Poor surface finish (Ra too high)
Ra is reading higher than the target, meaning the ground surface is too rough — scrap risk rises and functional surfaces like bores, journals, or sealing faces may fail in service.
How you can tell
- Profilometer reads Ra above the drawing callout after a normal finishing cycle
- Surface looks dull, matte, or visibly scratched under a work light
- Comma-shaped scratch marks running in the feed direction
- Finish was acceptable earlier in the shift but has degraded over time
- Rougher surface at the part edges than in the middle
Causes, most common first
Dress too coarse or overlap ratio too low
A coarse dress leaves large, open cutting points on the wheel face that produce a rough surface — dressing is the single biggest influence on Ra.
How to check: Review your dress lead and overlap ratio settings; if the ratio is below about 4, the dress is too coarse for finishing. Run a test part immediately after a finer dress and re-measure Ra.
Fixes:
- Increase the overlap ratio to 4–8 for finishing by reducing the dress lead per wheel revolution.
- Take a final light dress pass at zero or minimal depth to smooth the wheel face before finishing passes.
- Set up two dress programs — a coarse dress for roughing and a fine dress only before the final passes.
Insufficient or no spark-out passes
Without spark-out the wheel is still deflecting elastically at the end of the cut, leaving a rougher surface with residual peaks.
How to check: Listen for the grinding sound to fade to near-silence — if it doesn't, you have not reached true spark-out. Check your cycle program for the number and duration of zero-infeed passes.
Fixes:
- Add 2–4 spark-out passes at zero infeed at the end of the finishing cycle.
- Make spark-out time consistent every cycle so results are repeatable.
Dirty coolant scratching the surface
Particles larger than the target Ra scale get dragged through the grinding zone and leave comma scratches that raise Ra and Rz.
How to check: Inspect the coolant under a light — visible swarf or cloudiness signals contamination. Check your filter rating and compare it to the target finish: precision grinding needs roughly 10–25 µm filtration, fine finishing 5–10 µm.
Fixes:
- Service or replace the coolant filter to restore the correct filtration rating for the finish you need.
- Clean the coolant tank and remove settled swarf before refilling.
- Upgrade to a finer filter stage (paper/fleece band, cartridge, or centrifuge) if your current system cannot reach the required micron rating.
Wheel grit too coarse for target Ra
Each grit range has a practical Ra floor — even with perfect dressing, a coarse grit cannot produce the surface a finer grit delivers.
How to check: Compare your current grit to the Ra target: 46–60 grit is unlikely to reach below about 0.4 µm; 80–120 grit is needed for finer results. Check KB #67 ranges against your drawing callout.
Fixes:
- Step up one grit grade (e.g., 60 → 80 or 80 → 120) for the finishing operation.
- Lower work speed and traverse rate during the finishing pass to get more out of the current grit before changing wheels.
Work speed or traverse rate too high
High work speed and feed reduce the speed ratio and increase the chip load per grain, producing a rougher cut.
How to check: Check your finishing pass work speed and traverse — if they match roughing values rather than reduced finishing values, this is the cause. Compare the speed ratio for roughing versus finishing.
Fixes:
- Reduce work speed and traverse rate for the finishing passes to increase the grinding speed ratio.
- Split stock into a roughing stage and a separate light finishing stage with slower feeds.
Wheel imbalance causing waviness
An out-of-balance wheel vibrates against the part once per wheel revolution, producing waviness and a higher Rz and Ra.
How to check: Listen for a rhythmic hum or chatter at wheel speed. Touch the spindle housing lightly — regular vibration at grinding frequency points to imbalance. Check when you last balanced the wheel.
Fixes:
- Static or dynamic balance the wheel on the machine or a balancing stand.
- Re-balance after every dress if you have an automatic balancing system.
- Flush and brush the wheel face after dressing to remove loose grains that shift the balance.
Rougher finish at part edges (edge overrun or dwell)
Incorrect traverse overrun or reversal dwell causes the wheel edge to take a heavier, uncontrolled cut at the ends of the part.
How to check: Measure Ra at the part centre and at both ends — if the edges are significantly rougher, traverse settings are the cause. Check overrun distance and dwell time at reversal.
Fixes:
- Set traverse overrun so the wheel clears each end by roughly one-third to one-half of the wheel width — not more, not less.
- Shorten the dwell time at traverse reversal.
- Dress the full wheel width and chamfer the wheel edges evenly to prevent edge marks.
Safety
If a new or different wheel is fitted to address grit size, always verify the wheel's maximum operating speed is equal to or greater than the spindle speed before starting, and perform a free-running test before engaging the workpiece.
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