Out-of-round / lobing
The ground part is not truly round even though a micrometer reads the same diameter everywhere — lobing causes vibration, noise, and poor fit in service.
How you can tell
- Micrometer reads consistent diameter all around yet the part wobbles in a V-block
- Roundness tester or V-block indicator shows an odd number of high spots (3, 5, 7 lobes)
- Part has a faceted feel when rolled on a surface plate
- Finished bores spring back out of round after unclamping (internal grinding)
- Crankpins or camshafts show oval or lobed profile after chasing or orbital grinding
Causes, most common first
Part on or near center in centerless grinding
When the workpiece centre sits at or below the correct height, the geometry allows lobes to regenerate with every revolution instead of being ground away.
How to check: Measure the work-rest blade height relative to wheel-centre height; if the part centre is at or below wheel-centre height, the setup is wrong.
Fixes:
- Raise work height above centre line to the correct position for the part diameter — never set it on centre.
- Increase blade top angle to 30° or more and replace a worn blade.
- True the regulating wheel and check its speed.
- Add spark-out time and reduce stock per pass on infeed.
Three-jaw chuck or 3-point clamping distortion
Three clamping points squeeze a thin or compliant part into a triangle shape; when released it springs back out of round at exactly three lobes.
How to check: Measure the bore or OD on the machine under clamping load, then again after unclamping — a size change confirms clamping distortion.
Fixes:
- Switch to a 6-jaw chuck, collet, or diaphragm chuck to spread clamping force evenly.
- Reduce clamping pressure to the minimum needed to drive the part, especially on finishing passes.
- For thin-walled bores, use a face or diaphragm chuck and locate on a clean, true reference face.
Damaged or dirty center holes
Nicked, dirty, or incorrectly angled center holes cause the part to run eccentric or rock, producing repeating high spots as it rotates.
How to check: Remove the part and inspect center holes visually and with a loupe; run a finger around the cone surface and feel for grit, burrs, or damage.
Fixes:
- Clean center holes thoroughly before every setup.
- Re-lap or regrind center holes — always do this after heat treatment.
- Replace worn dead centers with carbide-tipped precision centers.
Wheel out of balance or out of round
An unbalanced or eccentric wheel vibrates at wheel-speed frequency, forcing repeating high and low spots into the part surface.
How to check: Listen for rhythmic vibration that matches wheel speed; check wheel with a static balancer or spin it up and watch for vibration on the spindle housing.
Fixes:
- True the wheel fully after every mounting using a fixed dresser path across the full face width.
- Balance the wheel on a static balancer before mounting, then re-balance after the first dress.
- Check spindle runout and flange seating; correct any spindle runout before proceeding.
Chatter at odd multiple of work speed
When chatter frequency is an odd multiple of work rotational speed, the waves stay in phase each revolution and lock into a stable lobe pattern.
How to check: Count the lobes on the roundness trace; if the number is odd (3, 5, 7) and the surface has a regular wave pattern, chatter regeneration is likely.
Fixes:
- Change work speed so that work rpm is not an integer fraction of wheel rpm, breaking the regenerative lock.
- Lower work speed to reduce chatter tendency if roundness is poor.
- Add steady rests and reduce infeed to lower cutting forces on long parts.
- Increase spark-out revolutions to let the wheel remove chatter waves at zero infeed.
Workhead spindle or chuck runout
Worn workhead bearings or a chuck with runout force the part to orbit off-axis, producing a repeating out-of-round error on every revolution.
How to check: Mount a test mandrel in the chuck or collet and sweep it with an indicator at the nose and further out; any runout reading confirms the problem.
Fixes:
- Clean the chuck and seating face and re-indicate the part true before grinding.
- Have maintenance check and correct workhead bearing condition.
- Use a collet or face chuck to eliminate jaw runout for precision finishing.
C-axis / X-axis sync error (orbital or cam grinding)
In orbital pin chasing or cam grinding, a phase lag between the C-axis (part rotation) and X-axis (wheel tracking) stamps an oval or lobed error directly onto the ground profile.
How to check: Measure the pin or cam lobe at multiple angular positions; if high spots fall exactly 180° apart it is dynamic tracking error, not a roundness issue.
Fixes:
- Reduce C-axis speed to give the X-axis linear drive time to track accurately.
- Reduce roughing infeed to lower cutting forces that overload the C-axis servo.
- Drive long crankshafts from both ends to eliminate torsional twist lag.
- Have maintenance tune C and X-axis servo gain to eliminate following error.
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