>Recently I hollowed pieces of Walnut and Holly. Both were fairly dry pieces. I turned them into closed forms with a small opening. I was using a rounded 1/8" wide HSS bit to turn out the inside. The tool is presented in a scraping mode.
I believe this scraping generated enough heat to further dry out the piece and cause 'heat cracks' to develop on the outside of the piece.
Wall thickness was still in the 1 1/2" range when the cracks appeared.
On the second piece when the first crack appeared, I sprayed water both inside and outside the piece and kept it wet. No other cracks appeared.
My question is; Were the cracks caused by heat and is adding water a solution that should be used to prevent it? Is there something else, or was I doing something wrong? (When the wood is green I haven't had this problem.)
I am not an expert at this but I wanted to give some answer before your question fell off the forum.
I doubt the "cracks" are caused by heat generated from within the turning. More likely internal stress is being released as the turning is progressing. The misting of water on the wood is likely to relax the wood so that the released stress manifest itself as warping instead of checking.
The heat check I have seen occurred from sanding with to fine a grit and with to much pressure. The surface of the piece was HOT to the touch. Hot enough that I couldn't touch the piece with my fingers. There were a LOT of very fine checks which went no more than 1/4" into the piece (Cocobolo).
I would suggest that hollowing in stages from the top of the piece towards the bottom such that the top couple of inches was completed before starting on the next couple of inches would restrict the release of stress to wood that was thin enough to distort rather than check.
That being said you appear to have a method which works for you. That is putting water in and outside of the piece as you hollow. Because that process is working for you I would continue with that process.
If someone else provides different advice they are mostly likely correct.
