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Stuart Batty On Tenons (Read 2,920 times)
 
robo_hippy
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Re: Stuart Batty On Tenons
Reply #15 - Nov 15th, 2013 at 12:18pm
 
Well, I may have not interpreted it correctly. He never mentions the word dove tail, and that is what he makes on the tenon to mount it into the Vicmark chucks. When I took the workshop with Stuart and his dad, 5 or so years ago, Stuart had me making tenons for some bowls we were turning. They were maybe 5 inches tall, and about the same diameter, so more of a closed form compared to what I normally turn. I made a tenon maybe 1/2 inch deep, and he had me round off the corners on the tenon for no more than 1/4 inch being in actual contact with the chuck jaws. He said it made the grip weaker. Not sure if I got an explanation or not. If the chuck jaw/tenon angles match perfectly, it should not be a problem. For sure if they are not a perfect match, the piece could rock as you are turning, and that would make it want to shear off. For my recesses that I use, I am seldom more than 1/8 inch deep, even for 16 inch diameter bowls. Of course, doing a lot of bowls and experimenting a lot helps me figure out what will or will not work.

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Re: Stuart Batty On Tenons
Reply #16 - Nov 16th, 2013 at 3:52pm
 
I also think Len got it.  I had a class with him years ago and checked my notes. I had a diagram of a foot with a chamfer and my notes said it provides additional mass. In other words technically the tenon only needed to be as long as the beginning of the chamfer. But the chamfer allows you to have a little more wood on the tenon - reducing the chance of the tenon splitting based on the grain direction or punky wood.  Clearly he does not put a chamfer on the spindles since he shows there is no chance of the them splitting if there is a bad catch.

I was not putting a chamfer on my bowl blanks. I now have a better understanding because of this thread, and will start adding a chamfer - just in case.
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Re: Stuart Batty On Tenons
Reply #17 - Nov 16th, 2013 at 5:10pm
 
Quote:
I had a diagram of a foot with a chamfer and my notes said it provides additional mass. In other words technically the tenon only needed to be as long as the beginning of the chamfer. But the chamfer allows you to have a little more wood on the tenon - reducing the chance of the tenon splitting based on the grain direction or punky wood.

For those of you wanting to try this method, you need to allow yourself the extra room for cutting a chamfer.
Example, if you usually cut a 1/8" tenon and now you want one with a chamfered edge, you need to cut a 1/4" tenon so that you will have enough area for your normal 1/8" tenon surface for the chuck to hold onto and an extra 1/8" area for the chamfer.
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Re: Stuart Batty On Tenons
Reply #18 - Nov 16th, 2013 at 8:00pm
 
There is nothing added in the grip line that would improve the strength of the tenon.  Having more wood 1/4" away from the stress point will not improve the strength of the stress point.
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Re: Stuart Batty On Tenons
Reply #19 - Nov 17th, 2013 at 11:03am
 
Quote:
There is nothing added in the grip line that would improve the strength of the tenon.  Having more wood 1/4" away from the stress point will not improve the strength of the stress point.

Jim, while I agree with you, some think adding the "additional mass" that Mike mentioned is some kind of benefit. I was just try to help
  As I have mentioned twice already, I think he is simply trying to salvage the tenon from splitting off.
He mentions and shows this at roughly 8:00 - 8:15 into the video

I know I'll most likely get grief for this but;
If this whole issue is about making proper tenins for safety reasons, there are two things that SB does that I would not.
#1. The tenons he cuts are too large for the jaws he is using.
You should not be able to see 3/8"-1/2" distance between Jaws.
#2, Over-tightening the Jaws. Anyone familiar with Vicmarc Jaws knows that they have a pretty sharp edge on them. The way he bears down on the chuck key when he's tightening, it's a wonder he doesn't shear off the tenon right there
Make the tenon fit the jaws and there is no need to tighten that much.
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Re: Stuart Batty On Tenons
Reply #20 - Nov 17th, 2013 at 12:33pm
 
Does that mean the if you are gluing a 1/2" board to something, you will have a stronger joint than if you are gluing a 1/4" board?
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Re: Stuart Batty On Tenons
Reply #21 - Nov 17th, 2013 at 1:22pm
 
Quote:
Does that mean the if you are gluing a 1/2" board to something, you will have a stronger joint than if you are gluing a 1/4" board?

Grin
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Re: Stuart Batty On Tenons
Reply #22 - Nov 17th, 2013 at 2:00pm
 
I will agree that Stuart puts way more pressure on the tenon than I ever would. The plastic on the hex wrench will break off, and I have several that have done it. Putting too much pressure can put the wood under so much stress that it can split if you have any tiny catch. Just the 'straw that broke the camel's back' thing. It may add to 'more metal in contact with the wood', but still, that is too much for me.

As for the chamfer adding mass to the tenon, well, it may do that, but nothing is added to the contact points. I think that is like people turning out the inside of the bowl and leaving the center high because it adds mass. Well, it does, but not at the point where it will add anything.

Stuart does make some good points in his turning methods, but I don't agree with all of them.

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Re: Stuart Batty On Tenons
Reply #23 - Nov 17th, 2013 at 2:13pm
 
Quote:
Stuart does make some good points in his turning methods, but I don't agree with all of them.

Yes he does, I should make clear that I am not trying to badmouth SB but point out some things that might help others have a better understanding.
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Re: Stuart Batty On Tenons
Reply #24 - Nov 17th, 2013 at 3:32pm
 
Ed Weber wrote on Nov 17th, 2013 at 2:13pm:
Quote:
Stuart does make some good points in his turning methods, but I don't agree with all of them.

Yes he does, I should make clear that I am not trying to badmouth SB but point out some things that might help others have a better understanding.


Absolutely.
Discussing techniques like this helps us all get a better understanding of our favorite pastime 
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Re: Stuart Batty On Tenons
Reply #25 - Nov 18th, 2013 at 11:39am
 
Ron Sardo wrote on Nov 11th, 2013 at 8:55am:
Len Layman wrote on Nov 10th, 2013 at 9:14pm:
I just learned something. I had never heard of putting a chamfer on my tenons.


Same here. I'm still trying to figure out why it works since the break would occur above the chamfer.


The chamfer changes the lever arm (shortens it) and the force is directed differently.  The effect is to avoid starting the splitting along the end grain.   The bowl will tear out of the jaws before it the grain splits.   

If you have bad wood, cracks, etc, all the chamfering in the world cannot avoid breaking/splitting.
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Re: Stuart Batty On Tenons
Reply #26 - Nov 18th, 2013 at 12:52pm
 
Hmm, I think most of my splits are on the side grain rather than on end grain. I do tend to orient my chuck jaws so they are not directly on the end grain, so not at 12/3/6/9 o'clock, but at 1:30/4:30/7:30/10:30. Just seems to hold a little bit better.

I still don't understand the use of a chamfer. I just make a shorter tenon. Same difference.

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Re: Stuart Batty On Tenons
Reply #27 - Nov 18th, 2013 at 1:01pm
 
It's very clear why SB uses this method, but the interesting thing to me is why it works.
It ONLY works if you have a tenon that is too large for the jaws.
You can see in the video, (8:20) after he forces the blank out of the chuck, there are really only 8 points of contact. The center of each jaw did little, if any damage to the tenon at all. If the tenon was cut closer to the same diameter as the jaws this would not work and the chamfer would not help. The closer the tenon and jaws match, the better the mechanical connection is and the less pressure is need to apply for a secure grip.
It does seem that he uses this method of a larger tenon intentionally for the purpose of re-chucking should a failure occur. I suppose some would call this planning ahead, I'm not sure.
I would prefer to have a more secure grip initially, than what I would consider a less secure grip with the chance of re-chucking.
JMHO
No matter what method you use, please turn safe.
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Re: Stuart Batty On Tenons
Reply #28 - Nov 18th, 2013 at 1:47pm
 
Ken Vaughan wrote on Nov 18th, 2013 at 11:39am:
Ron Sardo wrote on Nov 11th, 2013 at 8:55am:
Len Layman wrote on Nov 10th, 2013 at 9:14pm:
I just learned something. I had never heard of putting a chamfer on my tenons.


Same here. I'm still trying to figure out why it works since the break would occur above the chamfer.


The chamfer changes the lever arm (shortens it) and the force is directed differently.  The effect is to avoid starting the splitting along the end grain.   The bowl will tear out of the jaws before it the grain splits.   

If you have bad wood, cracks, etc, all the chamfering in the world cannot avoid breaking/splitting.


I've seen bowls break off at the tenon where the tenon is still firmly mounted in the jaws. (maybe the jaws were clamped down to tight and the bowl snapped off when a catch occurred.)

As we all know, the break happens along the grain. There is a different force on the bowl when the lathe is spinning compared to when the lathe is still. When the lathe is spinning it is almost like a twisting/rocking motion instead of the force being applied to one single spot.  Did you ever look the mark a gouge left after a catch? Its not centralized, instead its a few inches long covering 20%-60% of the circumference. The closer to the rim the more force is applied during a catch (think levers) and in a wider area as compared to a palm of a hand banging on a mounted bowl.

I think a better demonstration would be for Stuart to bang on the bowl then spin it an inch then bang on the bowl again (repeat if necessary).

I'm not convinced but I will still think about this some more.
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Re: Stuart Batty On Tenons
Reply #29 - Nov 18th, 2013 at 7:15pm
 

Let me try this another way ---

Good design and good work practice is to provide for the most safe failure.  Think of break away light poles along high speed highways.

The tenon design is to fail safe.  If the tenon is short, the champfer may not be needed to assure the failure will not likely be tenon separation.

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.   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
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