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Thread: Building a Table Top - Dealing with Bowing

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    Building a Table Top - Dealing with Bowing

    I purchased some 1.75" thick air dried black walnut planks from a friend of mine for a great price. I was hoping to make a dining table top out of the planks, but one or two of them have a slight bow in the middle of the planks? Slight here means roughly 1/4". The overall length of the planks is around 7ft. All of the planks were run through a planar at some point and are a consistent thickness. My question is what should I do about the bowing in one or two planks? If I had more stock, the obvious thing to do would be to swap them with straighter stock, but that really isn't an option. Given that I attempt this project with what I have, what is the best strategy for coping with the bowing? Note, I don't have much in the way of power tools to help - just a benchtop planar - which of course wont take out the bow by itself. If I use biscuits or splines, would that be enough to align the top so that the top is flat enough to make it work? Would cauls help?

  2. #2
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    Just my opinion, but I don’t think 1/4” over 7’ is much to worry about. I assume you can press out the bow against a flat surface? When doing the glue up, yes I would use cauls.

    Or, I have had luck adjusting boards as I clamp. It may not be the way many do a glue up, but it has worked pretty well for me. I clamp one end even, then move about 24” in with a clamp, adjust the boards until they are even in that spot then clamp tight enough to hold them in place, then proceed in this way every 24” until you reach the opposite end. I then go back and add a clamp inbetwen each clamp over the boards (so you have the original clamps under the boards, adding clamps over the boards). Then go back and tighten everything. I don’t use cauls when I do it this way.

    I might also consider gluing one bowed board to one straight board in sets first, then glue the sets together.

  3. #3
    Assuming the boards were at equilibrium when you bought them, the boards likely dried out over winter and cupped or bowed a little. If you live in an area where humidity changes winter to summer (nearly the whole USA) this will happen. One of the main things in furniture design, especially tables, is needing to take into account this summer to winter humidity and moisture content change. If you can get the top to glue up flat with biscuits that will probably work fine for assembly to the lower structure.

    If your top is 1.75 thick, you will need to make sure the rest of the table structure is strong enough to resist the top cupping and bowing throughout the seasons. One thing you will not be able to resist though is movement in width of the table top. That you will need to account for in design, with something to allow the table top width to move roughy 1/4 to 1/2 inch per foot from winter to summer. Finish will help some, but no finish will completely prevent moisture content change and wood movement; it will only slow it a bit.

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    Use the two boards. Put them towards the middle but not together nor on the edges. You should be fine.

  5. #5
    Quote Originally Posted by Andrew Seemann View Post
    Assuming the boards were at equilibrium when you bought them, the boards likely dried out over winter and cupped or bowed a little. If you live in an area where humidity changes winter to summer (nearly the whole USA) this will happen. One of the main things in furniture design, especially tables, is needing to take into account this summer to winter humidity and moisture content change. If you can get the top to glue up flat with biscuits that will probably work fine for assembly to the lower structure.

    If your top is 1.75 thick, you will need to make sure the rest of the table structure is strong enough to resist the top cupping and bowing throughout the seasons. One thing you will not be able to resist though is movement in width of the table top. That you will need to account for in design, with something to allow the table top width to move roughy 1/4 to 1/2 inch per foot from winter to summer. Finish will help some, but no finish will completely prevent moisture content change and wood movement; it will only slow it a bit.
    Yes, they were at equilibrium. They had been in the back of a garage since 08. Moisture meter was reading 7%.

    Great point about movement along the width. I failed to mention it, but I was also planning on doing breadboard ends.

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    John, 1/4" over 7' is nothing really, as someone said above. I've glued up boards more cupped that this.

    I have taken two cupped boards, straightened their edges with a track and saw, then sister'd them bowing in opposite directions using dominos to align them. You can do the glue-up in stages, a couple of boards at a time.

    I take the completed top to a wide belt sander, and once it's attached to it's base the tables are fine.

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    If you're planing these to a finish then you need to align them both so that the grain is running in the same direction (rising in the same direction so that they both plane the same way). If one is up and the other is down, in that arrangement, then glueing up like that can minimize the effect.

    If they are both in the same direction, then I would flatten them before gluing, since they have been sitting for so long and have only moved 1/4" you can be relatively assured that flattening them again will result in a fairly stable table top. Since they're 1/4" out you wind up with 1.5" thick tops, which is plenty thick.
    Bumbling forward into the unknown.

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    Quote Originally Posted by JohnM Martin View Post
    I purchased some 1.75" thick air dried black walnut planks from a friend of mine for a great price. I was hoping to make a dining table top out of the planks, but one or two of them have a slight bow in the middle of the planks? Slight here means roughly 1/4". The overall length of the planks is around 7ft. All of the planks were run through a planar at some point and are a consistent thickness. My question is what should I do about the bowing in one or two planks? If I had more stock, the obvious thing to do would be to swap them with straighter stock, but that really isn't an option. Given that I attempt this project with what I have, what is the best strategy for coping with the bowing? Note, I don't have much in the way of power tools to help - just a benchtop planar - which of course wont take out the bow by itself. If I use biscuits or splines, would that be enough to align the top so that the top is flat enough to make it work? Would cauls help?
    I almost hate to be the one to suggest this since we're on a neander forum, but: At that thickness you can in fact use your lunchbox to plane the bow out. The way you do it is to:

    1. Create a ply sled that's a little longer and wider than the board to be jointed, and securely fix the board in the middle of the sled (plenty of ways to do this, pick one) with its concave face down so that there's a gap in the middle.
    2. Insert shims between the sled and the board to prevent the planer's infeed/outfeed rollers from flattening the board against the sled. The spacing between the shims depends on the relative stiffness of the board and the infeed/outfeed springs. For a 1.75" board you can probably get away with a shim every few inches without too much waviness, but you'll want to experiment a bit. You'll be able to tell from the first couple passes if you don't have enough shims, and any waviness at that point will be entirely in the waste anyway.
    3. Feed the whole mess through your thickness planer until the top/convex surface is flat.
    4. Remove the board from the sled, flip it, and plane the concave side flat. No sleds or shims required.

    You can actually correct for just about any shape that way, limited only by your creativity and accuracy in the shimming step. Beware of loose shims, though. You'll want those positively affixed to the sled (if I were to hypothetically do such a thing I might use a nailer).

    You don't have to start with the concave face down, obviously, as it's a simple matter of where the shims go.

    Of course if it were me I'd hand-joint the board with a Jack and a #8. It's woodworking and workout, all rolled up into one :-).
    Last edited by Patrick Chase; 02-13-2018 at 2:00 AM.

  9. #9
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    I am assuming that these boards are bowed along their length and not cupped as some here may have spoken to. The way I have successfully dealt with this problem is to route a channel along each edge that is to be joined, then use a matching spline to align the boards during glue up. Be sure not to cut the channel for the spline all the way to the ends of the boards, or they will show in the end grain. You didn't mention if the top will have bread board ends or not, either way, no problem. The splines help with glue up also. Also, as previous mentioned be sure the have the grain oriented in the same direction to facilitate planing after glue up. Bob
    Life's too short to use old sandpaper.

  10. #10
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    I have straightened boards that were bowed by ripping the board down the middle and then gluing them back together with the bows reversed to each other.
    You will need to place dowels in the joint to maintain the strait finished board. I made board room doors out of walnut one time that was bowed.
    When I finished, the doors were straight and plumb.

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