The issue of spindle heating and declining rotational accuracy presents several challenges: machined workpieces exhibit low hole precision, poor cylindricality, rapid spindle overheating, and excessive operational noise. Upon analyzing the root cause, prolonged observation reveals that the centering cone hole of the spindle incurred damage over multiple tool changes. This damage primarily stems from improper tool insertion and extraction, which compromises the spindle's conical surface. Through meticulous inspection, it became evident that the spindle malfunction stems from four critical factors:
1. Insufficient quality of the spindle bearing’s lubricating grease, contaminated with dust, impurities, and moisture. These contaminants originate from the unfiltered and undried compressed air used in the machining center. During chip removal, dust and moisture infiltrate the spindle bearing's grease, leading to inadequate lubrication, increased heat generation, and significant operational noise.
2. Damage to the conical hole's positioning surface, preventing perfect alignment between the spindle's conical surface and the tool holder’s taper. This results in slight eccentricity in the machined holes.
3. Reduced preload force on the spindle’s front bearing, increasing bearing clearance.
4. Fatigue-induced failure of the spring in the spindle’s automatic clamping device, causing the tool to loosen improperly and deviate from its intended position.
To address these issues, corrective measures include replacing the spindle’s front bearing, using certified grease, adjusting bearing clearance, grinding the spindle’s conical hole’s positioning surface to ensure at least 90% contact with the tool holder, and replacing the clamping device spring while readjusting bearing preload. Additionally, regular inspections of the spindle’s shaft-hole and tool-holder cleanliness, along with the addition of an air filtration and drying system, are recommended. Properly scheduling processes and avoiding overloading the machine can further mitigate these issues.
Another common issue involves damage to the spindle's steel balls in the automatic clamping mechanism. Research shows that the spindle’s loosening action often misaligns with the robot’s pulling action due to a delayed response of the booster cylinder’s piston. This leads to violent tool pulling, severely damaging the puller steel ball and tensioning screw. To resolve this, clean and replace the oil cylinder, adjust the pressure settings, and ensure both actions are synchronized. Regular maintenance checks of the gas-liquid booster cylinder are essential to preemptively address potential hazards.
The third issue relates to the deformation of the spindle’s positioning keys during tool changes, accompanied by loud noises. Investigation reveals that these issues arise due to errors in the spindle’s stop position and drifting of the tool-change reference point. The Hall element, responsible for directional detection, experiences loose fixing screws after prolonged use, causing misalignment between the tool handle’s keyway and the spindle’s positioning key. Additionally, drifting of the reference point might result from poor circuit board contact, electrical parameter changes, or loose proximity switches. Corrective actions involve adjusting the Hall element’s position, securing it with anti-loosening glue, resetting the tool-change reference point, and replacing the spindle’s front-end positioning key. Regular monitoring of the spindle’s stop position and tool-change reference point is crucial to avoid such malfunctions.
Maintenance of mechanical spindles focuses on reducing bearing temperatures through proper lubrication. Two primary methods include oil and gas lubrication and oil circulation lubrication. With oil circulation lubrication, ensuring adequate oil volume in the spindle’s constant-temperature oil tank is vital. Conversely, the oil and gas lubrication method requires filling only 10% of the bearing’s space capacity. The advantages of oil circulation lie in reducing frictional heat and absorbing some of the spindle assembly’s heat.
Spindle lubrication also employs oil mist and injection methods. Cooling spindle components primarily targets reducing bearing heat generation and controlling heat sources. Sealing the spindle components prevents ingress of dust, chips, and coolant while also stopping lubricant leaks. Contact seals like linoleum rings and oil-resistant rubber seals require periodic checks for aging and damage, whereas non-contact seals rely on efficient oil drainage and unobstructed return oil holes. Proper lubrication reduces bearing temperature and extends lifespan. During operation, grease or oil circulation lubrication is preferred, while oil mist and oil gas lubrication suit higher speeds. Excessive grease can worsen spindle heating, so it’s best to limit the filling to 10% of the bearing space volume. Daily checks of the spindle’s lubrication constant temperature oil tank are necessary to maintain optimal oil levels and temperature ranges.
Mechanical spindles are characterized by their high speed, high accuracy, high efficiency, and low noise. However, addressing these issues ensures optimal performance and longevity.
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