I am turning the inside of the "holey bowl" discussed several weeks ago. To reduce waste, I glued a piece of birch to the bottom and turned a mortise into it while it was on the screw chuck mounted on the spindle. (I don't like to leave a mortise in the bottom of a finished bowl.) I was getting some chatter when I was only an inch into the inside. When I looked at the contact between mortise and the 4" jaws on my Nova2, I noticed that the jaws are contacting the bowl at the midpoint of the arc of each jaw. See photo.
My question is; Is that contat sufficient to hold the bowl securely?
I guess that I need to determine the perfect diameter of the mortise for gripping strength.