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Tenon Size

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Gregg Adams

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Feb 12, 2011
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Is there a rule of thumb on tenon size in relation to the workpiece or is it best to make your tenon as large as the chuck will handle?

Thanks!
Gregg
 
If you look at your typical four jaw chuck, and note the shaping, you will see that if you expand the jaw all the way open, and put a tenon in it that size, the chuck will only grip on eight points, the tips of the four jaws.

So you want to size the tenon to fit the jaws, when they are just open a little. This affords the maximum amount of contact between the jaws and the tenon.

As for size of the tenon to object, I've heard a figure of 25%. Now that is not a hard fast rule, but it's certainly a working number

TTFN
Ralph
 
Thanks Ralph. That makes alot of sense now that I look at it. After opening the chuck, I can see how the jaws lose their roundness. I've just got my chuck that I ordered and I'm a bit scared of flying objects.
 
Gregg, also the tendon should not bottom out and touch the bottom of the chuck, there should be a layer of wood for the top of the jaws to rest on, this layer can be turned away when you reverse your turning
 
Chucks, whether in mortise or tenon mode need to nose up tight to the piece you want to hold. A regular woodworker will recognize the advantage of shouldering to resist racking.

As to sizing, if you choose to make a tenon, you will want to make it about a quarter inch over the first point of circularity on your jaws on green wood. The wood will shrink, and if you want to use the same jaws, you'll need some extra to re-turn to circular. If you choose to make a mortise, you may make it optimum diameter on green wood immediately, because you will be able to remove wood and make it optimum size again after drying. How large a set of jaws you choose depends on you. If you want a broad base, start there. If you want narrow, start there.

I like mortises, because they allow greater depth on the traditional half-log form than tenons fitted to the same jaws. I like dovetail jaws for either application because they don't "grip by corners" when in a non-optimum expansion, they wedge the piece firmly to the chuck for a long way over optimum size. Not license to be sloppy, but certainly not as big a deal as some would like to make of it.

You mention getting the angle right. It doesn't have to be perfect. It really only needs a gap a bit smaller than the wedge to be effective.
 
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Tenon

I turn large pieces using a tenon to hold the work. I have a Oneway Stronghold that gives me a maximum of just under 6" at full expansion. It is important to make a flat shoulder where the jaws contact the bowl. The work I turn is sometimes over 20" in diameter and 12" deep. I put considerable torque on the pieces when I use the McNoughton Center Saver and have never had a piece slip or come loose. One other thing. Stop and retighten the chuck every few minutes, especially with green wood.
 
Thanks guys. I'll keep all of this info in mind. I'm getting a clearer understanding of all of this now. I hope to put it to the test this week. Take care!
 
The other guys are waay more experienced than I, but I've found, at least on Nova jaws, the jaw size from the package is usually pretty close to the ideal tenon size.

For example, on the 50mm(2") jaws, I use a 2" tenon and the jaws are pretty close to round at that point.

Don't know if it holds true for other brands, though.
 
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Mike, I see what you're saying. I'm guessing that it wouldn't hurt for me to invest in a couple of different jaws sizes. Then I could keep the tenon making max contact. I doubt I'll be turning anything soon that will require larger than 2".

Thanks Mike!
 
Extra big for green wood.

Anyone turning green bowls for drying and returning should leave extra size so that the dried and warped tenon can be turned round and still fit jaws you have.

When I turn tenons on bowls up to 14" diameter I use #2 jaws which close to 2". On my green bowls I measure (usually by eye) 1 " from my live center which is 5/8" diameter. This gives me a tenon a bit over 2.5" diameter. Using this method, I have always gotten a 2" diameter tenon when I true the warped one when returning the dried bowl.

happy turning,
AL
 
Nova? http://www.teknatool.com/products/Chuck_Accessories/50mm.htm
General information. http://www.teknatool.com/products/Chuck_Accessories/Index.htm

Seems there was a consolidated table for all somewhere, but this is a place to start. Another post reminds me of a further reason I use the mortise. If I have it properly sized, as I am able to do, even green, I have minimum force on any surface around the mortise. Doesn't dig in, so I'm not obliged to "Stop and retighten the chuck every few minutes, especially with green wood." as someone who was slowly crushing wood might.
 
My first chuck in the 80's was the Precsion Chuck it only expanded.

It held fine but limited what I could do with the bowl design.
so I used face plates and glue blocks when I wanted to turn a thin bowl, a bowl with a small (sit on a quarter) foot, a round bottom bowl, or even a 4" diameter bowl.

Once I got a chuck that gripped a tenon I could use it for everything except the pieces that didn't have enough waste for the tenon. Still use a glue block now and then.

I think it is soooo much easier to true a warped tenon than to true a warped recess. I think it also works a lot better if I'm doing reverse turning to finish the bottom.

I've been using MDF chucks mounter on 4 jaw chuck a lot. The recess hold on MDF is a must as the MDF tenons just break with normal turning pressure.

happy turning,
Al
 
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hockenbery said:
I think it is soooo much easier to true a warped tenon than to true a warped recess. I think it also works a lot better if I'm doing reverse turning to finish the bottom.
Click to expand...

Just in the way you turn. I use that pin chuck or pin jaws, so I can cantilever the piece out there and remove the little bit of wood it takes to get it back to round. I can even trim out and sand the bottom prior to reversing to re-hollow. Safe and efficient.

http://s35.photobucket.com/albums/d160/GoodOnesGone/?action=view&current=BottomHoldProcess.mp4
 
After watching Michaels' video, I'm interested in doing an inside tenon. I really don't have a way of cutting off the tenon safely. My only concern is that an inside tenon seems like a good way to make a workpiece break or split apart with too much pressure.
 
I'm not 100 percent on the source, though I believe it was one of Raffan's books for us beginners, but I read a suggestion of a minimum of 3/4" of material outside the recess for a bowl with a foot to limit the danger of splitting. A footless bowl like in Michael's video is different, of course, the recess being reinforced by a significant portion of the bowl.

I think the splitting danger is more from the turner than the physics and geometry of the hold. I am notorious for making things "farmer tight" but if the recess is sized properly there's no reason you need that much pressure, especially if you turn gently.

Interestingly, Teknatool's instructions for the Super Nova2 suggest a maximum bowl depth of 4" (100mm) for using the chuck in a recess. Of course, they also say not to exceed 600rpm in the same sentence that they call it a "strong holding method"...
 
As mentioned above, the mortise when properly configured wedges the nose of the jaws up against the bottom of the piece. That means that how much wood is around it is not truly important. The wedge would lift chips if properly configured and overtightened. Splits which might fool people into believing it's how much wood that's around that counts come when people make the angle too large and reef on the chuck. Not a wedge along the axis then, but across.

Of course I cheat openly, turning between centers until the bowl's nearly done. Safer for a hacker like me when I plunge and roll.

25mm jaws held this. http://i35.photobucket.com/albums/d160/GoodOnesGone/Beech-Wing.jpg where there were a lot of entries and exits.

And this, which has a very thick bottom to compensate for a narrow footprint. http://i35.photobucket.com/albums/d160/GoodOnesGone/Back-to-Unstable-Again.jpg
 
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