1. The issue of spindle heating and declining rotational accuracy: When machining a workpiece, the hole accuracy is low, cylindricity is poor, the spindle overheats rapidly, and the processing noise is excessive.
After observing the machine tool's spindle for an extended period, it was determined that the centering cone hole of the spindle was damaged due to multiple tool changes. The primary cause of this damage is the error in pulling and inserting tools, which harms the spindle’s cone surface. Upon thorough examination by professionals, four causes of this spindle malfunction were identified:
(1) The grease used for the spindle bearing does not meet the required standards and is contaminated with dust, impurities, and moisture. These contaminants primarily originate from the unfiltered and undried compressed air used in the processing center. During chip cleaning, dust and water infiltrate the spindle bearing's grease, leading to inadequate lubrication and significant heat generation accompanied by loud noise.
(2) Damage exists on the conical hole’s positioning surface used for tool positioning within the spindle. This results in imperfect fitting between the spindle’s conical surface and the tool holder, causing the processed holes to have slight eccentricities.
(3) The preload force of the spindle's front bearing has decreased, leading to greater bearing clearance.
(4) Fatigue failure of the spring in the spindle’s automatic clamping device causes the tool to fail to tighten properly and deviate from its original position.
To address these issues, the following solutions were implemented:
(1) Replace the front bearing of the spindle, use approved grease, and adjust the bearing clearance.
(2) Grind the spindle’s conical hole positioning surface for proper alignment, ensuring the contact surface with the tool holder is at least 90%.
(3) Replace the clamping device’s spring and adjust the bearing preload force.
Additionally, frequent checks of the spindle’s shaft hole and tool holder cleaning and coordination status are essential. An air filtration and drying device should be added, and processing techniques should be arranged rationally to avoid overloading the machine.
2. Damage to the steel balls in the spindle component of the machining center: The steel balls in the automatic clamping mechanism of the tool often get damaged, along with the tapered surface of the tool handle.
The investigation revealed that the spindle loosening action did not synchronize with the robot pulling action. Specifically, the limit switch was positioned at the tail of the booster cylinder. When the cylinder's piston reached its position, the booster cylinder's piston wasn’t in place, causing the robot to violently pull the tool before the clamping structure was fully loosened. This severely damaged the pulling steel ball and tensioning screw.
To resolve this, the oil cylinder and cylinder were cleaned, the sealing ring replaced, and the pressure adjusted to coordinate both actions. Regular checks of the gas-liquid booster cylinder are recommended to eliminate potential hazards.
3. Failure of the spindle parts’ positioning keys: The tool change generates loud noises, and the positioning keys that rotate the tool handle at the spindle's front end become partially deformed.
Research showed that the loud noise during tool change happened during the manipulator's tool insertion phase. This was due to errors in the spindle's correct stop position and drifting at the tool change reference point. The machining center typically uses Hall elements for directional detection. Over time, the fixing screws of the Hall elements loosen, misaligning the tool handle keyway with the spindle’s positioning key, damaging the key. Drifting of the reference point might result from poor contact of the CNC system’s circuit board, changes in electrical parameters, or loose proximity switch fixation. This drifting causes the tapered surface to hit the centering cone hole when inserting the tool handle into the spindle cone hole, generating abnormal noise.
Solutions include adjusting the Hall component’s installation position, adding anti-loosening glue, adjusting the tool change reference point, and replacing the spindle’s front-end positioning key. Regular checks of the spindle stop position and tool change reference point are recommended, with timely inspections for anomalies.
Maintenance of the mechanical spindle involves reducing the working temperature of the bearing through lubrication. Two methods—oil and gas lubrication and oil circulation—are available. Key considerations include ensuring sufficient oil volume in the spindle constant temperature oil tank for oil circulation and filling only 10% of the bearing space capacity for oil and gas lubrication.
Lubrication methods like oil mist and injection also play roles in spindle cooling. Seals prevent dust, chips, and coolant from entering while stopping lubricant leaks. Contact seals require checking for aging or damage, while non-contact seals must ensure quick oil return and unobstructed oil return holes.
Proper lubrication reduces bearing temperature and extends lifespan. During operation, grease or oil circulating lubrication is used, while oil mist or oil gas lubrication is preferred at high speeds. Excessive grease can worsen spindle heating, so the sealing amount is typically 10% of the bearing space volume. Daily checks of the spindle lubrication constant temperature oil tank are necessary to ensure adequate oil volume and appropriate temperature ranges.
Mechanical spindles are characterized by their “three highs and one low†traits: high speed, high precision, high efficiency, and low noise.
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