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Thread: Bent plywood

  1. #1
    Join Date
    Sep 2019
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    Lafayette, CA
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    851

    Bent plywood

    My two cabinet doors are all milled up and ready to glue and oil. Hard white maple frame with 1/2 prefinished birch plywood panels. They’re about 30” high by 20” wide, each.

    Problem is that the plywood is curved, making a bow going from top to bottom, along the long side. I thought if I just glued everything up with the panel snug in the frame’s groove, all would be well.

    Well, well it’s not. The plywood bow protrudes outward in the door fronts (both). This pulls out on the front of the grooves in the stiles. That makes the inward sides of the stiles deflect outward. If you look across the top and bottom of a clamped door, the stiles are angled backwards, toward the doors’ insides. No matter how hard I clamp, I can’t get a straight line across the tops and bottoms of the doors.

    If I try a sandwich of straight clamping cauls, the line might start out straight, but there would be a constant internal force on the stiles, and they’re only held together with 7/16” tongues and grooves.

    I’m not interested in gluing them up like this. I either have to permanently flatten the panels or replace them with new stock. I’m just a garage DIYer so I don’t want to buy another 4x8 sheet of ply.

    Can the panels be permanently flattened?
    Last edited by Bob Jones 5443; 07-09-2024 at 1:32 PM.

  2. #2
    It sounds like the pre finished 1/2" birch is an unbalanced product, it is a difficult problem to overcome. The heavy coat of industrial sealer on the face creates the imbalance.

    Here is a snip from fine homebuilding.

    Like the underside of a shriveling leaf, the plywood’s concave side has lost moisture and shrunk. Reversing the warp can be accomplished by adding moisture to this concave side and drying the convex humped side. I use a sponge or sprayer to wet the concave side of the plywood (hot water works best). Then I lay the sheet, moist side down, on a shop floor or a driveway. Now the sun, or the warm interior air, helps to dry out the convex, humped side of the plywood. At the same time, moisture is being absorbed into the concave side. This process works faster than you can imagine, so keep an eye on the material. If it warps the other way, just reverse the process.


    Last edited by Maurice Mcmurry; 07-09-2024 at 2:43 PM.

  3. #3
    Join Date
    Sep 2019
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    Ok. I’ve laid them out on the deck. We’ll see. I’m skeptical, though. Both sides of the plywood have been prefinished, so it’s not clear how the water will soak into the concave side.

  4. #4
    Join Date
    Nov 2009
    Location
    Peoria, IL
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    The only permanent solution I would have, since you don't want to replace the plywood, would be to cut some kerfs in the back of the panel to take away some strength of the wood.

  5. #5
    Just get a flat sheet, don't waste your time with warped material.

  6. #6
    Bob Jones, do you have any scraps of the 1/2" pre finished birch ply that you could photograph? Close-ups of the lamination stack and faces could help in figuring out what's going on. You are working with hello kitty brand plywood most likely. Two layers of 1/4 inch, back to back, will have a better chance of staying flat.
    Last edited by Maurice Mcmurry; 07-09-2024 at 8:44 PM. Reason: typo

  7. #7
    Join Date
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    Quote Originally Posted by Maurice Mcmurry View Post
    Bob Jones, do you have any scraps of the 1/2" pre finished birch ply that you could photograph? Close-ups of the lamination stack and faces could help in figuring out what's going on. You are working with hello kitty brand plywood most likely. Two layers of 1/4 inch, back to back, will have a better chance of staying flat.
    Adding where you bought it from also helps. Buying from a box store is hardly ever a good thing. If you stored the cut panels on a flat surface, one side may have absorbed a little moisture from the humidity. Panels always need to have air circulating around them all the time.

  8. #8
    the last ply I bought 3/4 which I think is their current baltic birch as close to I counted 12 plys. I emailed told them its made wrong and no response. Out layer of veneer so thin I can see through it if ive been eating carrots.

  9. #9
    I tried to find something helpful on the APA website about 1/2 inch pre-finished birch. I don't see much at a glance but will keep looking.

    https://www.apawood.org/plywood

    https://www.apawood.org/publication-search?q=&f=Plywood
    Last edited by Maurice Mcmurry; 07-10-2024 at 7:54 AM. Reason: https://www.apawood.org/publication-search?q=&f=Plywood

  10. #10
    Join Date
    Jul 2003
    Location
    Winterville, NC (eastern NC)
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    2,383
    Going to offer 2 options:
    1. Find a hardwood dealer and get some decent Birch plywood. Pricy but worth every penny. Forget the stuff from the big box stores; I have seen potato chips that were flatter than their offerings. I used this for some slide out drawers in my kitchen.
    2. Get some 1/4" plywood cut to your desired panel size, and make a glued sandwich which should cancel out any warpage. Good luck

  11. #11
    Join Date
    Sep 2019
    Location
    Lafayette, CA
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    851
    As always, I appreciate everyone’s support. No doubt I need to re-buy the 1/2” stock. I can buy a half sheet and not have too much waste; I need 33” x 26”.

    No Home Depot for this project! I bought all my lumber from Macbeath in Berkeley, CA. Those who are familiar with them know they generally sell premium stock. I bought several board feet of 10’ hard white maple boards, and every board was flat and straight. It machined like a dream into drawer boxes, drawer fronts, doors, and even a guitar stand on the side. Beautiful stuff. That and cherry are my two favorite woods to work with.

    I did store the 1/2” inch sheet leaning up against the 1/4” inch sheet for several weeks in my garage. I suppose that could have put a warp into it, but I kind of remember it was warpy from the start. I mistakenly thought that once I cut it down into smaller parts, the tongue and groove frames would take care of things. That did happen with the drawer fronts with their 4” and 6” high panels, but the doors couldn’t do the job.

    It’s just a fact: pre finished ply cannot be flattened by wetting in the sun. I also tried a heavy weight on the panel propped up on two boards: no help after days.

    I’ll get over to Macbeath for the new sheet, inspect it for flat, and use it right away.

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