causes for tenons breaking
#6
  
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??
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#7
  Re: causes for tenons breaking by riverturner01@gmail.com (I am new to turning ...)
Sorry I can’t help you with why but I had the same problem. Had a few bowls come flying off the lathe. I went to using a mortise instead and problem solved.
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#8
  Re: causes for tenons breaking by riverturner01@gmail.com (I am new to turning ...)
Mortise has been my solution as well--hoping for a better answer!!
earl
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#9
  Re: causes for tenons breaking by riverturner01@gmail.com (I am new to turning ...)
(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.
"the most important safety feature on any tool is the one between your ears." - Ken Vick

A wish for you all:  May you keep buying green bananas.
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#10
  Re: causes for tenons breaking by riverturner01@gmail.com (I am new to turning ...)
(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??

Two.

First, make sure your jaws shoulder against the bowl bottom to prevent racking and ripping.  Make a decent shoulder as wide as the jaw face and then don't squeeze the tenon, just snug it up to the shoulder.

Second, don't spin so fast.  Remember Newton says velocity squared, so twice as fast is four times the energy available to stress the tenon if you're a bit careless and "catch."  I turn between centers until the piece is as balanced and light as I can get it, but that's because my old Masterchuck - first I owned - used to throw things on a whim, much less a catch, when the piece was cantilevered. https://archive.org/details/themasterchu...1/mode/2up Don't think the Nova has seen more than 680 on a faceplate piece in its life.  Sometimes less.  

I too use a mortise, even when it's inside a "foot" for aesthetics.  There, make sure the jaws contact the bottom of the piece firmly as well to keep it from racking.
Better to follow the leader than the pack. Less to step in.
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