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Thread: Effect of wood expansion on design

  1. #1
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    Effect of wood expansion on design



    Hi guys, I'm working on a design for a coffee table with a lift top at the request of my wife, who is threatening to buy something from Ikea if I don't get this done. I've done a good amount of wood working, but this is my first attempt at designing and building a nice piece of furniture. Other than screwing on the top to the lift mechanisms, I want to use only traditional joinery techniques. I was hoping you guys could help make sure I'm doing this right.

    If you take a look at the picture, you will see the joints that I have questions about. Below are the types of joints I think belong in each location, but like I said, I'm inexperienced in furniture building, so I'd really appreciate someone pointing out any issues they see. Please note that there will be a web frame (is that the right term?) on the top of the case that mirrors the one on the bottom (to provide drawer kickers and reinforce the top of the front and back panel). Also, this will be made of solid hardwood (mahogany or walnut) and the front panel is one piece because I don't want to see any seams.

    1) I would prefer to do a sliding dovetail here to pull the front and back together, but I think it would take away too much material from the sides (3/4" thick), so I plan to do a mortise and tenon. The board running from front to back in this joint is 1" thick to better support the weight and racking of the lift mechanism when lifted.
    2) mortise and tenon - My question is, should the joint be a bit loose to allow for expansion of two rails (not sure if that's the correct term) running left and right in the front and back of the case?
    3) mortise and tenon - Same reasoning as #1
    4) dovetail - The front and back rails will pull in the two boards that the lift mechanisms are attached to. My thinking is this will hold better when dealing with racking forces if the top is moved side to side when lifted.
    5) I plan to have this rail pushed against the front and back but not glued or otherwise joined to the panel. My concern is that it will expand and push against the front and back panels and potentially crack them or more likely, open the exterior miter joints between the front/back and side panels. Would it help to put a dado into the front/back panels to allow for expansion?

    I definitely appreciate any help on these issues and if there's anything I'm missing here, please let me know.

    Thanks,
    Ben

  2. #2
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    In my opinion you shouldn't have to worry about wood movement in any of those joints, if the wood is properly dried to start with, that is 7-8% moisture. None of your pieces are wide enough for seasonal changes to affect the joints enough to be a problem. All of your pieces have the grain running in the same direction and pretty narrow in width. I've built many pieces of furniture with similar internal frames and they are all still functioning with no joint failures. I would advise coating all of the internal frame with finish to slow any moisture changes that might occur. However your top, if it is going to be one solid piece will expand and contract seasonally enough to be concerned with, so be sure to account for that movement in how it is fastened to your lift mechanism.

    There are tables that will tell you how much different types of wood react to seasonal moisture changes depending upon where you are in the world.
    Last edited by Lee Schierer; 09-13-2020 at 6:46 AM.
    Lee Schierer
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  3. #3
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    I agree with Lee. My question from a design point is why the lift top? Seems when you lift the top, all you see is the upper web frame and inside of the drawers? Is it that you want to access the drawers by pulling them out and from the top?

  4. #4
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    Thanks for the help guys. I plan to account for top expansion by making sure the lift attachment plates have elongated screw holes in the direction of expansion.

    Quote Originally Posted by Phil Mueller View Post
    I agree with Lee. My question from a design point is why the lift top? Seems when you lift the top, all you see is the upper web frame and inside of the drawers? Is it that you want to access the drawers by pulling them out and from the top?
    The lift top turns it into a table for eating or working while sitting on the couch (see picture). We previously had an Ikea coffee table that did this and it was so convenient. The drawers will pull out from the front, and for the most part, the top will be down and it will be used as a normal coffee table.

    Top up.jpg

  5. #5
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    Ahhh, thank you. Neat idea.

  6. #6
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    Quote Originally Posted by Benjamin Ellis View Post

    1) I would prefer to do a sliding dovetail here to pull the front and back together, but I think it would take away too much material from the sides (3/4" thick), so I plan to do a mortise and tenon. The board running from front to back in this joint is 1" thick to better support the weight and racking of the lift mechanism when lifted.
    You can do an effective sliding dovetail on 3/4" material. Cut the DT 3/8" deep. With thin material that you are using, that will provide more than enough mechanical interlock for your project.
    Brian

    "Any intelligent fool can make things bigger or more complicated...it takes a touch of genius and a lot of courage to move in the opposite direction." - E.F. Schumacher

  7. #7
    The basic design is flawed, as you are trying to cantilever a table top off an already top-heavy base. This type of top is normally cantilevered off a very substantial base that can be further weighted with stored items.

    Additionally I think you are making it unnecessarily difficult by calling for a sliding dovetail.
    "Anything seems possible when you don't know what you're doing."

  8. #8
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    Quote Originally Posted by andy bessette View Post
    The basic design is flawed, as you are trying to cantilever a table top off an already top-heavy base. This type of top is normally cantilevered off a very substantial base that can be further weighted with stored items.
    I think this is a valid concern, which I have put to the back of my mind until now. The table is 20" high and 22" front to back (tough 16" deep at the base). The top lifts about 5.25" and extends 10" forward (12" in front of the legs). I have seen other coffee tables on the market that are similar in size and weight that are just standard leg and apron tables with no base and the reviews don't mention tipping. But I would feel more comfortable if I had a way to calculate the weight at which it tips. I know there's a way to do it, but it's been a long time since I took physics. Do you have a resource for this?

    If there's a real tipping risk, I guess I could put some counter weights under the cabinet or a fold down leg under the top.

  9. #9
    It is a simple matter to estimate the center of gravity of the 2 components: table top and base. Then plot those points to see how initial stability has been affected by the cantilevered top. But the worst case is when people actually use that cantilevered surface for support of either their things or themselves.
    "Anything seems possible when you don't know what you're doing."

  10. #10
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    When figuring the tip over point you have to add in the effect of items that start sliding in the drawers which could double the tip over issue.

  11. #11
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    Quote Originally Posted by andy bessette View Post
    It is a simple matter to estimate the center of gravity of the 2 components: table top and base. Then plot those points to see how initial stability has been affected by the cantilevered top. But the worst case is when people actually use that cantilevered surface for support of either their things or themselves.
    Do you think it would make sense to move the lift mechanisms toward the back of the case so that the weight of the extended top would be focused at a point opposite the front of the top (where people will be putting things and their weight), or would the lever created by moving the top's point of attachment toward the back of the top offset the advantages gained from the shift in COG?

  12. #12
    Quote Originally Posted by Benjamin Ellis View Post
    Do you think it would make sense to move the lift mechanisms toward the back of the case so that the weight of the extended top would be focused at a point opposite the front of the top (where people will be putting things and their weight), or would the lever created by moving the top's point of attachment toward the back of the top offset the advantages gained from the shift in COG?
    The location of lift mechanism will not affect the instability one iota.
    "Anything seems possible when you don't know what you're doing."

  13. #13
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    Can you help me understand why that is? It seems like it would shift the COG toward the back, requiring more tipping force from the front (where any tipping force would occur) to tip the table.

    Do you think putting a 50-100lbs counterweight toward the back, but inside the cabinet, would prevent 50lbs or less put on the extended front from tipping the table forward?

  14. #14
    Quote Originally Posted by Benjamin Ellis View Post
    Can you help me understand why that is? It seems like it would shift the COG toward the back, requiring more tipping force from the front (where any tipping force would occur) to tip the table.
    That does not shift the CG (except for the weight of the mechanism itself).

    Quote Originally Posted by Benjamin Ellis View Post
    Do you think putting a 50-100lbs counterweight toward the back, but inside the cabinet, would prevent 50lbs or less put on the extended front from tipping the table forward?
    That would increase the initial stability with the table extended, but makes the whole even more top-heavy. And worsens initial stability with the table stowed.
    "Anything seems possible when you don't know what you're doing."

  15. #15
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    Quote Originally Posted by andy bessette View Post
    That does not shift the CG (except for the weight of the mechanism itself).



    That would increase the initial stability with the table extended, but makes the whole even more top-heavy. And worsens initial stability with the table stowed.
    Got it, thanks for the help.

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