I. Feed Transmission Chain Failures
In the feed transmission system of CNC machines, ball screw pairs, hydrostatic screw nut pairs, rolling guides, hydrostatic guides, and plastic guides are commonly used. Therefore, failures in the feed transmission chain are mainly reflected in the decline of motion quality, such as mechanical parts not moving to the specified position, interruption of operation, decline in positioning accuracy, increase in backlash, creep, and increased noise from bearings (after collision), etc.
To prevent such failures, the following measures can be taken:
(1) Improve transmission accuracy by adjusting the preload forces of each moving pair, adjusting loose parts, eliminating transmission gaps, shortening the transmission chain, and setting reduction gears in the transmission chain, which can also improve transmission accuracy.
(2) Improve motion accuracy. Under the premise of meeting the strength and stiffness of parts, try to reduce the mass of moving parts, reduce the diameter and mass of rotating parts to reduce the inertia of moving parts, and improve motion accuracy.
(3) Improve transmission rigidity. Adjusting the preload forces of screw nut pairs and support parts and reasonably selecting the dimensions of the screw itself are effective measures to improve transmission rigidity. Insufficient rigidity can also lead to creep and vibration of the worktable or drag plate and cause reverse dead zones, affecting transmission accuracy.
(4) Rolling guides are sensitive to dirt and must have good protective devices. The preload force of rolling guides should be properly selected; too large will significantly increase the traction force. Hydrostatic guides should have a well-filtered oil supply system.
II. Spindle Component Failures
Due to the use of speed-regulating motors, the structure of the CNC machine’s spindle box is relatively simple, and the parts that are prone to failures are the automatic tool clamping mechanism and the automatic speed regulation device inside the spindle. To ensure that the tool clamp does not loosen by itself during work or when the power is off, the automatic tool clamping mechanism uses spring clamping and is equipped with a limit switch to send clamping or releasing signals. If the tool cannot be released after clamping, consider adjusting the pressure and travel of the tool release hydraulic cylinder and the limit switch device, or changing the nut on the disc spring to reduce the spring compression. In addition, the issues of spindle heating and spindle box noise should not be overlooked. At this time, the main considerations are cleaning the spindle box, adjusting the amount of lubricating oil, ensuring the spindle box’s cleanliness, replacing the spindle bearings, repairing or replacing the spindle box gears, etc.
III. Faults in the Travel Switches of Each Axis’s Motion Position
On CNC machines, to ensure the reliability of automated work, a large number of travel switches for detecting motion positions are used. After long-term operation, the motion characteristics of moving parts change, and the reliability of the travel switch pressing mechanism and the quality characteristics of the travel switch itself change, which has a significant impact on the performance of the machine. Generally, timely inspection and replacement of travel switches can eliminate the impact of poor switches on the machine.
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