Burn / thermal damage

Burn or thermal damage means the wheel is putting more heat into the part than the material can handle, risking softened surfaces, brittle white layers, residual tensile stress, and early part failure in service.

How you can tell

• Discoloured (straw, blue, or black) patches or bands on the ground surface
• Dark or light spots revealed by nital etch — dark means temper burn, light patch with dark border means rehardening burn
• Burn appears shortly after dressing then worsens until the next dress
• Spindle power or amperage climbs steadily through the run
• Spiral or spotted burn pattern rather than uniform discolouration

Causes, most common first

1. Dull or glazed wheel rubbing instead of cutting

   Flattened grains slide over the surface instead of shearing chips, converting nearly all energy to heat.

   How to check: Look at the wheel face with a light — a shiny, smooth appearance confirms glazing; metal streaks in the pores indicate loading.

   Fixes:

   • Dress the wheel immediately with a coarser lead (faster traverse) to expose sharp, open abrasive.
   • Increase dressing frequency so the wheel never has time to glaze between dresses.
   • Rotate or replace the dressing diamond if it has worn flat — a dull diamond re-glazes the wheel during dressing.

2. Wheel grade too hard, retaining dull grains

   A hard bond holds worn grains in place instead of releasing them, so the wheel cannot self-sharpen and friction rises.

   How to check: Note when burn first appears after a dress — if it starts clean then degrades steadily, a hard grade that cannot self-sharpen is likely; also watch for rising spindle current during the run.

   Fixes:

   • Move one or two grades softer (e.g. K → J or I) so dull grains break out sooner.
   • Move to a more open structure to reduce contact friction and improve coolant access.
   • Lower wheel speed so each grain carries a larger chip load and breaks out sooner.

3. Coolant not reaching the contact zone

   Without coolant at the wheel-work interface, heat generated in the arc has nowhere to go and accumulates in the part surface.

   How to check: Watch where the coolant stream hits — it must enter the grinding gap, not the back of the wheel; check jet velocity is close to wheel surface speed so the air barrier is broken.

   Fixes:

   • Re-aim the nozzle so the jet enters directly into the wheel-work contact zone.
   • Increase jet velocity to match wheel surface speed so the coolant penetrates the air boundary layer.
   • Check that the nozzle covers the full wheel width, especially when burn appears on one side only.
   • Increase coolant concentration for better lubricity on heat-sensitive materials.

4. Too much stock per pass, high heat input

   A large depth of cut raises the specific material removal rate, generating more heat per unit time than the process can dissipate.

   How to check: Review your current depth of cut and pass count — if you are taking heavy roughing passes right to size, heat input is likely too high.

   Fixes:

   • Reduce depth of cut and spread the same total stock removal over more passes.
   • Finish with two to four spark-out passes to clear residual stress without adding fresh heat.
   • Increase work speed (table or part rotation speed) to lower the speed ratio and reduce arc contact time per grain.

5. Wheel speed too high for the condition

   Higher wheel speed makes each grain take a thinner chip, the wheel acts harder, grains rub more, and heat per volume removed increases.

   How to check: Check your current wheel speed against the wheel's rated maximum and compare to your work speed — a high speed ratio means thin chips and more heat.

   Fixes:

   • Reduce wheel speed within the machine's available range to make the wheel act softer and cut more freely.
   • If wheel speed cannot be lowered, increase work speed or feed rate to thicken the chip and restore self-sharpening.

6. Wrong abrasive type for heat-sensitive material

   Standard aluminium oxide on tool steels or carburized parts cuts less freely and generates more heat than a friable or superabrasive wheel designed for the job.

   How to check: Identify the workpiece material and compare it against your current wheel spec — hardened tool steels, bearing steel, and carburized parts all need cooler-cutting abrasives.

   Fixes:

   • Switch to white aluminium oxide or a ceramic (seeded-gel) abrasive for cooler, more friable cutting action.
   • Consider CBN for hardened tool steels (D2, M2, high-speed steel) in production — it runs much cooler and lasts longer.
   • Use the coarsest grit the required finish allows (46–60) to maximise chip space and reduce rubbing.

7. Spiral or spotted burn from uneven wheel face

   A wheel that is out of round, out of balance, or dressed unevenly creates high spots that contact harder each revolution, concentrating heat locally.

   How to check: Look at the burn pattern — a spiral suggests the wheel face is not flat (dressed from two directions or with a loose dresser); spots or one-sided burn suggests imbalance or misalignment.

   Fixes:

   • Dress in one direction only and confirm the diamond holder is tight before every dress.
   • True the wheel fully then balance it on its adaptor after truing.
   • Check wheelhead and table alignment if burn consistently appears on the same side.

Safety

If any fix involves changing wheel speed or fitting a different wheel, never exceed the maximum operating speed marked on the wheel label, and confirm the guard is rated for the wheel's maximum speed before starting.

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