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Thread: how to affix bench seat and back rest (for wood movement)

  1. #1
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    how to affix bench seat and back rest (for wood movement)

    I'm nearly finished with a bench for the dining table in our sunroom, and have some questions around affixing the seat and back rest. All parts are poplar.

    The back rest is 1.5" thick x 6" wide x 7 feet long, and the seat is 1.5" thick x 12" wide x 7 feet long.

    In both cases, a 1/4" deep dado in the back rest / seat will fit snugly over the back support / seat support.

    My concern is that if I glue the dado connection, wood movement will cause problems over time. So I'm instead considering drilling oversized holes through the back support / seat support that would allow fasteners to move (and then I would plug over those holes).

    Am I being too worried, or is this the right approach? (or is another approach preferred?)

    Thanks!
    - Bob R.
    Collegeville PA (30 minutes west of Philly)

  2. #2
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    Some photos would be helpful for us to actually visualize what you are asking about...
    --

    The most expensive tool is the one you buy "cheaply" and often...

  3. #3
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    The two spots where you see the green tape are where the back rest and seat will be affixed.

    If this were a table, this would be like the top sitting on the apron... which I would do with clips to allow movement. But clips in this case would be visible / not what I'm after.

    If I glue the seat and back rest to these spots, I'm worried it will not allow for wood movement.

    Does this make sense?

    dining bench 1.jpg
    - Bob R.
    Collegeville PA (30 minutes west of Philly)

  4. #4
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    Donít glue it.
    Use a mechanical connection allow some space for the seat part to move and the back rest.
    Very stylish bench Bob.
    Good Luck
    Aj

  5. #5
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    Thanks Andrew! I'm glad I confirmed my concerns.

    I think I will drill stepped holes through the seat support and back support (aka the areas with green tape in the pic above) - the outer hole would accept a plug, and the inner would be oversized to allow the fastener some movement. The fastener would go through the hole and into the no-show side of the seat or back rest respectively.
    - Bob R.
    Collegeville PA (30 minutes west of Philly)

  6. #6
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    Is there a mortise for a stretcher not shown in this photo? Simply insetting the back and set into a dado is not going to provide much lateral strength.
    I would mortise the seat and back rest into the supports and then add a stretcher on the lower part of the rear leg to get some lateral strength into this bench. You may have to get creative to raise the leg assembly to account for the offset of the bench seat otherwise the seat might get too low for comfort.

  7. #7
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    Hi Dwayne -
    I'm either going 1 of 2 ways on this (would love opinions)

    Diagonal brace from front leg to underside of the bench (on both left and right side) to prevent racking.

    or

    Stretcher from front leg to other front leg.


    I thought the diagonal would look more inline with the design as I would cut a gentle arc to match the rest of the piece.


    (any which way, I haven't cut those mortises yet)
    - Bob R.
    Collegeville PA (30 minutes west of Philly)

  8. #8
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    The seasonal moisture changes for the back will be minimal Assuming a 5% moisture change in your house and that the screws are located 1" in from each edge of the back.
    Seasonal Dimensional change = 5" x 5% x .00263 = .0657 (1/16") inches of change between the holes. For the 12" seat the over all dimension change would = 12 x 5 x .00263 = .158 inches or about 5/32" outside to outside.
    Lee Schierer
    Captain USN(Ret)

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  9. #9
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    Thank you Lee! I bookmarked that link for future. The wood movement being as minimal as this helps me understand how much "wiggle" is needed for each of the fasteners.
    - Bob R.
    Collegeville PA (30 minutes west of Philly)

  10. #10
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    Quote Originally Posted by Bob Riefer View Post
    Thank you Lee! I bookmarked that link for future. The wood movement being as minimal as this helps me understand how much "wiggle" is needed for each of the fasteners.
    You just need to figure out the max an min moisture levels in you house from season to season. I used 5% humidity variation, but your location may be different. The Philly area can be pretty humid in the summer.
    Lee Schierer
    Captain USN(Ret)

    My advice, comments and suggestions are free, but it costs money to run the site. If you found something of value here please give a little something back by becoming a contributor! Please Contribute

  11. #11
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    I would not put a stretcher at the front leg due to potential interference with the seated person's legs. The back leg is pulled in so far that a diagonal brace from it or the lower foot at the rear leg to the bench might look and function best. The brace could have a curve and angle to complement the rear leg. Thinking about it for a moment, the brace could be an arch from end to end of the bench. The arch could be fabricated by laminating (brick lay pattern) small-ish sections of 3/8" thick stock to approximate the general shape from which the arch could be cut. Mortise into the feet and into the bench then connect with floating tenons. Huge strength and complementary design.

  12. #12
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    To attach back, I made a stopped dado. So the back rest kind of hangs on the support. Dado itself was made less snug.

    I used epoxy (a blob towards top) and screws in oversized holes. Bench survived for a year at least outside and showed no signs of problems. Sold the house and left the bench back there so don't know the current state.

    You can see some photos at https://sawmillcreek.org/showthread....nch&highlight=

    Seat part is attached the same way.

  13. #13
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    Dwayne I like the way you are thinking. How about running the arch from the back leg just below the seat support joint up to the back rest and back down on the other side. It could be 1" x 3" and made as a glued lamination for better grain display. Probably need a regular tenon if only 1 x 3. The challenge of chopping angled mortices would be a new learning experience, at least for me.

  14. #14
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    An update... My first version (pictured above) has turned out to be the prototype, and I'm onto version 2 today.

    Long story short, I made the bench too tall by accident, and any efforts to shorten are only going to cheapen the look I'm after. I'm reminded that progressing at just about anything in life entails a willingness to fail and try again. So, I plan to re-use these for something else (tbd) and rebuild. Thankfully only using poplar so price impact is not too bad.

    (btw... the seat and backrest can be re-used for version 2)

    What I did learn is that with minor adjustment, the design can accommodate a simple stretcher under the front of the seat. Basically, if I make that front leg joint slightly bigger, I will be able to fit a stretcher there.

    To make the stretcher blend with the design, while adding the desired racking resistance, the plan is to start wide at the point where joined to the legs with an arc across. When I blend the joints (power carving) the front leg will essentially look like it has two tree branches growing from it... one connecting to the back leg, and the other connecting across.

    Rudimentary drawing:
    dining bench stretcher sketch.jpg
    - Bob R.
    Collegeville PA (30 minutes west of Philly)

  15. #15
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    Yeah, I've built a few chairs and the prototypes usually fail the sit test. One now serves as a plant stand.

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