During centerless external cylindrical grinding, the workpiece is positioned between the guide wheel and the grinding wheel. One of these wheels is used for grinding, while the other, known as the guide wheel, is responsible for transmitting motion. The lower part of the workpiece is supported by a support plate. The guide wheel is constructed with a rubber bonding agent, and its axis is inclined at an angle θ with respect to the grinding wheel in the vertical direction. This setup drives the workpiece to rotate and feed into the grinding process.
Common grinding defects of centerless grinders and their elimination methods are summarized as follows:
1. Out-of-round parts
Causes
- The guide wheel does not have a rounded edge.
- There are too few grinding cycles, or the ellipticity from the previous process is excessively large.
- The grinding wheel is dull.
- The grinding or cutting amount is too high.
Elimination methods
- Rebuild the guide wheel and wait until it is properly rounded. Generally, it will stop when there is no intermittent sound.
- Adjust the number of grinding cycles as needed.
- Rebuild the grinding wheel.
- Reduce both the grinding amount and the re-cutting speed.
2. Parts have edges (polygons)
Causes of Issues:
- The center height of the part is not sufficient.
- Excessive axial thrust on the part causes it to press against the stop pin, preventing even rotation.
- The grinding wheel is unbalanced.
- The center of the part is positioned too high.
Methods for Elimination:
- Accurately adjust the center of the part.
- Reduce the inclination of the grinder guide wheel to either 0.5° or 0.25°. If this does not resolve the issue, check the balance of the fulcrum.
- Ensure the grinding wheel is balanced.
- Appropriately lower the center height of the part.
3. Vibration marks on the surface of parts (i.e., fish spots and straight white lines appear on the surface of parts)
Causes
- Machine vibration caused by the unbalanced surface of the grinding wheel
- Part center moves forward and causes part to jump
- The grinding wheel is blunt, or the surface of the grinding wheel is too smooth
- Guide wheel rotates too fast
Eliminate methods
- Carefully balance the grinding wheel
- Appropriately reduce the center of the part
- Grinding wheel or appropriately increase the dressing speed of the grinding wheel
- Appropriately reduce the guide speed
4. Parts have taper
Causes
- The front section of the part is smaller because either the front guide plate and the guide wheel’s generatrix are positioned too low or the front guide plate is tilted towards the guide wheel.
- The rear section of the CNC machining aluminum parts is smaller because either the surface of the rear guide plate is lower than the guide wheel’s generatrix or the rear guide plate is tilted towards the guide wheel.
- The front or rear section of the part may have a taper for the following reasons:
① The grinding wheel has taper due to improper dressing
② The grinding wheel and the guide wheel surface are worn
Elimination method
- Carefully reposition the front guide plate and ensure it is parallel to the generatrix of the guide wheel.
- Adjust the guide surface of the rear guide plate so that it is parallel to the generatrix of the guide wheel and aligned on the same line.
① According to the direction of the part taper, adjust the angle of the grinding wheel in the grinding wheel modification
② The grinding wheel and the guide wheel
5. The center of the part is large, and the two ends are small
Cause:
- The front and rear guide plates are tilted evenly toward the grinding wheel.
- The grinding wheel has been shaped like a waist drum.
Elimination Method:
- Adjust the front and rear guide plates.
- Modify the grinding wheel, ensuring that no excessive allowance is made during each adjustment.
6. There are circular threads on the surface of the part
Causes
- The front and rear guide plates protrude from the guide wheel surface, causing parts to be scraped by the edges of the guide wheel at both the entrance and exit.
- The guide is too soft, which allows grinding chips to become embedded in the guide surface, creating protruding burrs that engrave thread lines on the parts’ surfaces.
- The coolant is not clean and contains chips or sand.
- Due to excessive grinding at the exit, the edge of the grinding wheel causes scraping.
- The center of the part is lower than the center of the grinding wheel, resulting in high vertical pressure that causes sand and chips to stick to the guide bristles.
- The grinding wheel is blunt.
- Excess material is ground off at once, or the grinding wheel is too coarse, which leads to extremely fine thread lines on the surfaces of the CNC lathe parts.
Elimination methods
- Adjust the front and rear guide plates.
- Replace the guide bristles with lubricated materials of higher hardness.
- Change the coolant.
- Round the edge of the grinding wheel, ensuring that approximately 20 mm at the exit of the part is left unground.
- Properly adjust the center height of the part.
- Ensure the grinding wheel is in good condition.
- Reduce the grinding amount and slow down the modification speed.
7. A small piece is cut off from the front of the part
Cause
- The front guide plate extends beyond the surface of the guide wheel.
- There is a significant misalignment between the front surface of the grinding wheel and the guide wheel.
- Excessive grinding is occurring at the entrance.
Solutions:
- Slightly reposition the front guide plate backward.
- Replace or modify the longer of the two components.
- Decrease the amount of grinding at the entrance.
8. The middle or tail of the part is cut badly. There are several types of cuts:
1. The cut is rectangular
Cause
- The rear guide plate is not aligned with the surface of the guide wheel, which prevents the part from rotating and halts the grinding of the tread surface.
- The rear support pad is extended too far, causing the ground part to remain in place and preventing it from rotating or moving forward.
Eliminate
- Adjust the rear guide plate to the correct position.
- Reinstall the support pad.
2. The cut is angular or has many micro-shaped marks
Cause
- The rear guide plate lags behind the surface of the guide wheel
- The center of the part moves too high, causing the part to jump at the exit
Eliminate
- Move the rear guide plate slightly forward
- Properly reduce the center height of the part
9. The surface brightness of the part is not zero
Cause
- The inclination of the guide wheel is excessive, causing the part to move too quickly.
- The grinding wheel is adjusted too quickly, resulting in a dull surface.
- Additionally, the guide wheel is modified too roughly.
Solution
- Decrease the angle of inclination.
- Lower the modification speed and start modifying the grinding wheel from the beginning.
- Reconstruct the guide wheel.
Note: When the grinding wheel is not in operation, it is prohibited to open the coolant. If the coolant must be opened first to prevent any faults from occurring, it should be turned on and off intermittently (i.e., on, off, on, off). Wait for the coolant to disperse from all sides before starting the work.
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Post time: Dec-10-2024