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I am making two matching table tops out of 5/4 ash. This is my first attempt at table tops. The boards were straightened and jointed (after straightening they are 7/8" thick). I thought I had done a good job with the glue-up using cauls to keep everything flat for 24+ hours. After glue-up I cleaned up the surface and joints with a jack plane. But then I got busy and left them alone laying flat on my assembly table for a month. When I returned to finish them I noticed a 1/8" bow in each table. Why did they bow? Is it because I didnt protect the wood by finish it and they absorbed moisture in the air (or maybe dried). Or was there something wrong with my glue-up?

Are there any tricks to flattening them or must I resort to the very labor intensive jack plane?
 

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All the above. Laying them flat would cause more moisture to absorb to one side vs the other. I typically get a finish on within a few hours if possible to prevent that. How big are the tops? An 1/8 bow isn't to bad and depending on their size maybe fixable when attached to base or you could try to acliment the opposite side.
 

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Could potentially be lots of root causes, like internal stress. Most likely root cause is the top side lost more moisture than the underneath which was in contact with the assembly table.

What is the wood? Was it kiln dried? Did it have time to acclimate to your shop?

If you are lucky you could try putting the bowed side down on the assembly table and see if the tops get any flatter by themselves.

If the wood has not reached equilibrium with your shop, there is the potential for more movement.
 

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They warped because of the way you stored them for that long period. A glued up panel should never be placed flat on another flat surface. That causes an unequal moisture content in the two surfaces of the panel which leads to warping. Always sticker or set the panes on edge so that air can freely get to both sides.

Sometimes, you can now sticker the panel in an area where there is some air movement and the two surfaces will equalize and the warping may be eliminated or, at least, minimized.
 

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I had a similar issue a few months ago. I was flattening a case top after glueup. I got one side flat and flipped it over to work on the other side. I ended up leaving it flat on the workbench for a week or two. When I got back to it, It had cupped pretty bad, if I remember right it was significantly more than 1/8 inch. I started, by flipping it over for a few days. Then, I stood it up on end so that it could get even air on both sides. I'm pretty sure it was less than a week and it had straightened itself out. (I probably could have skipped the first step, but I figured it would speed up the process. I did the second step for two reasons. One, to know that the warping was only caused by uneven moisture loss and it would stabilize to the flat condition. Two, so I wouldn't over-correct and end up with it warped the other direction. )
 

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I need to throw in too... Finish both(all) sides, and at the same time or close to one another. If you did the face( good side) and waited a few days to do the underside, you may be back to square 1. I usually finish the underside first, flip it. Then do the face.
 

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They warped because they were laid flat where air couldn't get to one side. You could stack them with sticks or dowels between them so air could circulate around them until the flatten again or if you are in a hurry you could wet the cup side of the top with water to balance the moisture content. Just be sure when you get them flat you mount them to the table soon and finish both sides.
 

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This wood was kiln dried and in my shop for about 4 weeks before I started to work with them. How long should I let boards acclimate?
4 weeks sounds reasonable. The unknown is the difference in moisture content of the as-received wood and your shop. The greater the difference the more time you need to wait.

No easy calculation, too many variables.

Air dried lumber will have higher moisture content than kiln dried.

Without a moisture meter, the best way to know is to cut off a small piece, weigh the piece every week and note the data. When the weight stops reducing, you are at equilibrium.
 

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Look at the ends of the panels. If there are boards with significant arches or curves indicating they are from near the center of the tree, there is a certainty that there will be a cup in the top with even a modest change in humidity. Some workers put all the curves the same way to allow forcing it flat.. Some alternate the curves and live with a slight wavey top when the moisture content/humidity changes.
If this is not the case , then the issue of change in humidity on one side greater than the other would give a temporary cup. This will correct by returning to the original humidity on both sides.
A small cup can be pulled flat and fixed with the hold down system. If you can push the table flat with 20-40 pounds its not a bid deal. There are other options but they involve more info and work.
Keep us informed.
 
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