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Tenon size

Discussion in 'Woodturning Discussion Forum' started by Roger Wiegand, Jul 17, 2020.

  1. Roger Wiegand

    Roger Wiegand

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    Last night I attended another online remote demo, one of many I've been able to do in the last couple of months. The presenter made some fairly dogmatic assertions about tenon size, echoing comments made in other, earlier presentations I've heard. The assertion, in this case presented as an ironbound rule, was that a tenon must be 40% of the diameter of the workpiece. Interestingly, in all three case where I've heard this put forward recently, the justification for the rule was "as Dale Nish told us". With all respect to Mr. Nish, what are the data?

    From a practical point of view most folks don't own a chuck large enough to make that happen for bowls bigger than 8-10", and in most cases that gives you a base way too big for the piece, something that you then need to deal with while reverse turning, generally a more fraught situation with regard to holding on to the workpiece.

    In practice it seems to be a rule that few seem to actually follow. Since I didn't know the rule I certainly haven't followed it, turning bowls up to 20" on a 3-4" tenon. I've made maybe 100 large bowls in the last two years and I did have a broken tenon on one; that was a flawed piece of wood that I should never have put on the lathe. I also had one spectacular failure with a piece securely screwed to a large faceplate that came apart into three pieces due to a hidden flaw in the wood. Watching many demonstrations recently none of the demonstrators working on larger pieces made tenons that large.

    My limited experience suggests that a tenon in the ~20-30% size range seems to work fine. Am I tempting fate in some foolhardy way? What data led to the 40% value?
     
  2. hockenbery

    hockenbery AAW Advisor Staff Member

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    I probably use too small a tenon and too small a faceplate.
    Turning technique and the quality of the tenon or quality of the faceplate mount let these smallish sizes work for me.

    I turn most of my face grain hollow forms using a 3” faceplate and #12 screws.
    The larger of these are 15” diameter.

    Bowls up to 16” diameter I do with #2 jaws and a 2.5” tenon.

    -:) as too foolhardiness - you need to judge :)
     
  3. odie

    odie

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    Al is correct......There can't be a rule like this that applies to everyone.....a critical evaluation is required. The 40% rule for tenon size is much more applicable to turners with limited experience, than those who are very experienced. The difference here, is the likelihood of a catch.

    The smaller tenon is much less resistant to losing it's grip, and having a dangerous outcome. I'd say most of my bowls are in the 40% range, but under many individual cases, the smaller tenon does have a certain artistic appeal. I do take advantage of that "artistic appeal" on some bowls.

    -----odie-----
     
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  4. Brandon Sloan

    Brandon Sloan

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    I have heard of a formula by Dale Nish that suggests a safe lathe speed is determined by the product of the diameter of the bowl times the RPM falling between 6000 and 9000. That is a 10 inch bowl should not be turned faster than 900 RPM (10 X 900 = 9000) for safety and 600 RPM for cutting efficiency (10 X 600 = 6000)

    @hockenbery you break all the rules.
     
  5. Randy Anderson

    Randy Anderson

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    I pay more attention to the quality of the wood than the tenon than the size. I keep turning down until I feel I have a good solid piece to grab onto. My failures, which are very very few, are due to a wood defect I missed or ignored. I try to make sure no cracks, pith area or weak looking spots then turn to a size that fits my chuck well. I turn 16" bowls with a 3 1/4" tenon all the time. I don't use faceplates at all. I know that doesn't mean it's the best thing to do but with my VERY limited experience relative to others here it seems a solid tenon matters more than whether it's 3" or 5". I'm sure there's a lower limit. I wouldn't attempt a 16" bowl on a 2" tenon but yikes, a 16" bowl would require a 6 1/2" tenon per the 40% rule . No way I'm doing that.
     
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  6. Donovan Bailey

    Donovan Bailey

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    As with all things in turning "it depends." Having a 40% rule-of-thumb is useful. However, I too am more tuned into the properties of the wood than just about anything else. For instance if I'm turning something like locust, frankly, I don't sweat the size of the tenon near as much due to the hardness of the wood as say something like soft maple. Also, if I'm using a tenon while I'm coring a blank (and sometimes I don't) then this requires an entirely different train of thought on the size and strength of the tenon that I want to use...because one can rip off a tenon in a heartbeat if applying too much pressure on the coring bar (don't ask...). So having the 40% rule is fine as a starting point, however, there are certainly other factors that come into play when it comes to settling on the size of a tenon...not to mention the tenon as it impacts the shape of the vessel that you would like.
     
    Last edited: Jul 17, 2020
    odie likes this.
  7. hockenbery

    hockenbery AAW Advisor Staff Member

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    I try never to break the laws of physics.... every time I try I get a negative reward.

    I saw Dale do a NE Bowl demo a long time back. His bowl was about 10” diameter. I’m quite sure he was at at least 901 rpm maybe 1100+

    I used Dale’s book “creative Woodturning” a lot when I was learning.
     
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  8. robo hippy

    robo hippy

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    Maybe time to dig this one up again....


    View: https://www.youtube.com/watch?v=5KHkkws9lWA


    Tenon size matters, and it matters more depending on the wood and what you are turning. I still prefer a recess, as I say in the video. Since I core, and turn at production speeds, I would never attempt a 16 inch bowl on a 2 1/2 inch tenon. It would not survive. I would probably opt for some thing at least 25% of the diameter, or slightly more. If you are turning very dainty, then yes, that could work, as long as you don't have any catches. The 40% rule would work for my style of turning on just about any size and any wood, but I consider it overkill for most of what I do. With my recess, a 2 5/8 inch recess works on bowls to 14 inch easily, and that includes coring. I can get away with it on 16 inch bowls, and probably would use it since I don't have bigger jaws on my chucks.

    robo hippy
     
  9. Dean Center

    Dean Center

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    When I was first starting, our local guru said the FOOT of a bowl should be 40% of the diameter or less. Since some folks make their foot out of their tenon with a little cleaning up, the 40% rule could easily have gotten ported over to the tenon size. Could that be where the rule came from?
     
    Last edited: Jul 18, 2020
  10. Randy Anderson

    Randy Anderson

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    Dean, maybe so. For a long time I did just that and would use my tenon as my foot on a traditional or on a natural edge. On natural edge the bottom warp means there is some blending and reshaping of the foot to get it round and even height and then smoothed out to the wing profile. Having the tenon as the foot made this very difficult. That also meant the tenon size dictated a lot about the foot size. A while back I started rough turning the foot area in addition to the tenon and quit using the tenon stub as my foot. I can then turn the foot to the size I want, usually 1/3 of the bowl width or a bit larger depending on how it looks. Also gives me a lot more flexibility to finish out the bowl down to the foot on the outside without coming up against the chuck. Sure this is common sense to many but some of us are slow learners.
     
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  11. Ric Williams

    Ric Williams

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    A tenon is supposed to be 40% of the diameter??? I thought it was 140%, and my 2 chucks are both Super Novas, so I haven't turned anything bigger than a shot glass! ;)
     
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  12. JeffSmith

    JeffSmith

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    I’ve got 4 dovetail jaw sizes always mounted and ready to go - 2”, 2-1/2”, 3” and 4”. Everything fits. anything getting cored goes in the 4” regardless of size. Tenons sized by the design (intended) for the piece. The tenon with its support shelf for the jaw face rarely winds up the same size of the foot or base in my case.
     
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  13. John Torchick

    John Torchick

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    Interesting thread. I read/saw/heard that when the chuck jaws are closed, there should be just a small bit of wood showing between the jaws when fully closed on the tenon. Also, heard of the 40% "rule." I have a Supernova2 chuck and really don't, at this time, turn anything real big.
     
  14. hockenbery

    hockenbery AAW Advisor Staff Member

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    That is where the perfect circle grip is. That is the curvature of the jaws is in a perfect circle. About an 1/8” gap.
    My vicmark #2 jaws have a Perfect circle at 48mm (1.9”). This is the strongest hold.

    A 2.5” tenon in green wood Centered on the grain will dry oval at about 2.5 x 2.25 in most woods.
    The 2.5 “ green tenon can always be turned round when it is dried to get a 2” tenon to return the bowl.

    The jaws hold a 2.5” tenon extremely well and although less strong than the perfect circle hold, it is strong enough.

    also with dovetail jaws the perfect circle grip does not mark the wood in a noticeable way.
     
  15. Mike Johnson

    Mike Johnson

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    Anytime someone commits to, or puts in writing a "rule", they tend to cover themselves by allowing for a margin of error and other factors that come into play. In the world of OSHA, Insurance and Legal Actions you generally want to make sure a statement or publication does not come back on you. In the industrial world if you are lifting an object that weighs 10,000 pounds you usually use rigging that is rated at least 4X the weight 40,000 pounds. It really comes down to the individual on the lathe knowing their own limitations and using safe practices in supporting the wood blank properly and not exceeding the ability of the method used in securing the wood to the spindle. A novice turner does not any of these limitations so you need to create a "rule of thumb" that covers the lowest common denominator. That is the world we live in these days.
     
  16. William Rogers

    William Rogers

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    I don’t consider the 40% rule a rule. To me it is a guideline. There are so many variables involved. If I was coring I would use a larger tenon than I would use for turning. End grain vs side grain, I have never had an end grain tenon fail. I guessing I have had maybe 3 side grain tenons fail in the last 7 years, all have been soft maple less than 10". I usually set up a large bowl blank between centers to determine the best position and then use a worm screw for initial roughing. This will take the initial rounding up cuts that produce the most force. My HTC 125 standard jaws are 2 1/4” and feel they are fine for end grain or hardwoods up to 16”. My largest Axminister jaws are 4” and use those on diameters up to 20” and on soft woods. I don’t think one size fits all because of all the variables. And like Randy, I don’t let the tenon dictate the foot size.
     
  17. Ron Vasser

    Ron Vasser

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    In my short turning career, one thing that helps me decide tenon size on bowls is if the tenon is cut on the sapwood or heartwood side of the tree. Maybe some of you experienced turners could expound on that theory.
     
  18. Roger Wiegand

    Roger Wiegand

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    I do try to use a faceplate whenever possible on big, out of balance, pieces of wood. Not sure I trust screws into end grain though; for end grain pieces I rough between centers and then hold the piece in a chuck.
     
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  19. John Tisdale

    John Tisdale

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    You might consider "diving under". (you might want to start at bullet-10)
    I am assuming most start between centers with a spur-drive and then, when it's "like we like it", cut a tenon. I always use a 6" faceplate on my up to 22" hollow-forms - why go smaller? But the base I cut is never, ever, over 4".
    • I'll shape the outside between centers and then make the 6" tenon, maybe a bit more to allow for warping 8-mos later.
    • The 6" tenon surface must be "dead-nuts" flat - a teeny bit concave is ok.
    • My Oneway faceplates accept #14 screws - I use SS sheet-metal screws - oval head gives a bit more grip
    • Hinge "self centering" bits mark the hole centers - this is critical. I then drill with an 11/64 bit with a "stop" attached
    • I use 1-1/4" screws in inner-6 and 1" screws in the outer-12 - there's a reason.
    • I then can re-true the outside and then start hollowing. Even on large pieces, the screws hold.
    • When done hollowing, time to boil and let dry
    • When 6% to 7.5% MC, I mount on the Kelton mandrel and re-true / remount the faceplate.
    • After re-turned / re-hollowed to final, it's time to "dive under"
    • I remove the outer-12 screws and remount on the lathe between centers with a cone on the live-center. I can then follow the profile with push cuts and reduce the tenon to 4" - I like to include a 1/4" annular ring (the actual base before the tenon).
    • Then I can then remove the faceplate and attach a "floor flange for 1/2" galvanized pipe" - insert a 10" nipple and put on my "finishing lathe" (you can build one out of 2x6 scrap in a few minutes) - all finishing and sanding is on the "finishing lathe".
    • When finishing is done, I put on the Kelton mandrel to remove tenon / cut base
    • Sign, mount on the umbrella fixture, and apply two more clearcoats - then rub out.
    • Off to the gallery
     
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  20. Mike Adams

    Mike Adams

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    [QUOTE="John Tisdale, post: 163947, member: 34749"

    • When finishing is done, I put on the Kelton mandrel to remove tenon / cut base
    [/QUOTE]

    Who's selling those mandrels in the US? The Kelton's site says Craft Supplies, but Craft Supplies is 404ing that page
     
  21. John Tisdale

    John Tisdale

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    You scared me so I went and looked - googled Kelton Woodchucker Mandred - there it is: $99.95 - cheap at twice the price.
    There are knock-offs out there.
     
  22. Emiliano Achaval

    Emiliano Achaval Administrator Staff Member

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    I do not pay much attention to tenon size, I do it instinctively I know what a safe size is for whatever I'm doing. It is a bit scary seeing a novice turner turning a bowl that is maxing out his 12 in lathe with a small little tenon, mostly due because they bought a small tiny nova chuck for $49 on Amazon. I start 95% or more of my work in between centers.
     
  23. Leo Van Der Loo

    Leo Van Der Loo

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    No problem with tenon size, I just use a recess, and no not 40% either :rolleyes:
     
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  24. Mike Adams

    Mike Adams

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    Anywhere in the US?

    All I see is a 404:

    upload_2020-7-28_13-54-15.png
     
  25. Jerry Bochenek

    Jerry Bochenek

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    Mike, I found the Kelton woodchucker mandrel at Craft Supplies for $99.95. Not sure why you are getting a 404 error.
     
  26. Mike Adams

    Mike Adams

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    Man, that's crazy! I'll email and ask them about it.

    EDIT: Now, with added crazy: I tried to visit https://www.woodturnerscatalog.com and I get a nice blank page with "Forbidden" in the top left-hand corner.
     
  27. Gerald Lawrence

    Gerald Lawrence

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    Don't know where you saw a Nova of any kind for 49 but let me know. As to small I have turned 10 to 12 inch bowls regularly with a 2 inch tenon . No I would not try that with a 25mm but that size jaw is a extra and does not come with the chuck.
     
  28. Timothy White

    Timothy White

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