Tennon for Bowl turning - Woodworking Talk - Woodworkers Forum
 
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post #1 of 18 Old 12-07-2011, 04:40 PM Thread Starter
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Have seen a couple of ways to attach a bowl blank to the chuck for hollowing the inside. One is a simple tenon. The other is a recess is cut in the bottom of the blank and the chuck jaws expand in this recess. Is there an advantage to one over the other? It look like you could incorporate the recess in the bowl design verses cutting off the tenon. Also, I'm not sure how to support tail stock end while cutting the recess. As you can tell I'm new to bowl turning.
Thanks, Tom

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Last edited by TomC; 12-07-2011 at 04:44 PM.
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post #2 of 18 Old 12-07-2011, 04:50 PM
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To cut a recess to mount the bowl, if the tailstock is in place you turn away most of the recess and then just carve of the little remaining nub. This is assuming you are turning the outside of the bowl between centers. If you are face plate mounting or screw chuck mounting the blank, you can just take the tail stock out of the way when you get to that point. As far as an advantage, it's more a of a preference thing. How do you like/want the bowl to look?

That bowl was perfect right up until that last cut...
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post #3 of 18 Old 12-07-2011, 04:52 PM
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Tom,
the tenon is definitely the stronger method of the two. With a recess, you run the risk of overtightening and cracking the bowl. Not tight enough and too aggressive of a cut and the bowl can fly off. You can mount your blank between centers and cut either a tenon or a recess. You'll have to leave just a small nub of wood where the tailcenter is and then trim it off after removing the blank from the lathe. A small carving chisel works well for that.
Mike Hawkins
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post #4 of 18 Old 12-07-2011, 05:22 PM
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I agree with sawdustfactory except that I always leave the little nipple in the center whether I form the recess between centers or with the tailstock removed as when a faceplate or screw chuck is used. Forming a tenon without the tailstock is much easier but you still need to leave the dimple/nipple to re-center the work. For between centers I made my dovetail tool at 45* and pull it back to me. It can get really close if you have a 1.5 live center to work around cutting a dovetail into a 2 recess.

Whether tenon or recess you will gravitate towards one or the other as you turn more and decide on a style.

With a tenon you can get to more of the bottom, IF you need to get to more of it for your design. I make about 80% of mine with a recess and generally stay about the 1/3 base to diameter ratio.
Mike may be correct that a tenon is stronger. However, unless it has changed; area = pi (radius squared). A 2 diameter tenon has 3.14 square inches of wood holding it. A 2 diameter recess with one inch of wood surrounding the recess (and minus the 3.14 in the interior) has three times the area of wood holding it on (9.42).

For final turning I often decrease the amount of wood surrounding the recess. Even one half inch around the recess has more area than the tenon. This is for the standard 50mm jaws, as you move up in size the difference decreases.

They who can give up essential liberty to obtain a little temporary safety, deserve neither liberty nor safety. Benjamin Franklin
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post #5 of 18 Old 12-07-2011, 05:43 PM
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agree with all the above, but always beware of your wall thickness and depth of recess jaws so you don't hollow in to your chuck jaws.

Jeff,

"Just because your not bleeding, don't mean your turning safely"..
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post #6 of 18 Old 12-07-2011, 05:53 PM
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Whether to turn a tenon or recess in bottom of bowl depends upon your blank. Both methods have advantages and disadvantages. Chucks designed to hold by contraction or expansion of the chuck jaws.

Turn mostly green bowls so easy to drill a pilot hole, screw blank onto wood screw in chuck then drill recess in base with forstner bit installed in drill chuck in tailstock or use gouge and parting tool to cut a recess.

Cannot emphasize enough drill under sized hole when using chuck screw center. On dry/wet bank use one or two-drill bit sizes smaller than diameter of screw center for pilot hole. Normally takes moderate force to screw blank on, and seat. Check before turning lathe on blank is tight, If blank spins by hand don’t use this method.

If want a tenon normally start rough turning between centers turning out side leaving a tenon then reverse turn inside mounted in chuck.

You never want to over tighten chuck jaws whether using a recess or tenon.
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post #7 of 18 Old 12-07-2011, 06:23 PM Thread Starter
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Thanks all for the info. I have been using a tenon and for now will continue to do so. I want to try the recess sometime in the future; after I gain more experience.
Tom
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post #8 of 18 Old 12-09-2011, 09:36 AM
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I usually use the recess, only because it's easier and a little quicker to do. Before I chuck a blank, I put it on my drill press and use a 2 1/8" forstner bit to drill the recess. It saves me a step on the lathe.
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post #9 of 18 Old 12-09-2011, 04:55 PM Thread Starter
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Quote:
Originally Posted by b00kemdano
I usually use the recess, only because it's easier and a little quicker to do. Before I chuck a blank, I put it on my drill press and use a 2 1/8" forstner bit to drill the recess. It saves me a step on the lathe.
Are you saying you don't do any turning between centers; just drill the recess and mount on the chuck?
Tom
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post #10 of 18 Old 12-09-2011, 06:04 PM
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I don't use centers for bowls or hollow forms, only spindle type projects.
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post #11 of 18 Old 12-09-2011, 06:55 PM
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[QUOTE=NCPaladin;269592]

Mike may be correct that a tenon is stronger. However, unless it has changed; area = pi (radius squared). A 2 diameter tenon has 3.14 square inches of wood holding it. A 2 diameter recess with one inch of wood surrounding the recess (and minus the 3.14 in the interior) has three times the area of wood holding it on (9.42).

NC,
I wasn't really referring to area. I was referring to compression vs. expansion. And only in the respect that if you try to expand a chuck with the same force you compress a tenon, you stand a pretty good chance of breaking the bowl. Don't ask me how I know.....
Mike Hawkins
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post #12 of 18 Old 12-10-2011, 10:51 AM
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Still think bowl blank should determine whether to use a tenon or recess.

I can mount a blank between centers, take a parting tool and cut a recess in bottom of blank that allows me to grip a tenon or expand into the recess. Which method would allows jaws stronger hold? Whether contract or expand jaws of a chuck holding force is the same. My serrated jaws might grip tad better, but does not change mechanical advantage of chuck ability to hold wood.

Depending upon species, spalted wood, how dry/wet wood is all can affect holding power. Over tightening, chuck jaws just as bad as not tightening enough whether using a tenon or recess.

When in doubt read manual that came with your chuck! Some go into details on how to make a tenon or recess many do not.
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post #13 of 18 Old 12-10-2011, 01:27 PM
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I'm not an engineer, so will not get into the "which is better" arguement...

I turn 80%-20%, recess-tenon, 'cause that's what I prefer at this point in my turning career...

On some really soft woods, the tenon seems to hold the blank better, FOR ME...

I haven't the skill, currently, to do alot of cleaning up the base after turning the inside...

I pretty-up the recess, to make it look like a part of the design, which it actually is...

The folks who buy my stuff may see the bottom detail as a kind of signature design, which I probably stole from someone much better at this stuff than me!

I DO plan on getting the skills to turn off the recess one day, but is not a big concern currently...

Do what you're comfortable with!

Good luck and post pics!

p






...ever notice how "I'm sorry" and "I apologize" mean the same thing, unless you are at a funeral..?

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post #14 of 18 Old 12-10-2011, 06:15 PM Thread Starter
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Thanks txpaulie,
I was wondering if anyone incorporated the recess in the bowl design. It look like that would be the way to go on a blank with limited depth. I also Iunderstand what other have said about the possibility of damaging the blank by applying to much force in the recess verses compressing the tenon.
Tom
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post #15 of 18 Old 12-10-2011, 07:51 PM
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Quote:
Originally Posted by TomC View Post
Thanks txpaulie,
It look like that would be the way to go on a blank with limited depth.
I don't think so.

By definition, if you can turn a recess, you can turn a tenon -- all it requires is that you don't extend the recess all the way to the center of the blank.

Making a tenon doesn't require you to turn away the whole base of the blank.
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post #16 of 18 Old 12-11-2011, 01:09 AM
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Quote:
Making a tenon doesn't require you to turn away the whole base of the blank.
Exactly that!

There is really no functional difference at all!

p

...ever notice how "I'm sorry" and "I apologize" mean the same thing, unless you are at a funeral..?

Last edited by txpaulie; 12-11-2011 at 01:09 AM. Reason: Bacon, the other white meat.
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post #17 of 18 Old 12-11-2011, 01:19 AM
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Okay, the previous post was not entirely accurate...
the difference is that with a recess, you must leave enough wood for the chuck to hold...

Red=tenon v. yellow=recess...

p

...ever notice how "I'm sorry" and "I apologize" mean the same thing, unless you are at a funeral..?
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post #18 of 18 Old 12-11-2011, 09:47 AM
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TomC, how we turn bottom of bowl important as form, rim and foot if there is one. Only objective when reverse turn bottom/ foot of our bowls is to show you cared enough about what you turned to give every area the same attention.

So yes, can cut a recess and add a foot if you want one before we reverse turn a bowl to hollow out inside.
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