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Thread: What joinery method is best for crosspiece support

  1. #1

    What joinery method is best for crosspiece support

    I'm currently working on a mission style couch and plan on having several crosspieces connecting the front and back lower rails. I'm not sure what joinery technique would be best. A traditional mortise and tenon would weaken the rails too much and I'd prefer not to use metal angle brackets. I thought about making the crosspieces a little taller and using horizontal stub mortises at the top and bottom and pinning them with a dowel, but am not sure if that will weaken the front & rear rail too much as well. Suggestions will be welcome. The front and back rails will be 1" thick and 6" tall in white Q-sawn Oak.




    pic.jpg

  2. #2
    Join Date
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    Jim, what about a sliding dovetail. Since it is so short it does not need to be tapered.

    Link: http://www.inthewoodshop.com/Furnitu...IEndRails.html





    There is nothing special about the sliding dovetail on the lower rails. They are short and, therefore, were left parallel ..





    The top rail is simply dovetailed in




    Regards from Perth

    Derek

  3. #3
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    Many of what I have seen have the cushion sitting on top of the back rail. The slats run behind the cushion. If you want a rail showing you add another slat rail. Your picture shows the cross piece in the lower part of the rail. It should be at the top of the rail, top 1/3 or so. If done that way when loaded the tenon is in compression at little cost to the rail. If done in the lower part the mortise opening is in the tension section of the rail. If you make the cross piece as wide as the rail than the mortise would indeed weaken the rail. The other way is to use a ledger on the rail and slats for the cross pieces. You could use sliding dovetails as Derek suggested but need them in the top part of the rail and you have a thru opening in the top instead of a closed mortise.
    Jim

  4. #4
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    What is the purpose for the cross rails. Are they supports for the seating or merely intended to keep the rails from spreading? If its the latter, then not much force will be required. If they are supports for the seating then the shear forces applied by the occupants need to be factored into tenon sizing.

  5. #5
    A little more information.

    1) The crosspieces serve two purposes. To hold the rails together, and to support the cushion frames the webbing will be attached to. (The webbing is the seat-belt looking material woven in fore and aft, that the cushions rest on.)

    2) I plan on having the frames sit a little below the top of the rails (1/2-1") in order to avoid a visual "reveal" between the cushion and frame.

    3) The crosspiece portion of the design is not finished (obviously ) It can be located so that the frames sit on top, or rebated on each side, so it's top surface is on the same plane as the frames top surface. If the latter is best, I could also just screw in ledgers on the crosspieces like shown on the back rail in the additional jpeg shown below

    A sliding dovetails sounds like nice way to solve the problem, except I do not know how to keep the joint from being visible, assuming you would cut the dovetail in the rail starting at the top.

    The additional jpeg offers a little more detail.

    pic2.jpg
    Last edited by Jim Foster; 02-10-2019 at 10:58 AM.

  6. #6
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    If the mortise and tenon are sized properly they will weaken the rails less than sliding dovetails in my opinion.
    Bumbling forward into the unknown.

  7. #7
    Brian,

    Can you suggest how I should size this? As I mentioned, the rails are 1"x 6" and the crosspiece is flexible in how it's sized. I know that for a beam in bending, the top and bottom of the beam take the load in compression and tension, and the center of the beam has zero load. I've got it in my brain, that if a mortise is used, it should be pinned, to keep the joint tight over time, but maybe not. I could get creative on how to pin it, but am interested in advice on this. It's a fairly big project for me, and almost every joint is exposed.

    Quote Originally Posted by Brian Holcombe View Post
    If the mortise and tenon are sized properly they will weaken the rails less than sliding dovetails in my opinion.
    Last edited by Jim Foster; 02-10-2019 at 11:17 AM.

  8. #8
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    On second look the skirt is pretty light duty.

    On beds I run a heavy frame around the inside of the rails, the frame can be inserted into a groove, this groove bears the load and places it more equally around the piece. It stiffens the skirt against bending. Thin pieces break by bending and twisting.

    The cross pieces can be joined to this frame via mortise and tenon. The corners can be made as bridle joints.
    Bumbling forward into the unknown.

  9. #9
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    This is an example from my recent work.



    Bumbling forward into the unknown.

  10. #10
    Looking at the photos Brian provided and the advice so far, I think wedged through mortise & tenon joints might work well. Brian's photo shows a good example of this in the same orientation I would expect to use.

  11. #11
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    That's a good way to go, if you want a little more strength, add glue blocks. If they will be visible, get creative and make them a feature. Good spot for a little artwork.

  12. #12
    I suggest a simple solution: make a separate frame that fits inside the "show frame" and attach it with screws or rest it on cleats that are screwed to the inside of the lower rails. Quick and easy as well as how it was done "back in the day"

    Bob Lang

  13. #13
    Bob,

    I think your right, I like the idea of trying a joint I have not used yet, but what I was planning makes assembly very complicated. The cleats sound like a great way to solve this.

    PS: I have the book on mission furniture you put together a few years ago am planning a number of projects from it. Thank you for sharing your knowledge and passion.

    Quote Originally Posted by Bob Lang View Post
    I suggest a simple solution: make a separate frame that fits inside the "show frame" and attach it with screws or rest it on cleats that are screwed to the inside of the lower rails. Quick and easy as well as how it was done "back in the day"

    Bob Lang

  14. #14
    Sorry to kill the fun everyone was having trying to complicate this.

    Bob Lang

  15. #15
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    A cleat is best with a groove to locate it, especially if it’s going to be screwed in place.
    Bumbling forward into the unknown.

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