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Stuart Batty On Tenons (Read 2,928 times)
 
Ed Weber
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Re: Stuart Batty On Tenons
Reply #30 - Nov 18th, 2013 at 8:06pm
 
As is always the case, there are many schools of thought.
It doesn't appear that in the video SB is targeting safety with this method, as much as not wasting wood. Whether the tenon breaks off in the jaws, or comes out of the chuck, it makes little difference, the blank is still flying free. So after changing your underwear, the only difference is that in SB's method, you may still be able to re-chuck your piece using the same tenon. Except now your chamfered tenon has damage from being pulled out of the chuck and most likely won't have as secure of a connection as it did originally.

As to Ron's point, I had similar thoughts.
I think trying to leverage a blank out of the chuck would make a more sensible test than hitting it from the headstock side. Maybe try something like screwing in two lag bolts, one in the center and another two or three inches from center. Then lock the spindle, get a pry-bar and try to twist the blank out. This might better mimic the twisting and the forces of leverage when you get a catch.
Whatever your methods, turn Safely
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John Teichert
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Re: Stuart Batty On Tenons
Reply #31 - Nov 18th, 2013 at 8:36pm
 
Ken Vaughan wrote on Nov 18th, 2013 at 7:15pm:
Look at the champfer as a way to shorten an overly long tenon, and look more at the effective contact length of the tenon. Also note that the hinge point is at the outside contzct point of the outside of the jaws and the flat contact point on the bowl.


It does seem as if Batty is reducing the effective length of the tenon, possibly to allow it to come out of the chuck more easily in order to save the tenon, and that the chamfer still allows more wood behind the tenon, perhaps to provide additional support for the dovetail helping it to remain more intact in case of the bowl coming out of the chuck. In the tenon that failed, the dovetail was the full length of the tenon, and thereby I would think provide more gripping power. In addition the tenon that failed had no supporting edge on the bowl, providing a more acute angle of moment at the leverage point, contributing to the forces that caused the tenon to fail. These two factors would seem to provide a higher failure rate at the tenon, than the shorter effective length of dovetail of the chamfered tenon and it's supporting flat surface on the bowl that changes the angle at which the forces are applied on the tenon that did not fail.

Ken Vaughan wrote on Nov 18th, 2013 at 7:15pm:
Batty uses the chamfer to set the failure to the less risk mode of the tenon coming out of the chuck if an intense catch occurs

I not sure you can make that point. The flat surface that provides the contact point must surely help the stability of the blank, as pointed out by Batty when mounting the bowl without a contact surface on the bowl. The shorter effective tenon length of the chamfered tenon, would seem to me to allow the bowl to come off the chuck easier than a longer tenon, but help in leaving an intact tenon.

As I interpreted Batty, he is recommending (1) that the bowl have a flat surface for the face of the chuck jaws to provide stability and thus far less prone to stress the tenon, and (2) that the chamfered tenon provides a stronger attachment. I'm not sure that it provides a "stronger" attachment, but perhaps one that is less susceptible to damage, and is sufficiently strong enough to provide the means by which the bowl can be worked. He has certainly had far more experience than I will have accumulated for the rest of my turning years, and the points would seem to me in my very limited turning experience to have validity.
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« Last Edit: Nov 18th, 2013 at 8:38pm by John Teichert »  
 
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Re: Stuart Batty On Tenons
Reply #32 - Nov 18th, 2013 at 9:37pm
 
Ken Vaughan wrote on Nov 18th, 2013 at 7:15pm:
Good design and good work practice is to provide for the most safe failure.

Safety is always #1. The safest way to prevent a bowl from coming out of a chuck would be to use a tailstock. Like Ed pointed out, Stuart's technique is more about saving the blank than it is about safety.

Its been my experience that tenons will shear at the top of the jaws.

There have been times when I have a 1" long tenon with a shoulder 1/8" to 1/4" from the end. If the the blank shears off at the shoulder there is still enough tenon left to cut a new shoulder. It wouldn't be hard to reverse the piece and cut a new shoulder.

Take a look at the timestamp 8:47, notice there is a shoulder. Now look at timestamp 9:01, Stuart points out that there is no shoulder and mentions the problems working without a shoulder, good advice there. Shoulders are very important on a tenon and its were the blank should sit on.

Take a look at timestamp 9:36, Stuart is cranking down the jaws to a point where he might be helping the tenon to fail by making the jaws to tight, add a a minor catch (or in Stuart's case a few raps) and the tenon breaks off.

John Teichert wrote on Nov 18th, 2013 at 8:36pm:
As I interpreted Batty, he is recommending (1) that the bowl have a flat surface for the face of the chuck jaws to provide stability and thus far less prone to stress the tenon, and (2) that the chamfered tenon provides a stronger attachment.  I'm not sure that it provides a "stronger" attachment, but perhaps one that is less susceptible to damage,

I think that is it in a nut shell.

Good conversation everyone.

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