1. The issue of spindle overheating and declining rotational accuracy: Machined workpieces exhibit poor hole accuracy, poor cylindricity, rapid spindle heating, and excessive operational noise.
Upon analyzing the root cause, prolonged observation revealed that the spindle's centering cone hole sustained damage over multiple tool changes. This was primarily due to errors in tool insertion and removal, causing wear on the spindle's conical surface. Through meticulous inspection, it was identified that there were four primary causes contributing to the spindle malfunction:
1) The lubricating grease used for the spindle bearings did not meet quality standards and was contaminated with dust, impurities, and moisture. These contaminants primarily originated from inadequately distilled and dried compressed air used in the machining center. During chip cleaning, dust and moisture infiltrated the spindle bearings' grease, leading to poor lubrication, overheating, and significant noise.
2) Damage to the conical hole's positioning surface, which prevented perfect alignment between the spindle's conical surface and the tool holder's taper, resulting in minor eccentricities in the machined holes.
3) A reduction in the preload force of the spindle's front bearing, increasing bearing clearance.
4) Fatigue failure of the spring in the spindle's automatic clamping device, causing improper tool tightening and deviation from the original position.
To address these issues, the following solutions were implemented:
- Replace the front bearing of the spindle, use qualified grease, and adjust the bearing clearance.
- Grind the spindle's conical hole to ensure proper contact with the tool holder, achieving a contact surface of no less than 90%.
- Replace the clamping device spring and adjust the bearing preload force.
Additionally, regular checks of the spindle's shaft-hole and tool-holder coordination should be conducted during operation, along with adding an air-filtration and drying device, and arranging processing techniques to avoid overloading the machine.
2. Damage to the steel balls in the spindle assembly of the machining center: The steel balls in the automatic clamping mechanism of the tool spindle frequently fail, and the tapered surface of the tool holder is also prone to damage.
Research indicated that the spindle loosening action was out of sync with the robot pulling action. Specifically, the limit switch was positioned at the tail of the booster cylinder, and when the cylinder's piston reached its endpoint, the booster cylinder's piston did not align in time, causing violent pulling of the tool before the robot fully loosened the clamp. This severely damaged the pulling steel ball and tensioning screw.
To resolve this issue, the oil cylinder and booster cylinder were cleaned, sealing rings replaced, and pressure adjusted to synchronize both actions. Additionally, the gas-liquid booster cylinder must be regularly inspected to eliminate potential hazards.
3. Failure of the spindle component's positioning keys: Tool changes produce loud noises, and the positioning keys at the front end of the spindle, which rotate the tool handle, are partially deformed.
Research showed that the loud noise during tool changes occurred during the manipulator's insertion phase, caused by 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, but after prolonged use, the fixing screws of the Hall elements become loose, misaligning the tool handle's keyway with the spindle's positioning key. This damages the positioning key. Drifting of the reference point might be due to poor contact of the CNC system's circuit board, changes in electrical parameters, or loose proximity switches. This drifting causes the tool handle's tapered surface to directly impact the spindle's centering cone hole, producing abnormal noise.
To fix this, adjust the Hall component's installation position, secure it with anti-loosening adhesive, and recalibrate the tool-change reference point while replacing the spindle's positioning key. Regularly check the spindle's stop position and tool-change reference point during machining center usage, addressing any anomalies promptly.
Maintenance of the mechanical spindle involves reducing bearing temperatures through lubrication. Two methods are oil and gas lubrication and oil circulation lubrication. When using these methods, ensure sufficient oil volume in the spindle constant temperature oil tank for oil circulation lubrication. Conversely, the oil and gas lubrication method requires filling only ten percent of the bearing space capacity. Circulating lubrication reduces frictional heat generation and absorbs some of the spindle assembly's heat.
Spindle lubrication includes oil mist and injection methods. Cooling spindle components aims to minimize bearing heat generation and control heat sources effectively. Spindle component sealing prevents dust, chips, and coolant ingress while stopping lubricant leakage. Seals are either contact or non-contact. For contact seals like lip seals and oil-resistant rubber, monitor their aging and damage. Non-contact seals require ensuring quick oil drainage and clear oil return holes to prevent leaks. Proper lubrication reduces bearing temperature and extends lifespan. During operation, use grease and oil circulation lubrication at low speeds, while oil mist and oil gas lubrication are suitable for high speeds. Avoid overfilling grease, as excess exacerbates spindle heating. For oil circulation lubrication, daily checks of the spindle lubrication constant temperature oil tank are necessary to ensure sufficient oil volume and appropriate temperature ranges.
The characteristics of mechanical spindles include three highs and one low: high speed, high accuracy, high efficiency, and low noise.
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