Centerless cylindrical grinding occurs when the workpiece is placed between the guide wheel and the grinding wheel. One of the wheels grinds while the other, known as the guide wheel, transmits motion.
The workpiece is supported at the bottom by a support plate. The guide wheel is made from a rubber binder, and its axis forms an angle θ with the grinding wheel in the vertical direction, causing the workpiece to rotate and feed.
Here is a list of common grinding defects on centerless grinders and their remedies:
1. Non-round parts
Causes:
(a) Guide wheel not trued
(b) Insufficient grinding cycles or excessive ovality from the previous process
(c) Dull grinding wheel
(d) Excessive grinding or feed rate
Remedies:
(a) Retrue the guide wheel until it is round (usually until there is no intermittent sound)
(b) Appropriately increase the number of grinding cycles
(c) Retrue the grinding wheel
(d) Reduce the grinding and feed speed
II. Parts are Polygonal (Multi-Sided)
Causes:
(1) The part’s center height is insufficient.
(2) Excessive axial thrust on the part, causing it to press against the stop pin and rotate unevenly.
(3) The grinding wheel is unbalanced.
(4) The part’s center is too high.
Remedies:
(1) Precisely adjust the part’s centering.
(2) Reduce the grinding wheel’s swivel angle to 0.5° or 0.25°. If the stop pin cannot resolve the issue, check the balance of the pivot point.
(3) Balance the grinding wheel.
(4) Appropriately lower the part’s center height.
III. There are Vibration Marks on the Surface of Parts (i.e., the part’s surface showing mottle and white straight lines)
Causes:
(1) Machine vibration caused by an unbalanced grinding wheel surface.
(2) Jumping of the part due to the part’s center being pushed forward.
(3) The grinding wheel is blunt or the surface of the grinding wheel is too smooth.
(4) The feed wheel is spinning too fast.
Remedies:
(1) Carefully balance the grinding wheel.
(2) Appropriately lower the part’s center.
(3) Increase the grinding wheel’s dressing speed as required.
(4) Appropriately lower the feed speed.
IV. Component Tapering
Causes:
1) The front guide plate’s guideway line is excessively low, or the guide plate tilts toward the guide wheel, leading to a smaller front part of the component.
2) The back guide plate’s surface is low or tilted towards the guide wheel, causing a smaller rear part of the component.
3) The front or rear part of the component tapers due to the following reasons:
① The grinding wheel is incorrectly trimmed, inherently causing tapering.
② The grinding wheel and guide wheel surface have worn out.
Remediation Methods:
1) Appropriately move the front guide plate and adjust it to be parallel with the guideway line of the guide wheel.
2) Adjust the guiding surface of the back guide plate to be parallel with the guideway line of the guide wheel and in alignment.
①Based on the direction of the component tapering, adjust the angle of the grinding wheel in the modification.
② The grinding wheel and guide wheel.
V. Part center is larger than both ends
Causes:
(1) Front and rear guide plates are tilted uniformly towards one side of the grinding wheel
(2) The grinding wheel is dressed into a barrel shape
Solutions:
(1) Adjust the front and rear guide plates
(2) Modify the grinding wheel, each adjustment should not be too large
VI. Part surface has annular thread lines
Causes:
(1) Front and rear guide plates protrude beyond the surface of the guide wheel, causing the part to be scraped by the edge of the guide wheel at the exit or entrance
(2) The work rest blade is too soft, and the cuttings embedded on the blade surface form protruding burrs, which engrave thread lines on the surface of the part
(3) The coolant is not clean, containing chips or sand particles
(4) At the exit, the part is scraped by the edge of the grinding wheel due to excessive grinding
(5) The center of the part is lower than the center of the grinding wheel, resulting in large downward pressure, which causes the sand particles and chips to stick to the work rest blade
(6) The grinding wheel is dull
(7) Excessive material removal in one grinding pass or the grinding wheel is too coarse, resulting in very fine thread lines on the part surface
Solutions:
(1) Adjust the front and rear guide plates
(2) Replace with a work rest blade that has a higher hardness and provides surface lubrication
(3) Replace the coolant
(4) Round off the edge of the grinding wheel, and ensure that the last 20mm of the part at the exit is not ground
(5) Adjust the center height of the part properly
(6) Dress the grinding wheel
(7) Appropriately reduce the amount of grinding and slow down the adjustment speed
VII. A small chunk is cut off from the front of the part
Causes:
(1) The front guide plate protrudes beyond the surface of the guide wheel
(2) The front end faces of the grinding wheel and the guide wheel are not aligned, and the difference is large
(3) Too much grinding at the entrance
Solutions:
(1) Move the front guide plate a bit backward
(2) Replace or adjust the longer one of the two
(3) Reduce the amount of grinding at the entrance
VIII. When the middle or end of the part gets cut, the cuts are categorized as follows:
1. The cut is rectangular in shape.
Reasons for occurrence:
(a) The rear guide plate is outside the guide wheel’s surface, preventing the part from rotating and continuing the traverse grinding.
(b) The back support pad is extended too far, and the ground part doesn’t fall off, hindering the rotation and advance of the ground part.
Methods of elimination:
(a) Appropriately shift the rear guide plate backward.
(b) Reinstall the support pad.
2. The cut is angular or possibly has numerous minute traces.
Reasons for occurrence:
(a) The rear guide plate is behind the surface of the guide wheel.
(b) The center of the part advances too high, causing the part to bounce at the exit.
Methods of elimination:
(a) Slightly shift the rear guide plate forward.
(b) Properly lower the height of the part’s center.
IX. The surface brightness of the part is inadequate.
Reasons for occurrence:
(a) The lean angle of the guide wheel is too large, causing the part to cut too quickly.
(b) The wheel modification speed is too fast, leading to a less shiny wheel surface.
(c) The guide wheel modification is too coarse.
Methods of elimination:
(a) Reduce the lean angle.
(b) Lower the modification speed and re-modify the wheel.
(c) Recondition the guide wheel.
