The servo and feed system is the core component for positioning and feeding of CNC machine tools. Common faults include excessive position deviation, intermittent feed motion and overload alarms. Mastering troubleshooting techniques helps solve problems rapidly.
1. Excessive Position Deviation
The dimensional error of machined workpieces exceeds the tolerance range, and accuracy cannot be stabilized even after repeated adjustment of tool compensation. This problem is mainly caused by excessive backlash. Long-term operation will wear the ball screw pair and increase the clearance between the screw and nut. Besides, loose bolts on the coupling connecting the screw and servo motor shaft will create gaps during reverse movement and damage positioning accuracy.
Troubleshooting: Fix a dial indicator on the machine bed with the probe against the side of the worktable. Manually move the table in reverse direction and record the reading to measure backlash.
If the clearance is small, modify the backlash compensation parameters in the CNC system and recheck the precision.
If the backlash is too large to meet accuracy requirements after parameter compensation, disassemble the screw and nut assembly, adjust the preload or replace the worn nut. Carry out compensation again after reinstallation and calibration.
2. Intermittent Feed Motion
The worktable stutters and runs unevenly during feeding, which seriously impairs surface finish.
First, check whether the wiring of the servo motor encoder is loose. Poor signal transmission will lead to unstable feed control. Next, verify servo drive parameters to rule out parameter drift caused by electromagnetic interference.
If no electrical fault is found, inspect guide rails and lead screws for jamming. Scratches on rail surfaces caused by metal chips or insufficient lubrication will change feed resistance and produce vibration.
Solutions: Fasten encoder cables and add shielding protection. Reconfirm servo parameters. Clean metal scraps from guide rails and lead screws and replenish lubricant. For scratched rails, conduct grinding repair or replace the rail plastic strip to eliminate the fault.
3. Overload Alarm of Feed System
The numerical control system triggers an overload alarm and halts feed movement during machining.
First, check the machining load. Unreasonable cutting parameters and excessive cutting depth will exceed the rated load. Reduce cutting depth to clear the alarm.
If the load is normal, check for jamming on screws and guide rails. Increased resistance due to poor lubrication will also trigger overload, which can be fixed by cleaning and lubrication.
In other cases, the power module of the servo motor is damaged and causes false overload alarms; replace the defective power module to fix the problem.
Routine Maintenance
Regularly clean metal chips on guide rails and lead screws and replenish lubricating oil strictly in accordance with specifications. Inspect the tightness of couplings every month. These measures can greatly reduce feed system failures and maintain stable machining precision.
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