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  1. #1
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    May 2019
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    Default Glue-up cupping due to over-tightening clamps

    Quick question - it seems as if I've gotten a panel to cup from clamping it too tightly. I'm pretty sure that the cupping is primarily due to clamping pressure, as the pieces were all quarter-sawn, and had been stable prior to the glue-up. The fact that I had brought it indoors while the glue was setting may have contributed to it. Is there benefit in leaving for a few days for it return to its original state, or should I just flatten it again now?

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  3. #2
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    Did it cup when in the clamps or after taking it out? When clamping panels, it can be useful to put clamps on the bottom and top to act against each other.
    Also, how have you stored the glued boards , I like to sticker it to allow airflow between the pieces.

  4. #3
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    It's warped after taking it out. Only slightly, but a straight edge pivots on it. The boards were very dry to start with. Also, the glue-up was done in stages, so the final glue up comprised of gluing up 2 halves. The half glue-ups have been very stable.

  5. #4
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    May 2015
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    Brisbane
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    This happens to me all the time, SEQ weather is constantly humid one day and dry the next

    sticker the board both sides with the bow up and put a lot of weight on it. It may also help to dampen both faces.

  6. #5
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    I’m getting too old and memory not so good but I remember something about putting cupped boards out on the early morning grass cupped side down.
    It would suck up some dew (expand)and the sun would warm up the other side(shrink).
    H.
    Jimcracks for the rich and/or wealthy. (aka GKB '88)

  7. #6
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    Strathalbyn South Australia
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    Default

    Were all glued edges completely square to each other?

  8. #7
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    May 2019
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    Quote Originally Posted by Cal View Post
    Were all glued edges completely square to each other?
    yeap, dead square

  9. #8
    Join Date
    Apr 2001
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    Perth
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    Default

    Quote Originally Posted by yoboseyo View Post
    Quick question - it seems as if I've gotten a panel to cup from clamping it too tightly. I'm pretty sure that the cupping is primarily due to clamping pressure, as the pieces were all quarter-sawn, and had been stable prior to the glue-up. The fact that I had brought it indoors while the glue was setting may have contributed to it. Is there benefit in leaving for a few days for it return to its original state, or should I just flatten it again now?

    It depends what you plan to do with the panel. If the cup is very slight, and the panel will be joined at the ends to another panel, such as with a case, especially if dovetailed, then I would not bother with straightening it. The joinery will hold it flat. If the panel is to stand on its own, then I might try adding breadboard ends to reinforce and flatten the panel. The worst case scenario is to rip all, re-joint and add cauls when glueing.

    Regards from Perth

    Derek
    Visit www.inthewoodshop.com for tutorials on constructing handtools, handtool reviews, and my trials and tribulations with furniture builds.

  10. #9
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    Aug 2020
    Location
    Sunshine Coast
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    Quote Originally Posted by lewisc View Post
    Did it cup when in the clamps or after taking it out? When clamping panels, it can be useful to put clamps on the bottom and top to act against each other.
    Also, how have you stored the glued boards , I like to sticker it to allow airflow between the pieces.
    It's more than useful. It's necessary to avoid putting undue pressure on one side to the boards. If they don't sort themselves out you may need to cut them along the glue lines and re-glue up

  11. #10
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    Aug 2020
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    Sunshine Coast
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    Quote Originally Posted by clear out View Post
    I’m getting too old and memory not so good but I remember something about putting cupped boards out on the early morning grass cupped side down.
    It would suck up some dew (expand)and the sun would warm up the other side(shrink).
    H.
    Probably better on the concrete than the grass. Have done it there and it can work.

  12. #11
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    May 2015
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    Also, if the cup side up is the face you could do some relief cuts lengthwise on the underside.

  13. #12
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    Jan 2008
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    Yeah, I’m feeling it too, just edge joined two sassafrass boards perfectly flat, they cupped in the wet weather here in Perth after I took them out of the clamps and before I could put breadboard ends on. So back in the clamps for a few days, hopefully they will flatten when the weather dries out. Sassafrass always does that, a beautiful timber but behaves badly! The clamps are the “squeeze ‘em both ways” type.
    Attached Images Attached Images
    Swifty

  14. #13
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    Jul 2014
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    Brisbane
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    Some time ago I was shocked by how well the leave out on the grass method worked. I cut a 10mm thick slice off a burl and left it flat on my bench overnight, it looked like a potato chip/crisp the next day. Left it out in the sun with the side that was on the bench facing the sun for a few hours and it had cupped in the opposite direction... I flipped it over and left it for another hour and it was almost flat afterwards. I need to cut another slice of that burl and take some photos, the severity of the cupping was crazy.

    Edit: If the panel is going to restrained by joinery or be floating in grooves, I wouldn't fret too much about a small cup. If you're cutting dovetails into it for a carcass, you can clamp your slightly cupped board to a backer board to get the cup out. Once assembled, the joinery will constrain and guide the cupped panel.

  15. #14
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    Jan 2008
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    I just chucked it on the grass as suggested for a couple of hours, worked fine and I managed to fit the breadboards to boot. Thanks for the tips!
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    Swifty

  16. #15
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    May 2019
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    Canberra, Australia
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    I'll give the leaving on the lawn thing a go. Hopefully it actually stays that way!

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