(01-29-2021, 01:20 PM)riverturner01@gmail.com Wrote: I am new to turning bowls and have a problem with tenons breaking off. I believe they are formed correctly with dovetail side and dimensions which do not allow the tenon to bottom out on the chuck. Using a Record Power SC4 chuck. Wood is hickory form log cut 5 months ago. Lathe speed 1,000-1,200. Any ideas??
You do not mention what jaws you are using or how big of a bowl you are trying to turn.
That speed seems a little high for roughing.
Can you post any pics of the failed tenons?
There are some wood flaws that can cause problems if you don't see them. One of those is a shake (separation) in a particular wood ring. If that sort of flaw is the reason for the breaks, it ought to be fairly obvious in a picture.
Trying to use a dovetailed tenon in Nova 50mm chuck jaws has gotten more than a few of us in trouble. The Record jaws fit the Nova chucks as well. So, Record may also be using that same tenon-gripping profile as well on some of their jaws.
When using a dovetailed tenon with dovetailed jaws, you want the wood sitting tight against the "top" (side toward the tailstock) of the jaws and the wood dovetail touching the jaws at the bottom of the dovetail (furthest away from the tailstock).
If the wood only touches the jaws at the "top", then the inner corner of the jaws is trying to shear away the tenon. The more one tightens the chuck, the more it is like trying to drive a dull chisel into the corner of the dovetail. Add vibration and stress from turning and the dovetail will start propagating a crack along the grain from the edge of those 4 chisels.