Common mechanical failures of CNC machine tools and preventive measures

1、 Spindle component failure

Due to the use of speed regulating motor, the structure of the spindle box of CNC machine tools is relatively simple. The parts that are prone to failure are the automatic tool clamping mechanism and automatic speed regulating device inside the spindle. In order to ensure that the tool holder will not loose automatically during operation or power failure, the automatic tool clamping mechanism adopts spring clamping and is equipped with a travel switch to send a clamping or loosening signal. If the tool cannot be loosened after clamping, consider adjusting the pressure of the tool loosening hydraulic cylinder and the travel switch device or adjusting the nut on the disc spring to reduce the spring compression. In addition, the problem of spindle heating and spindle box noise cannot be ignored. At this time, the main consideration is to clean the spindle box, adjust the amount of lubricating oil, ensure the cleanliness of the spindle box, replace the spindle bearing, repair or replace the spindle box gear, etc.

2、 Feed drive chain failure

In the feed transmission system of CNC machine tools, ball screw pairs, hydrostatic screw nut pairs, rolling guides, hydrostatic guides and plastic guides are commonly used. Therefore, the failure of the feed transmission chain is mainly reflected in the decline of the motion quality. For example, the mechanical parts do not move to the specified position, the operation is interrupted, the positioning accuracy decreases, the reverse clearance increases, the creeping, and the bearing noise increases (after the crash).

Such failures can be prevented by the following measures:

(1) The transmission accuracy can also be improved by improving the transmission accuracy, adjusting the preload of each motion pair, adjusting the loose links, eliminating the transmission clearance, shortening the transmission chain and setting reduction gears in the transmission chain.

(2) Improve transmission stiffness. It is an effective measure to improve the transmission stiffness by adjusting the pre tightening force of the screw nut pair and supporting parts and selecting the size of the screw itself reasonably. Insufficient stiffness will also lead to crawling and vibration of worktable or pallet, and cause reverse dead zone, affecting transmission accuracy.

(3) Improve motion accuracy. On the premise of meeting the strength and stiffness of components, reduce the mass of moving components as much as possible, and reduce the diameter and mass of rotating components to reduce the inertia of moving components and improve the motion accuracy.

(4) The guide rail and rolling guide rail are sensitive to dirt and must be equipped with good protective devices, and the pre tightening force of the rolling guide rail must be properly selected, which will significantly increase the traction force. The hydrostatic guide rail shall have a set of oil supply system with good filtering effect.

3、 Automatic tool changer failure

The failures of the automatic tool changer are mainly manifested in: the movement failure of the tool magazine, the excessive positioning error, the instability of the tool handle held by the manipulator, and the large movement error of the manipulator. In case of serious failure, the tool change action will be stuck and the machine tool will be forced to stop working.

1. Magazine movement fault

If the coupling connecting the motor shaft and worm shaft is loose or the mechanical connection is too tight, the tool magazine will not rotate. At this time, the screws on the coupling must be tightened. If the tool magazine does not rotate properly, it is caused by motor rotation fault or transmission error. If the tool sleeve cannot clamp the tool, adjust the adjusting screw on the tool sleeve, compress the spring, and tighten the clamping pin. When the tool sleeve does not go up/down in place, check the fork position or the installation and adjustment of the limit switch.

2. Tool change manipulator failure

If the tool clamp is not tight and the tool drops, adjust the spring of the clamping jaw to increase its pressure, or replace the clamping pin of the manipulator. If the tool cannot be loosened after being clamped, adjust the nut behind the locking spring so that the maximum load does not exceed the rated value. If the tool drops during tool exchange, it is caused by the spindle box not returning to the tool change point or the tool change point drifting during tool change. The spindle box should be re operated to return to the tool change position and reset the tool change point.

4、 Pressing failure of travel switch at each axis movement position

In order to ensure the reliability of automation, a large number of travel switch machine tools are used to detect the movement position on CNC machine tools. After long-term operation, the motion characteristics of the moving parts change. The reliability of the pressure closing device of the travel switch and the change of the quality characteristics of the travel switch itself have a great impact on the overall performance. Generally, the travel switch should be checked and replaced in time to eliminate the impact of poor switches on the machine tool.

5、 Auxiliary device failure

1. Hydraulic system

The hydraulic pump shall be a variable pump to reduce the heat of the hydraulic system. The filter installed in the oil tank shall be cleaned regularly with gasoline or ultrasonic vibration. Common faults mainly include wear, cracks and mechanical damage of the pump body. At this time, it is generally necessary to overhaul or replace parts.

2. Pneumatic system

In the pneumatic system used for tool or workpiece clamping, safety protection door switch and spindle taper hole blowing, the water separator filter shall be drained and cleaned regularly to ensure the sensitivity of moving parts in pneumatic components. Failure of valve core action, air leakage, pneumatic element damage and action failure are all caused by poor lubrication, so the oil mist atomizer should be cleaned regularly. In addition, check the tightness of the pneumatic system frequently.

3. Lubrication system

Including the lubrication of machine tool guide rail, transmission gear, ball screw, spindle box, etc. The filter in the lubricating pump shall be cleaned and replaced regularly, and shall be replaced once a year generally.

4. Cooling system

It cools the tool and workpiece and washes chips. The coolant nozzle should be cleaned regularly.

5. Chip removal device

The chip removal device is an accessory with independent functions, which mainly ensures the smooth operation of automatic cutting and reduces the heating of CNC machine tools. Therefore, the chip removal device shall be able to automatically remove chips in a timely manner, and its installation position shall generally be as close to the cutting area of the tool as possible.

6、 Conclusion

CNC machine tool is an automatic processing equipment controlled by computer and electromechanical integration. The use of CNC machine tool is a technical application project. Correct prevention and effective maintenance are the basic guarantee to improve the efficiency of CNC machine tools. For common mechanical failures, although there are few opportunities, we must not neglect to comprehensively analyze and judge the root causes of failures, and try to shorten the downtime of failures, so as to facilitate the efficient performance of CNC machine tools.