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Using biscuits in plywood butt joints

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  • Using biscuits in plywood butt joints

    My son is restoring a toy box I made for him 32 years ago. The box is plywood. The lid is 2 pieces of plywood butted together. I used a 1x2 along the length of the plywood butt joint on the inside of the lid to hold it together (remember, it was 32 yrs. ago and I was new to woodworking). He wants to remove the 1x2 and use biscuits & glue to butt the plywood together. I wondered how strong the joint will be by cutting into the plys of the plywood. I have used biscuits in hardwood to make a panel but never plywood. I suggested if he does it he may want to use Kreg pocket hole screws in addition to the biscuits. Your thoughts? Thanks, Craig
    Never outsmart your common sense

  • #2
    Re: Using biscuits in plywood butt joints

    I would go with the Kreg Jig. I have yet to have a joint fail with this system. The Biscuits would also work but you will have to be carefull what size you use.

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    • #3
      Re: Using biscuits in plywood butt joints

      The biscuits should hold if he is generous with them (the more, the better). If he gets a good smooth glue edge, the matching long-grain plys will add to the strength.

      I would not try kreg joinery to butt plywood. Not enough tear-out strength around the screw heads.

      JMTCW

      Go
      Practicing at practical wood working

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      • #4
        Re: Using biscuits in plywood butt joints

        I wouldn't use the Kreg system either and I'm not sure exactly how solid biscuits will be, although I think it should be stronger than pocket screws.

        I might think about using and edge cutting bit with my router and then using 1/8- or 1/4-inch ply as a spline across the full length of the mating edges. In my mind, that would be the strongest possible joint.

        I hope this helps,

        CWS
        Last edited by CWSmith; 01-23-2010, 10:12 PM.

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        • #5
          Re: Using biscuits in plywood butt joints

          Using a router, glue and a strip of some solid wood that's what I would do along the entire length of the joint:
          Attached Files
          In order to understand recursion, one must first understand recursion.

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          • #6
            Re: Using biscuits in plywood butt joints

            I've tried to butt-splice plywood panels with bicuits and been disappointed in the results. When gluing up a solid-wood panel, the strength comes from the glue along the long grain of the edge, not really the biscuit - it just provides alignment. Because the integrity of the plywood edge is not as good as solid wood, and the plies alternate long grain / end grain, you're unlikely to get a great joint with just biscuits and glue.

            If your son is bound and determined to use biscuits and glue only, get him to consider using a good epoxy rather than yellow woodworker's glue. West Systems epoxy is the best I've found for wood projects... but very pricey. Follow the directions exactly and use one of their recommended fillers (no fillers are needed for solid wood joinery with tight fitting parts - you need it for plywood).

            I've used pocket screws on ply panels and wouldn't hesitate to add some to seriously strengthen the joint. Depending on the quality of the plywood (probably pretty good since it's old) it's very likely to work fine - I've used them on plywood and never had a problem. You do need to use the coarse thread washer head kreg screws, not the fine thread screws used for solid hardwood. Don't overtighten - a common mistake with Kreg screws.

            I agree with the idea of cutting a slot for a spline. It's my preferred method of but-splicing plywood most of the time because it's fast, easy, works much better than biscuits, and you don't have to look at the pocket screws. You can get a lot of glue area doing this - it's essentially a floating tenon. You can use a tablesaw as well as the router if you don't mind seeing the spline on the ends of the piece.

            If you want the strongest possible joint, use just a couple of biscuits for alignment, glue & let dry, then rout a dado centered on and full length of the seam line on each side of the panel. Then glue in a strip of solid wood about 1/8" thick on each side of the panel. When dry, scrape or plane it to flush. This is more work but considerably stronger than the spline, because moment loading of the panel (probably the main type of loading the joint will see) is much better resisted by adding strength to the surfaces of the panel than the center. Make the reinforcing strips as wide as possible (use a couple of passes with the router) and it will be very robust. This is also a good excuse to add a contrasting wood... a bit of inlay to spruce up the panel.

            Good luck, and congrats on having built an heirloom piece 32 years ago! Nice job!
            Last edited by Andy_M; 01-24-2010, 12:45 AM.

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            • #7
              Re: Using biscuits in plywood butt joints

              Originally posted by Andy_M View Post
              If you want the strongest possible joint, use just a couple of biscuits for alignment, glue & let dry, then rout a dado centered on and full length of the seam line on each side of the panel. Then glue in a strip of solid wood about 1/8" thick on each side of the panel.
              I also use that method. I just thought that for a legacy piece this might take away a little too much from the original material. The "too much" can be only defined by the owner of the chest though so if applicable, I'd also recommend that route (no pun intended).
              In order to understand recursion, one must first understand recursion.

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              • #8
                Re: Using biscuits in plywood butt joints

                Well, I just talked to my son and found out this plywood isn't like plywood today. It is birch plywood, 3/4 thick, but looks like hardwood. What I'm trying to say is it doesn't have plys, is that possible? I don't know if this makes for a better case to use pocket screws or not, and if so should he go with fine thread instead of course thread like you would normally would with plywood.

                Thanks for the good feedback.

                Craig
                Never outsmart your common sense

                Comment


                • #9
                  Re: Using biscuits in plywood butt joints

                  Originally posted by darius View Post
                  I also use that method. I just thought that for a legacy piece this might take away a little too much from the original material. The "too much" can be only defined by the owner of the chest though so if applicable, I'd also recommend that route (no pun intended).
                  Good point. Maybe if the inlaid pieces were the same species as the ply (softwood? hardwood face veneers? I don't think the OP specified) it wouldn't be too noticeable.

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                  • #10
                    Re: Using biscuits in plywood butt joints

                    You people are sick. Putting biscuits in butts.

                    J.C.

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                    • #11
                      Re: Using biscuits in plywood butt joints

                      Originally posted by Craig Moore View Post
                      Well, I just talked to my son and found out this plywood isn't like plywood today. It is birch plywood, 3/4 thick, but looks like hardwood. What I'm trying to say is it doesn't have plys, is that possible? I don't know if this makes for a better case to use pocket screws or not, and if so should he go with fine thread instead of course thread like you would normally would with plywood.

                      Thanks for the good feedback.

                      Craig
                      Craig, there is a type of plywood that is not made of plies. It's called "lumber core" plywood. The core is made of a cheaper grade of solid lumber for a core, stuff that was usually a cheaper grade of lumber, such as luan, or whatever the mill had a lot of. Then the face veneer was applied. You don't see much of it anymore, although it's still available from my supplier and I imagine many others as well - just not as common as it used to be. It doesn't have the stabilty we like to get from "reglar ol'plywood".

                      There is also a variant that uses an MDF core. It's very stable, but I don't think they had that 32 years ago.

                      If you have lumber core and the joint will happen along the long grain of the core, you can just go right ahead and glue it with your biscuits and it will be fine. Screws or spines or all the rest of the ideas will work fine too, but the plain butt joint would be plenty strong. Just mill off 5 or 10 thousandths to expose a fresh gluing surface.

                      If you're gluing end grain of the core, no way.... got to go with screws, splines, or the inlaid strips method. Even at that, prime the end grain with glue 15 minutes before the final joint, and use a lot of glue (messy yes, but you need a lot of glue with end grain). Or epoxy.

                      I would still go with coarse thread kreg screws. The core is usually not a great grade of lumber...

                      Good luck to your son!

                      Yes JC, I'm ignoring you.
                      Last edited by Andy_M; 01-24-2010, 09:10 PM.

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                      • #12
                        Re: Using biscuits in plywood butt joints

                        Sounds like blockboard to me:

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                        • #13
                          Re: Using biscuits in plywood butt joints

                          Yup, that's lumbercore plywood. Never heard it called "blockboard" before, but that's the stuff.

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                          • #14
                            Re: Using biscuits in plywood butt joints

                            Are you talking about using those "wooden" type biscuits, oval shaped--or the one's my wife made last Saturday morning? David

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