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FOT - flattening cupped plywood

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  • FOT - flattening cupped plywood

    Some time ago, someone gave me some plywood. I didn't have the space to store it properly and some of it cupped.

    I have four pieces that are 3' x 4' x 1/2"

    My intention is to laminate them into one 2" thick top and use them as a workbench top.

    Two of the pieces are badly cupped, however. They were stored together, so the cup in each is quite similar.

    The workbench frame is assembled. There's a 1 1/2" lip all the way around for the top to sit on and 3 joists. I put the plywood in and clamped one end. This made the cupped boards stick upwards at the other end. I could barely get the plywood to seat using all my weight - 1" of plywood gets springy, apparently.

    Any suggestions on how I can flatten these? My intention, even if the plywood was flat, was to put glue between each sheet and screw it down to the bench using a generous number of screws. The idea was that the screws would hold the plywood together and allow the glue to set.

    If I put the cupped pieces together with the cupping pointing opposite ways, at least the forces will tend to balance.

    Anyway, any ideas?

  • #2
    cupped ply

    no easy fix. be patient. depending on the amount of cup, respring. I've cleaned up 3/4" ply, full size, but not tomorrow. I hang the UP side, up, up between supports, put weights and MAYBE a little moisture on both sides, watch and maybe more moisture. Depending on the moisture in the air and amount of cup, it may take two weeks or, forever!!! Sorry. Oh yes, monitor daily. Wayne.


    • #3
      If they are cupped the sane amount then there is a good chance if you clamped them together convex side to convex side they would be pretty flat. If that works then undo it, glue it, and clamp.


      • #4
        Hi all again:

        Macona: Sorry to argue: IF the cup is too much, the gluing and screwing might pop or pull apart in the future. Better to try and respring first. Wayne.


        • #5
          Spray with water with sprayer bottle on the concave sides. Make it good and wet, set it with the concave sides together with weight on it and wait a day. It will straighten out quite a bit as the wet sides expand. Then put together with water based wood glue, wet side to wet side and screw, clamp or park a truck on it.
          Free software for calculating bolt circles and similar: Click Here


          • #6
            How is the cupping oriented? ...along the length of the grain? ...across the width? Or both?
            If it's cupped both ways, I'd be inclined to lay them on the ground, concave side down and let the ground moisture expand the concave sides. Check on it every few hours, so it doesn't overdo it. Might take a day or so.

            To laminate them together, I'd first laminate two, with the convex sides faceing, and clamp around all sides. When those have cured then laminate the third one against those two. Depending on how much cupping remains in the two already laminated and the other one, it might be helpful to introduce some "counter cupping" or bending during the glue up, in hopes that they'd spring back to flat when dried and clamps removed.

            If you don't get most of the distortion out before attachment to the frame, they'll more than likely pull the frame into a warp.


            • #7
              Way easier with concave sides facing. It only needs weight in the middle and the edges are dead tight.
              Free software for calculating bolt circles and similar: Click Here


              • #8
                I clamp them convex to convex all around the edges, and the center takes care of itself. Now, I do have dozens of clamps lying around, so that made my choice easier. I suppose I might use the weight method if I had some easily moveable tonnage I could park there. I did it once with my '71 Dodge, but it turned out to be more of a pain than I expected.

                As to the glue not holding once the glue-up is complete, I can't imagine how that would be a problem. With that many square inches of contact, the glue could be mighty weak stuff and still hold incredibly well. IF I do have a problem with flatness, I take care of that with some hand planing after the bench is complete.
                Last edited by Frank Ford; 12-14-2011, 01:22 AM.

                Frank Ford


                • #9

                  Evan: Moisture both sides. BE patient. The cupping did not happen fast. This will take time. BTW, Sorry, I forgot this forum goes around the planet. Down here on the coast, BC, we are used to moisture on all our parts!! Wayne.


                  • #10
                    More force? A bottle jack can prolly press it down. Use some angle iron under the bottle jack to distribute the pressure.

                    So then the problem is how to support the opposite end of the jack?

                    There are many ways. Depends on what is surrounding the table. I have a large overhead beam and would use that in my case. I have a portable power jacking system so there are plenty of extensions to reach the beam. But a bottle jack and a length of steel could be used.

                    One way would be to use a long piece of steel, angle iron will work, rectangular heavy wall tube would be better. Place the tube on top of the bottle jack with the tube overhanging the table frame then make up some links that hook to the frame rails. Could be 2x4s screwed to the frame and attached to the steel. You would do this on the narrow side of the table. Jack the divot down and secure.

                    Another way is to use an engine lift (cherry picker). I have had to make a means of providing downward force before when I didnt have any solid beams above to press against. So I installed some machine screw inserts into my garage floor. Some 1/2" thread inserts were good for several hundred pounds of force. I used a simple pulley attached to an eyebolt that screwed into the insert and ran a nylon line from the engine lift, through the pulley then up to the item I needed to pull down. Sounds wasteful to set inserts just for that but Ive used them many times after that for other purposes. Its kinda handy having some threaded inserts in the garage slab. And when not in us I pulg the holes with some plastic threaded plugs that have a philips drive on the head. They screw in flush to the concrete floor.

                    Want some more goofy ideas? I have bags of them.

                    Do you have a porta power type hydraulic jack? If so you prolly have the 10 ton pull back ram too? You could drill a hole through the wood, run some rod or all thread through the hole, place a massive washer or just a drilled 2x4 over the all thread, nuther washer and nut. Hook the other end of the all thread to the pullback ram and support the other end of the ram to again, a floor mounted insert or steel tube thats holding up an arrangement of blocks going upward to the frames underside. Put the squeeze..

                    At first heating the panels was a thought but that was ruled out just as fast. I dont think it would be good for the glue in the panels. JR
                    My old yahoo group. Bridgeport Mill Group



                    • #11
                      Get lots of clamps ready. Paint both crowned sides of the plywood with some runny white glue. Stack the pieces and clamp one long end loosely in the middle, then go to the other long end. Force it together and place another clamp. Then do both sides in the middle, then add a couple more to each side. If you have enough clamps, add two more to each end. When you have the first four placed, square up the pieces and bring those clamps up snug. Then add the others.

                      If you're short on clamps, you will still need at least four. One each end, one each side. Once they are in place, snug them up. Start placing screws one each side of a clamp. Then you can move the clamps, bring the plywood together, add a screw either side of the clamp again. Do this in a sequence working out from the center of the ends and sides. Once you make it to the corners you might go back and snug all the screws again.

                      One thing about using screws- if you rely solely on a screw to pull the plywood together, it would be best if you pre-drilled some pilot holes in one side. Otherwise the screws will tend to create a gap between the plywood pieces, and you'll end up over-driving the screws to get the gap to close up. That weakens the hold and looks poor, and probably will get you a lot of screw ends sticking out the other side. That's one of the reasons I suggest to tighten a clamp, then place a screw close to it on either side.

                      Plywood edges are not pretty, nor are they safe. It would be good to saw them all square if you can, then apply an edging. This could be cut from some 2x4 or 2x3, or could be some kind of hardwood like maple. This could be your perimeter frame, so you decide how substantial you would like to make it. Rabbeting an edge of some kiln-dried 2x4s to drop the edge of the laminated plywood into would be a good idea. Having some temporary support blocks levelled up to support the plywood while the glue dries is also a good idea. You might have to add weights to take out the twisting, and you'd want to do this before adding the screws.

                      Plywood generally makes a poor work surface. I like to use a 3/8 thick mdf add-on layer, as it's smooth and doesn't splinter or give you slivers. If you would be gluing this on, then put one layer on both sides of the plywood sandwich. If you would just be laying one on top of the plywood, then just put several small screws around to keep in in place and be done.
                      I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-


                      • #12
                        Some woodworking bends wood by scoring the concave side every inch or so with a skill saw, ?? maybe 1/2 way through, this should relieve a lot of tension, then liberaly glue that side and then weight it heavily and screw as much as you can of the surface to the rest of the sheets.
                        Probably take a lot of glue.

                        May be better to just get a flat sheet or two and fasten them down to a top of 2x8 or so.


                        • #13
                          Ply compression

                          Do you know someone with a medium sized forklift ,they are very good mobile weights .
                          Assemble the ply sections with a covering of good quality glue and place a 3/8 steel sheet on the ply and then park the forklift on the sandwich for a weekend .
                          Failing that ask a crane hire company if they could put a heavy block of concrete on the glued ply ,most have large concrete blocks for testing or other purposes.


                          • #14
                            Wow, thanks for all the ideas.

                            After grunting these pieces into the frame last night, I think the using some water to flatten the plywood is prudent.

                            +1 on plywood edges being nasty. The top sits into the bench's apron so the edges are not exposed.


                            • #15
                              We use plywood for covers for our bait tanks. After a year they cup, we just flip them over and after a few weeks they are flat again. The moister from the tanks may have something to do with it as well.