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Thread: Table Top Cracking - What Did I do Wrong

  1. #16
    Join Date
    May 2016
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    Northeast PA
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    527
    When I built my dining table a couple years ago, I drawbored the breadboard ends using dowels that were only very slightly tapered, if at all. I made the hole in the tenon 2x as wide as the diameter of the dowel. The top moves very noticeably in width from summer to winter and the drawbore does not restrict its movement at all.
    ---Trudging the Road of Happy Destiny---

  2. #17
    Join Date
    Aug 2010
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    USA
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    5,582
    Do you happen to have a picture of the breadboard and tenons before final assembly and glueup? I dont buy either the theory of the dowel pin cause or the pith crack.

  3. #18
    Your board is plain sawn. It cracked right at the most vulnerable spot, where the radial surface is perpendicular to the face of the board. The radial surface is the easiest to split, which is also why quartersawn walnut boards are prone to tear out. So the board was restricted from shrinking somehow as the humidity dropped and it split at a weak spot, not necessarily any relation to the tenon. This particular split is not near the pith, which for walnut is about 1/8 inch wide.

    If the humidity drops more, the crack could get worse, but otherwise will not get worse on its own. I would not worry about it.

    This is an example where work that is easily disassembled would be advantageous. If it has been made with repair in mind you can easily take it apart and see where the problem might have occurred. Tapered pins pop out somewhat easier when hit from the back side and actually in this case you would not have needed glue.

  4. #19
    Quote Originally Posted by Charles Guest View Post
    If you haven't experienced a failure, consider it a gift from the woodworking gods.
    I have not experienced any breadboard end failures because of the attention I paid to the holes, width of mortises relative to the tenons, and gluing surface in the center tenon. My continued success has nothing to do with the dowels themselves. I do not taper any dowels/pins; occasionally, I may chamfer very slightly one end of the dowels to ease the entry.

    If the tenon holes are not properly elongated, tapered pins may reduce but not eliminate the risk of failure.

    Simon

  5. #20
    Quote Originally Posted by Pat Barry View Post
    Do you happen to have a picture of the breadboard and tenons before final assembly and glueup? I dont buy either the theory of the dowel pin cause or the pith crack.
    I'm not sure, but I will check my phone and see. Probably would be good to post here for learning purposes.

  6. #21
    Join Date
    Aug 2013
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    Princeton, NJ
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    I think this is due to not enough room for expansion.
    Bumbling forward into the unknown.

  7. #22
    Quote Originally Posted by JohnM Martin View Post
    I think my error might have been in not elongating the dowel holes enough on the tenons. I did make the mortises wider for the sub tenons. I don't remember exact measurements.
    My guess is that this is the most likely explanation. If I am visually scaling your picture right, you have about 12 inches between the two tenons in the picture. For flat sawn walnut, you could be looking at somewhere between 1/4" and 3/8" shrinkage between summer and winter, possibly more if the lumber was a little on the damp side when you built it.

    My rough guide for wood expansion is to figure about 1/4 inch per foot from summer to winter for quarter sawn wood, and 1/2 inch per foot for flat sawn for summer to winter. I up this for less stable species like maple and birch, and sometimes cheat it down for more stable like oak and white pine. I'm also in a place with warm humid summers and cold bone dry winters, so seasonal movement can be extreme here.

    When I do a joint like that, I drill it through, and then pull the pieces apart and then drill another hole (or two) adjacent to it in the likely direction of movement and chisel it out to a slot (spring and fall I go both ways). For summer and winter, I also remove some wood in the opposite direction of likely wood movement, in case the wood is drier or wetter than anticipated.

    Wood movement is just one of those things you seem to have to learn the hard way. I know I have several pieces in my past that exhibited spectacular failures from not anticipating and accommodating wood movement.

  8. #23
    Join Date
    Apr 2015
    Location
    New England area
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    588
    It's possible the tabletop would have cracked completely on its own even if set aside in your house and not mounted on the undercarriage.

    That said, I'd still rive and taper pins in the future. Aesthetically I don't care for the perfectly round pin look, it screams "I'm a dowel," and you can avoid this easily if you make your own pins. You can whittle dowel stock to a rectangular head, but if you're going to the effort to do that you might as well make your own pins.

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