observe the I/O status through CRT, and analyze the PLC ladder diagram (or flow chart) of the machine tool to determine whether the starting conditions of the feed shaft, such as lubrication and cooling, are met;
③ For the servo motor with electromagnetic brake, check whether the electromagnetic brake is released;
④ feed drive unit failure;
⑤ Servo motor fault.

7. Position error
When the servo shaft movement exceeds the position tolerance range, the CNC system will produce an alarm of excessive position error, including following error, contour error and positioning error. The main reasons are:
① The tolerance range set by the system is small;
② The servo system gain is set improperly;
③ The position detection device is polluted;
④ the cumulative error of the feed transmission chain is too large;
⑤ When the headstock moves vertically, the balancing device (such as the balancing hydraulic cylinder, etc.) is unstable.

8. Drift
When the instruction value is zero, the axis still moves, resulting in a positional error. It is eliminated by error compensation and zero speed adjustment of the drive unit.

9. Clearance and loosening of mechanical transmission components
In the feed transmission chain of CNC machine tools, the transmission is often damaged due to the gap between the keyway and the key, so in addition to carefully choosing the key connection mechanism in the design, the processing and assembly must be carefully inspected. When assembling the ball screw, the preload of the bearing should be checked to prevent the axial movement of the ball screw, because the clearance is also another reason for the obvious transmission clearance