Tenon tear out after drying

Discussion in 'Newbie' started by Jon Klobofski, Jul 12, 2016.

  1. Jon Klobofski

    Jon Klobofski

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    IMG_0404.JPG
    I was checking my rough turned stock and noticed this separation around the tenon. Would this be from aggressive hollowing, grain orientation, or a combination of both? I do not remember the separation after turning this piece.
     
  2. john lucas

    john lucas

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    I'm guessing it was there after you turned but wasn't very noticeable. When the bowl dried it further opened up the crack.
     
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  3. Owen Lowe

    Owen Lowe

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    It kind of looks like you tore the fibers by really, really tightening the chuck jaws. Could that be the case? Being end-grain, the fibers aren’t all that strong in resisting separation in that orientation.
     
  4. Jon Klobofski

    Jon Klobofski

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    I am constantly checking and tightening the chuck. I had a couple of pieces come off the chuck a year or so ago. That left me a little paranoid. After watching a couple of videos, I have learned to make tenons larger, shallow, and square .
     
  5. robo hippy

    robo hippy

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    Hard to tell. It could have been a stress crack from a catch where it didn't come all the way off the chuck. It could be ring shake where the wood cracks along the growth rings rather than off the pith, and it opened up as it dried. I don't see any chuck marks on the tenon, so that may not be the problem. If you return it, I would put some thin CA glue in the crack, then some thick or medium on top of that, and then stand it on edge so the glue will soak all the way down in, let it stand over night, and then make sure to stand out of the line of fire when doing the second turning. Keep the speed down too, not more than 500 or so.

    What kind of wood? Black locust?

    robo hippy
     
  6. Richard Coers

    Richard Coers

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    Is the wood punky in that area? If so it's likely a shrinkage issue. If that tenon for a chuck, I'm concerned about that radius on the face of it, and what appears to be flats on the diameter of the tenon. Are those crushed wood flats? If so you are flirting with some very marginal wood. You need the tenon surface to be very smooth, with a tight corner where the jaws meet the bowl. The jaws must butt against the vessel and not the base of the tenon. The blurry picture doesn't help with the diagnosis.
     
  7. Zach LaPerriere

    Zach LaPerriere

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    Maybe it's just the photo, but the tenon doesn't look round...I understand that in drying our tenons turn oval, but I'm seeing a reasonably sharp angle at the widest point of the crack.

    It's also worth noting that rounding the tenon again after drying is important, especially on bigger bowls and tenons. In my experience at least, cranking the chuck down on an ovoid tenon will (probably) hold, but the cutting isn't as clean—which I am guessing is because the bowl tends to vibrate a bit more.
     
  8. Jon Klobofski

    Jon Klobofski

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    Please excuse the tool marks.( I was lazy) I rough turned this to a general shape with the intention of refining it after it dried. My tool handling has improved since I turned this sometime ago.
    The wood is local Cottonwood. In this picture you can see the tenon was mashed by the chuck jaws. My plan is CA and re true the tenon. I have noticed a separation near the bottom edge (also CA). I am hoping that I can slope or turn the separation away.
    IMG_0406.JPG
     
  9. Bill Boehme

    Bill Boehme Administrator Staff Member

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    That last picture helps a lot more than the earlier ones. I would definitely not reuse that tenon. Turn a new round one with the piece mounted between centers. Make the new tenon longer, get rid of that hump on the tenon, and create a shoulder for the top of the jaws. The shoulder is very important because that is what provides the alignment. I agree with what Richard says.

    Also, the rough turning of the vase shouldn't be cylindrical shaped on the exterior. It should be roughly the final shape so that the wall thickness is fairly uniform from rim to center of the bottom. However, if a cylindrical shape is what you want there will be a strong likelihood of cracking because of the abrupt change in how the flat bottom wants to shrink compared to how the cylindrical doesn't have the same internal forces.

    I always create a very slight dovetail on the tenon when using my Talon or Stronghold chucks ... and make certain that you get a crisp intersection where the tenon meets the shoulder. A good length for the tenon is just short of bottoming out.
     
  10. Steve Worcester

    Steve Worcester Admin Emeritus

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    I will add to what Bill Sid.
    The tenon show looks to be a bit angled with the narrow at the end. That would mean the wood could rock in the chuck, hence his dovetail comment.
    The crack could also be caused from stress in drying. The grain appears to be oriented so you re cutting end grain. With end grain, the walls are as thick as the piece is tall and the tenon section is as thick as the tenon thickness plus the bottom thickness. So to very different drying speeds. Add to that Cottonwood is very soft, at least what we get down in Texas, too soft for my taste, and drys very rapidly.
     
  11. Jon Klobofski

    Jon Klobofski

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    Thank you, I see the tenon is angled wrong for my Talon chuck. This piece was wrapped in brown paper and left to dry awhile ago. (Didn't write the date on the paper). Wall thickness on this piece is 3\4 of an inch, with a base of about an inch.
    I have been following Robo's video with my bowls. Turning to final thickness, wrapping with plastic wrap, and let air dry. So far my bowls are drying too fast and I am getting ring separation. With my hollow forms, I wrap in paper or box with shavings.
    On this piece, I will let the CA set before returning to centers. I will follow your advice Bill, and try to turn a new and correct tenon. Am I tightening the chuck too much?
     
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  12. MichaelMouse

    MichaelMouse

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    Use a dovetail tenon, if you have the jaws to support making one. Make the angle a bit less than the included dovetail angle, MAKE A SHOULDER to bear against, then snug, don't grab with the chuck. Holds against rotation by wedging the nose of the jaws up against the shoulder, and requires no squash or chewing to work the inside. I have turned willow and aspen, cousins of your cottonwood, and definitely prefer a wedged hold.
     
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  13. Steve Worcester

    Steve Worcester Admin Emeritus

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    Let me say this about CA glue.
    It works holding in one axis and not another. It is very brittle, even the flexible. Which works against the constantly moving properties of wood.
    And lastly..
    It is not archival. The bond is not meant to be permanent in the sense of passing your turnings on to future generations.
    Epoxy is better, but won't flow into the cracks. So with a ring check like this, put it in the firewood pile, and move on. Understand why it happened and go get some more wood and turn some more stuff.
    There are a lot of stories about pieces coming apart on the lathe and doing bodily harm, it isn't worth it.
     
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  14. Zach LaPerriere

    Zach LaPerriere

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    For what it's worth, the cottonwood here in Alaska is VERY prone to ring checking (ie. separating between the growth rings.) A few months back I brought a couple hundred pounds back to the shop and couldn't get a single bowl out of it. My limited experience suggests that if one chunk in a given cottonwood has ring shake, the rest of the tree is likely to be riddled with it...and it tends to pop up while turning sometimes. Take extra precaution, and be wary.

    That said, cottonwood can be very beautiful, despite its difficulty to turn.
     
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  15. Jamie Straw

    Jamie Straw

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    Bears repeating -- from what I've read, chucks often get blamed for failures that are due to tenon/shoulder problems. Might want to expand/elucidate your sentence. Tenons larger = diameter of tenon, yes? But not so large it expands the chuck too much -- close to a circle is what people seem to say. "Shallow" -- tenon not too long, don't want it to ride on the bottom of the chuck jaws. "Square" meaning square to a good shoulder (where the tenon meets the bowl) so the jaws have something to seat on. Correct me if I'm wrong, plzzzz.
     
  16. Jon Klobofski

    Jon Klobofski

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    Jamie you nailed it. When I first bought my chuck, everything you are not suppose to, do I did. My first tenons were sized just bigger in diameter than my chuck fully closed. I would square the bottom of the tenon to rest on the chuck, and there was no flat shoulder for the jaws. I learned the hard way. I forget who's video finally caused the "Ah-Ha" moment . Sometimes I'm a little denser than the wood I turn.
    Now I size the diameter of the tenon to the size of the piece. Shoulders are squared to the tenon and the bottom of the tenon never touches the chuck.
     
  17. Dean Center

    Dean Center

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    One of the biggest lesson I've learned from working with high school students in our after school turning club is the value of clean junctions between shoulder and tenon. The effect of a little debris or a little rounded/curved corner is huge.
     

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