Titanium swarf catching fire - not fun

[VertigoCycles]

I'm prepping for a run of about 20 bikes and was turning head tubes for tapered steerer bikes on Tuesday night. Every time I started turning down the O.D. of a new head tube, the swarf caught fire. I turned down the speed, caught fire. Turned up the speed, caught fire and threw flaming swarf (truly an impressive and frightening sight). Cranked up the feed, caught fire. Tried skimming, fire. Even if I sprayed it with coolant, it caught fire. This only happened on the first cut and once I took off the first 0.010 of depth it was smooth sailing. There weren't oils on the surface but these are 2" O.D. hollows with an acid etched surface rather than the polished surface of the more typical 1.5 x 0.087 I normally use. No problems on the 1.5 on the same evening.

FWIW I was using a carbide insert with a soft nose radius (I don't know the spec but it's about 0.020") because it gives a very nice surface finish. It didn't matter if the tool was on center or below center either.

Any hypotheses as to why this was happening?

[Freddy]

Yup? I have seen this happen, and I wondered, so I will look into it and post back.
I suspect the carbide is chemically reacting but to what, maybe the acid soaked up in the metal?

I think Dr Sen will know so I will call him.

Stay tuned
Freddy