Fault Diagnosis and Repair for Irregular Dimensional Deviation

Fault Phenomenon: During the inspection of a part processed on a CNC milling machine with a FANUC 0i system, irregular dimensional deviations were found in the Y-axis direction. The actual dimensions of the workpiece were inconsistent with the theoretical data programmed.

Analysis and Processing:
    1.    Fault Analysis: From the perspective of CNC machine control, the Y-axis dimensional deviation was caused by the Y-axis position loop deviation.
    •    1) Check the Y-axis relevant position parameters, and find that reverse backlash, clamping tolerance, and other factors are within the required range, so the fault caused by improper parameter settings can be excluded.
    •    2) Check the Y-axis feed transmission chain. Any gap or looseness in the transmission chain could cause position deviation, resulting in out-of-tolerance part dimensions.
    2.    Fault Diagnosis:
    •    1) As shown in the diagram, place a micrometer stand on the crossbeam, align the dial indicator to the negative direction of the main spindle Y-axis movement, and compress the indicator to about 50µm. Then reset the indicator to zero.
    •    2) Select the incremental work mode on the machine control panel, set the axis selector to the Y-axis, and manually feed the Y-axis in the negative direction using the handwheel. Observe the micrometer reading changes. The measurement revealed that both positive and negative incremental movements on the Y-axis showed irregular deviations.
    •    3) Place a ball bearing at the center of the ball screw end and contact the dial indicator with the bearing. Select the manual work mode on the machine control panel and use the feed buttons in both the positive and negative directions. The spindle moves continuously along the Y-axis, and the micrometer reading remains unchanged, excluding the possibility of axial movement of the ball screw.
    •    4) Check the synchronous toothed pulley connecting the Y-axis servo motor and the ball screw. Found that the pulley conical sleeve connected to the servo motor rotor shaft was loose, causing the feed drive to be out of sync with the servo motor. The looseness was irregular during operation, leading to position deviations and eventually causing irregular dimensional deviations in the processed part.

Maintenance Experience and Key Repair Points:
Since the Y-axis uses an encoder to form a semi-closed loop position control system, the position value detected by the encoder does not truly reflect the actual position of the Y-axis. The position control accuracy is largely determined by the feed transmission chain’s transmission accuracy.
    •    1) In daily maintenance, pay attention to checking the feed transmission chain, especially related coupling elements such as couplings and conical sleeves for looseness.
    •    2) Use a step-by-step inspection approach based on the transmission chain’s structural form to eliminate possible fault causes and ultimately pinpoint the fault location.
    •    3) Through part inspections, continuously monitor the CNC machine’s dynamic accuracy to determine whether mechanical adjustments to the CNC machine are necessary.