Sparks when grinding hard carbon steel

[Bud]

I think this from Mark Reich above may speak to your question Mr. Sharpco.

"Files are hard, so the particles coming off are very small. Small particles have a lot of surface area (to react with oxygen) compared to their mass, so they immediately burst into flame even at low speed. The softer the steel the larger the particles, and large particles need more speed (perhaps more friction...?) to oxidize fast enough to light up."

And Merry Christmas to everyone!