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  • How thick of tennon?

    What thickness of tennon/mortise would you recommend for 7/8" stock?

    I was thinking I'd use 1/2" thick, likely about 1" deep, but wanted to ask before cutting into my pieces. I need considerable strength at the join.


  • #2
    Re: How thick of tennon?

    Normal rule of thumb is one half the thickness of your stock. In your case this would be 7/16ths.

    Big G


    • #3
      Re: How thick of tennon?

      I would do a 3/8" tenor with 1/4" left on all side's.


      • #4
        Re: How thick of tennon?

        The "1/2 thickness" rule that Big G mentioned gives pretty close to optimum strength. If you decide to go up to 1/2 as you planned, it'll be fine. I would go longer than 1" if possible, as this will improve strength. Don't cut the mortise much deeper than the tenon, but leave a definite path for glue squeeze out.

        On big M-T joints like this one, if possible give the joint an extra week to dry before any sanding. The wood will swell due to the moisture fronm the glue. You sand it perfectly, then it shrinks as the moisture evaporates, and the joints become visible. I use West Systems epoxy on M-T joints when I need extra strength. There's no moisture issue with that stuff, as it is 100% reactive (no solvents at all). If your fit isn't perfect, mix in one of their fillers (I use #404 but others work as well) and it will be a great joint.


        • #5
          Re: How thick of tennon?

          Pretty much what everyone else has said, about half the total thickness of your stock. If you want even more strength, you can pin your tenons using dowels, that way the only place you can have a failure is the wood itself.


          • #6
            Re: How thick of tennon?

            Between 1/3 and 1/2 the thickness of the wood. I personally go closer to the 1/3 if both the tenoned piece and the mortised piece are the same thickness (i,e paneled door, etc). 1/2 + if the mortised piece is thicker (like a table leg/apron joint).

            Making the tenon too thick in matched-thickness parts will not strengthen the joint for lateral forces, because you are at the same time making the mortise walls thinner. However, my final determination is based on the tools I have for making the mortise. For 3/4" stock, I go 1/4" tenon. For 1" thick, I go 3/8".

            For 7/8, it would be a toss-up, but I would probably go with a "thick" 1/4 if cutting the mortise with a chisel (allowing more thickness in the mortise walls while chiseling, but would probably widen it to closer to 5/16", 5/16 if using a drill bit, and 3/8 if using a plunge router. I would not go 1/2" unless the mortised piece is at least 1 1/4" thick.

            If the pressure is trying to pull the joint apart, I would also lengthen the tenon to about 1 1/2" if possible, and even consider a draw-bored dowel if the pull is extreme. In this case I would go 3/8 tenon.

            It would help if you could give some more info as to what the joint's purpose is and what material you are using. For instance, I would use a 1/4 - 5/16" tenon in quarter-sawn oak, but would go 3/8 in flat-sawn.


            Last edited by Gofor; 12-31-2009, 10:21 PM.
            Practicing at practical wood working


            • #7
              Re: How thick of tennon?

              1/3 is the general rule