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Thread: Tenon Size: Is jaw contact area really all that matters?

  1. #1
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    Question Tenon Size: Is jaw contact area really all that matters?

    I've heard turners claim on a number of occasions that all that really matters when it comes to tenon size is the amount of contact that the jaws are able to make with the wood. That is, that diameter doesn't matter. Is this really true?

    Is there no benefit to a larger diameter tenon once the jaws are contacting the wood as much as they ever will?

    Under this assumption, if I'm going to use my Oneway Stronghold Number 2 jaws to hold a tenon for my bowl, I might as well always use a tenon that is just barely above their minimum external gripping capacity (2"). A tenon that is 2.2" in diameter would be just as effective and have just as much holding power as one that is 3.3" in diameter--as long as either is being held with the number 2 jaws, the maximum contact between the jaws and the tenon is already reached at the minimum diameter. Likewise, if I'm going to instead use my number 3 jaws, there's no reason to allow the tenon to be more than around 3 7/8", even though the maximum external gripping capacity is 4 1/2".

    I've always calculated my tenon size as 30% the diameter of the bowl, and assumed that there was some sort of inertia benefit to having a larger-diameter tenon. What do you think?

    Is it true that for a particular size of chuck jaws, a tenon that is just above the minimum external capacity is just as good as one that is close to the max?

  2. #2
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    Quote Originally Posted by Chris Gunsolley View Post
    I've heard turners claim on a number of occasions that all that really matters when it comes to tenon size is the amount of contact that the jaws are able to make with the wood. That is, that diameter doesn't matter. Is this really true?

    Is there no benefit to a larger diameter tenon once the jaws are contacting the wood as much as they ever will?

    Under this assumption, if I'm going to use my Oneway Stronghold Number 2 jaws to hold a tenon for my bowl, I might as well always use a tenon that is just barely above their minimum external gripping capacity (2"). A tenon that is 2.2" in diameter would be just as effective and have just as much holding power as one that is 3.3" in diameter--as long as either is being held with the number 2 jaws, the maximum contact between the jaws and the tenon is already reached at the minimum diameter. Likewise, if I'm going to instead use my number 3 jaws, there's no reason to allow the tenon to be more than around 3 7/8", even though the maximum external gripping capacity is 4 1/2".

    I've always calculated my tenon size as 30% the diameter of the bowl, and assumed that there was some sort of inertia benefit to having a larger-diameter tenon. What do you think?

    Is it true that for a particular size of chuck jaws, a tenon that is just above the minimum external capacity is just as good as one that is close to the max?
    Not true, with the profiled jaws holding, the leverage needed to break the hold/wood at a larger diameter is greater than at a smaller diameter.

    A larger tenon or recess is stronger for that reason.
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  3. #3
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    Lots of opinions on tenon size vs strength. Personally I make mine larger, mainly because I turn wet wood and some woods move a lot. Having a larger tenon allows truing that tenon up when re-turning the bowl after drying........ I only have dovetailed jaws, so the general consensus on DT jaws is smaller is more holding power. I think.

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    Looks like we're talking apples and oranges. A larger tenon should be harder to shire off the bottom of a bowl than a smaller one. Thus larger the bowl the larger diameter the tenon. Jaw gripping power generally is greater close to it's closed diameter. This leads to larger jaws and tenons as the size and wieght increases.
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  5. #5
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    Quote Originally Posted by Allan Ferguson View Post
    Looks like we're talking apples and oranges. A larger tenon should be harder to shire off the bottom of a bowl than a smaller one. Thus larger the bowl the larger diameter the tenon. Jaw gripping power generally is greater close to it's closed diameter. This leads to larger jaws and tenons as the size and wieght increases.
    Yes, there are two different factors being discussed here (and I've really appreciated the input thus far on both). One is how easily the tenon will break. The other is the gripping power. What if I am confident that my tenon won't break? Speaking strictly on gripping power, does the diameter matter above the minimum external capacity?

    Sometimes, for example, I'll use a hefty tenon (.5"-.75" deep) with tower jaws so that I can be more aggressive while hollowing than I would be with a shallower tenon. I'm not concerned about the tenon breaking here.
    Last edited by Chris Gunsolley; 09-26-2017 at 2:02 PM.

  6. #6
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    Are you asking for opinions? I've heard opinions both way, some based on feelings, some on something someone told them, maybe some on experience where they lost a piece.

    The manufacturer of the chucks I use clearly recommends making the tenon and recess diameters very close to the same as the jaws when almost fully closed since that is when they form a circle. I follow this advice since I believe they recommend this for a reason and they are probably smarter then me. I teach this for the same reason.

    So far, I've not seen any objective experiments so show one way is better than the other. Cut some different sized tenons in long pieces of crossgrain wood. Use wood as identical as possible, at least from the same tree. Insert each one at a time into the same jaws on the same chuck. Apply measured side force to a measured point from the jaws. Record the force needed to pry the wood from the jaws. Repeat several times for an average. Do this with both green wood and dry, hard and soft. Of course this is a lot of work so it's easier to just state an opinion! Maybe someone knows of such experiments and can provide a reference.

    I think the issue is more the wood deforming at the jaw than the tenon breaking off the blank. Most that I've seen come out of lathes left the tenon attached. In the case of the wood deforming, it seems to me that wood held by the corners of the jaws might not be as secure as wood held by the circumference of the inside of the jaws.

    Another issue is the softness of the wood. Very soft wood like soft maple or buckeye might deform and pull out of the chuck easier than oak or black locust. Of course, the "corners" of each jaw segment are more likely to press deeply into soft wood so the center part of the segment may grip better than with hard wood.

    Another issue would be the dryness of the wood. Just like it is easy to cut wet wood and harder to cut dry wood, wet wood may be easier to pry out of jaws. Here again, wet wood may deform from jaws spread wide and hold tighter than dry wood.

    It may be even more important to make a recess diameter close to the minimum jaw diameter since small jaws opened wide contact the wood only on the rounded centers of each jaw segment.

    A larger tenon, of course, does have a stronger attachment to the bottom of the bowl due to the surface area. The length of the tenon also makes a difference - all else being the same a longer tenon will be gripped by more steel than a shorter tenon, as long as it is properly shaped.

    This is my opinion.

    JKJ

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    Excellent response from John.
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    Quote Originally Posted by John K Jordan View Post
    The manufacturer of the chucks I use clearly recommends making the tenon and recess diameters very close to the same as the jaws when almost fully closed since that is when they form a circle.
    Strong point. The closer they are to the minimum external capacity, the more contact the jaws will have on the wood. This does suggest that the smaller the tenon in diameter, the better its gripping power will be.

    It's looking to me that a larger-diameter tenon is stronger, but a smaller-diameter tenon has more holding power...
    Last edited by Chris Gunsolley; 09-26-2017 at 2:21 PM.

  9. #9
    I prefer to make my tenons as large as possible. The reason is that I perceive that it reduces the vibration - especially on finish turning near the rim. With a small tenon, I tend to get more chatter.

    Now, I'm comparing tenons made with two different sets of jaws (~2.25" and ~4"). I have not noticed any appreciable difference in vibration or holding power of tenons within the range of either set of jaws. The couple few times (perhaps 2 / 250) times I've had a tenon break or slip, it's been because my tenon was too thick or the taper was not proper. But then again, I'm using a midi lathe and my lathe may halt before it busts a tenon as a larger lathe may.

  10. #10
    This is the type of thread that will gain opinions a diverse as there are woodturners!! But, in an effort to add to the mix I will offer a few thoughts.

    First, I agree with your bold font statement - a tenon should be close to the diameter of the jaws when closed.

    Secondly, I think folks are prone to over tighten the chuck on tenons. While it certainly needs to be snug, and re-checked periodically, it doesn't need to be cranked down with all the force one can muster. There is a significant amount of mechanical advantage happening with a scroll jaw chuck. That may be as much to blame for severed tenons as anything - save for severe catches. Those can break about any tenon.

    Finally, there are actually two "holding forces" at play with a chuck tenon - the clamping pressure AND the opposing force offered by the outside edge of the jaw resting against the bottom of the vessel. For this reason, I feel it is important to assure that the surface of the "shoulder" for the chuck jaws be either parallel to the top surface of the jaws or slightly acute to the jaw surface thereby assuring the outside top edge of the jaws contact the wood. That will help prevent the piece from shifting in the chuck or launching if/when a catch occurs.

    As JKJ stated in another thread, with experience catches will usually be limited to those times of inattention. I find that is usually when you allow the tool to engage the wood before the tool is fully rested on the toolrest. These will happen to everyone - eventually. Shop tip - if you don't have your eyes on the tool, don't get it close to the wood! Seems there is a magnetic force that draws the gouge in against the wood.

    Without catches, a 2" tenon about 3/16" deep, with a proper angle such that the outer jaw edges contact the wood, is a very secure arrangement. I turn nearly 100% of what I do with that setup, including 20"+ platters and bowls that are 12+". Most of my bowls are 3" deep, or so, but if I was doing a really deep bowl without a steady, then I would switch to larger jaws with a tenon appropriate to the jaw size.

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  11. #11
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    I think it is difficult to just compare strength vs holding power. Such as 2" jaws almost closed and 2" jaws almost fully open. I'm lucky to have a good selection of jaws so I can make mine as close to closed as possible ... I just need to move up to the 3" jaws instead of the 2". A 2" tenon would have 3.14 square inch of area of wood to shear off; a 3" would not be 50% more but 225% more (7sq inch)
    I can I rough turn with maybe 1/8 over the min with 3" and when remounted trim to 1/8" over 2".

    As far as shearing off I am not sure the length matters. If the jaws hold I assume the wood will shear off with a 1/4" length tenon or a 1-1/4" tenon.
    As to dislodging I do think a longer tenon is beneficial. My daughter was turning one a few months back with about a 3/8" tenon. It came out except for about 1/16 on one side. If it had been a 1/4" tenon it probably would have orbited. This was a close fitting tenon so it had to press against the full jaw. If it had been oversized where only the corners gripped the tenon then again it probably would have been orbited.
    My procedure is as large diameter a tenon as possible with the maximum jaw contact. In some cases the tenon is larger than I want but if it is a rough turn so what? I trim it down to the next size when remounted dry. I may mount with a 4" tenon and remount with a 3".
    My analogy would be a baseball catcher. Would you want to be behind the batter with his hand wrapping the bat or the batter who holds the bat with his finger tips?
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  12. #12
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    Quote Originally Posted by Chris Gunsolley View Post
    Yes, there are two different factors being discussed here
    ...
    There's another factor I haven't seen mentioned yet and I forgot to mention it myself - the grain orientation. If turning bowls and things in a side grain orientation, where the grain runs across the face (perpendicular to the axis of the lathe) the tenon strength can be less than with turning in the end grain orientation. In the end grain orientation (with the grain running parallel to the lathe ways) the tenon is extremely strong and will not shear or break off without setting off a stick of dynamite. I don't see many bowls turned in the end grain orientation, probably since you can get a bigger bowl from the same log with face turning and the figure is better in most wood. End grain is often preferred, of course, for vessels, vases, hollow forms, boxes, and such, especially things you want to stay round, maybe with a lid. (I probably cut 95% of my hundreds of blanks for end grain turning.)

    For example, recommended for the Nova Titan Powergrip jaws:
    Endgrain wood: 350mm diameter x 350mmin length (14” x 14”)
    Crossgrain wood: 200mm diameter x 200mm length (8” x 8”)

    JKJ

  13. #13
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    Quote Originally Posted by Chris Gunsolley View Post
    ...Sometimes, for example, I'll use a hefty tenon (.5"-.75" deep) with tower jaws so that I can be more aggressive while hollowing than I would be with a shallower tenon. I'm not concerned about the tenon breaking here.
    The other factor on tenon holding strength is the shape of the blank that contacts the face of the jaws. A well-formed (cut surface that mates cleanly with the face of the jaws - with a clean ineer corner) adds to the resistance to side-loading - as when you're 'aggressively hollowing'. In this case the forces are acting with leverage to essentially pry the tenon out of the jaws. I've always been told that a tenon that's correctly sized for maximum contact with the jaws, and squarely faced to the front surface of the jaws will maximize the holding power of the jaws no matter what size they are.

  14. #14
    Quote Originally Posted by Chris Gunsolley View Post
    Yes, there are two different factors being discussed here (and I've really appreciated the input thus far on both). One is how easily the tenon will break. The other is the gripping power. What if I am confident that my tenon won't break? Speaking strictly on gripping power, does the diameter matter above the minimum external capacity?
    You nailed it there.
    * gripping power is greatest when the tenon is small enough for full contact with the jaws. With hard strong wood, this is the most important factor.
    * strength of the tenon is greater when the tenon is larger. With softer woods, this can be more important than strength of the grip.

    An additional factor when rough turning a piece that will be twice turned, it is wise to make the tenon large enough that it can be trued up after drying and still fit the chuck.
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  15. #15
    Geometrically, the most contact would occur when the curve of the tennon is as close as possible to the curve of the jaws., Whether inside or out. When the jaws are expanded for a large tennon, there is less likely to be as much contact. Granted wood has some compression, but unless it is a softer wood, the curve of the jaws can't be compressed all the way into the larger tenon. Now in a way the inside jaws' corners will make up for some loss of contact on a large tenon because those corners dig in like 8 points. But the outside of the jaws would just have the curved arc expanding into the wood. On wet/green wood that seems like runners of a sled on slush. Not much friction there.

    But I am just a newbie at turning, what do I know.

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