Analysis of Causes for Unstable Size of CNC Machine Tools

Mechanical cause analysis:

1. The connection between the servo motor shaft and the lead screw is loose, causing the lead screw pre-motor to be out of sync and a dimensional error occurs. Only need to mark the coupling between the servo motor and the lead screw during the test. Moving the table (or tool holder) back and forth with a faster rate will cause significant relative movement at both ends of the coupling due to the inertia of the table (or turret). Such faults are usually realized by changing the machining dimensions in only one direction, and only need to tighten the coupling screws evenly to eliminate them.
2. The lubrication between the ball screw and the nut is poor, so that the movement resistance of the table (or the tool holder) is increased, and the movement command cannot be completely and accurately executed. Such failures usually manifest as irregular changes in the size of the part within a few filaments, and the lubrication can be improved to eliminate the fault.
3. The movement resistance of the machine table (or tool holder) is too large, which is generally caused by the over-tightening of the strip and the poor lubrication of the surface of the machine guide. This failure phenomenon generally shows that the size of the part changes irregularly within a few filaments, and it is only necessary to readjust the strip and improve the lubrication of the guide rail.
4. The rolling bearing is worn or improperly adjusted, resulting in excessive movement resistance. This failure phenomenon is also usually manifested by irregular changes in the size of a few filaments. Simply replace the worn bearing and carefully adjust it to eliminate the fault.
5. If the screw clearance or clearance compensation amount is improper, the fault can be eliminated by adjusting the clearance or changing the clearance compensation value.

Processing size instability class fault diagnosis and maintenance

1. The workpiece size is accurate and the surface finish is poor.

The cause of the failure: the tool tip is damaged, not sharp; the machine tool generates resonance, the placement is not stable; the machine tool has crawling phenomenon; the processing technology is not good.

Solution: If the tool is not sharp after being worn or damaged, re-grind the knife or choose a better tool to re-align the tool; the machine will generate resonance or place it is not stable, adjust the level, lay the foundation, and fix it smoothly; the reason for the mechanical creep is drag The guide rail of the plate is very worn, the screw of the lead screw is worn or loose, and the machine should pay attention to maintenance. After the work, the wire should be cleaned and the lubricant should be added in time to reduce the friction. The coolant suitable for the workpiece processing can be selected to meet the processing requirements of other processes. In the case, try to use a higher spindle speed.

1. The workpiece produces a taper size

The cause of the fault: the level of the machine tool is not adjusted well, one high and one low, the placement is not stable; when turning the long axis, the contribution material is relatively hard, the tool eats the knife deeper, causing the knife phenomenon; the tailstock thimble is not concentric with the main shaft.

Solution: Use the level to adjust the level of the machine, lay a solid foundation, fix the machine to improve its toughness; choose a reasonable process and appropriate cutting feed to avoid the tool force to make the knife; adjust the tailstock.

1. The driver phase lamp is normal, and the processed workpiece size is large and small.

The cause of the failure: the long-term high-speed operation of the machine tool carriage causes the lead screw and the bearing to wear out; the repeated positioning accuracy of the tool holder is deviated in the long-term use; the carriage can accurately return to the machining starting point every time, but the size of the processed workpiece still changes. This phenomenon is generally caused by the main shaft, and the high-speed rotation of the main shaft causes the bearing to wear seriously, resulting in a change in the machining size.