Flattening a bowed tabletop glue up

[Kyle Brookshire]

I have a 72x42, 1.25” thick white ash tabletop that just came out of clamps yesterday - boards are around 6 inches wide. (Started as 8/4, kiln dried S2S) I did the glue up in two panels first, then joined the two larger panels together, using dominos for alignment. The glue joints came out pretty well (for not having a jointer), and individually each seam is quite level from one board to the next.

But when I got it out of the clamps and wrestled it up to the bench, it’s bowed in the middle of the face side about 1/8”-1/4” to each side. The outer most boards seem to stay fairly flat and the bow seems to be most pronounced in the middle where the two “sub” panels were joined.

There’s a lot of conflicting advice on here and elsewhere about a solution; so before doing anything drastic, I flipped the panel over, (now concave side facing up) put it on some stickers for airflow and clamped some cauls on the ends and middle - which seemed to flatten it out pretty well, and pretty quickly. To the point it could probably be fine for use.

Just curious how long I should leave it like this, and if I can expect it to stay flat when the cauls come off? Or if there’s anything I should do or help it stay this way? I’ve seen suggestions of steaming, wetting the concave side, ironing, etc.

Hoping just a day or so pulled flat and letting the moisture level out will do the trick - But want to act now to give the best chance for success.

[Gordon Stump]

In my experience, it will bounce back after the cauls are removed. I breadboard the ends on large tables like that. If you do not want breadboard ends you could design legs with caul type tops and even a hidden one in the middle. It is odd to me for a glued up panel that thick to bow like that. If the edges were only slightly off square it could happen.

Good luck

[Russell Hayes]

If it's just the last joint that caused the problem, I would rip it at the joint and try again. You really need a jointer to prepare the joint for glue up on a job that large.

[John C Cox]

Wood can move a lot during glue ups. I would let it acclimate and see what happens before you go making big changes.

If all the boards started out pretty flat and you're sure your jointer is square, I might be tempted to wipe down the concave side with a wet rag and let it sit a couple days.

If it's still like that after a while - find a local cabinet shop and pay them to run it through the wide belt sander.

[mike calabrese]

Usually using the opposite clamp energy as you have already applied works better if the work allows to "bend" the works a little beyond the flat condition. Expectation is it will bounce back some and not over correcting may result in some gain but not eliminate the entire condition.
You also might try adding sone hot or warm water to the conves side then clamping as you described. This allows the wet side to stretch some additional compared to a dry clamp up. Be careful that your wood glue joint will not be comprimised my the added moisture.
Titebond III if youe used that product would be pretty resistant to the added moisture.
Lastly as suggested above he best fix might be to rip the top at the central joint , joint the edges clean and and reclamp . It would be best if you dry fit any re-rip / clamp process by carefully checking the fit of the new joint. The concept here is to apply as little reclamping pressure as needed to bring the joint tight.
If you use high clamping pressure to compel the joint tight you most likely end upp right back where you are.

Be sure to use as many clamps as needed alternating them top and bottom of the piece so not to induce all the energy in one "bowing direction".

lastly here are some links that may be of some help...Youtube is your friend....
calabrese55

https://www.youtube.com/watch?v=2Pd6rEbkfo0
https://www.youtube.com/watch?v=MNCHVzBZH3Y
https://www.youtube.com/watch?v=9V9O06-AxiU
https://www.youtube.com/watch?v=FqQUlLJOTMg
https://www.youtube.com/watch?v=jjmcYHeBv4Q
https://www.youtube.com/watch?v=2Pd6rEbkfo0

[Kyle Brookshire]

Thank's for the reply, guy's - helpful advice. I'm definitely of the mind to not do anything too quickly and see how it works out before cutting/re-gluing, installing C-channel, etc. I'll probably wet down the concave side today, being careful of the glue lines and let that sit in the clamps for a day. I took some readings with a basic two-pin moiusture meter and the convex side was 1-2% higher than the other - though readings varied widely all across the top. (6%-8.5% vs 7%-10% more or less).

A couple quick questions in response to some of your suggestions:

- I don't have a jointer, but I do have a track saw: do ya'll think that would give me a good enough glue line down the middle if I went that route? This would only cost me an 1/8 inch of
width and some time. I do think too much clamping pressure probably caused this - trying to close up a less than perfect joint.

- What would the minimal width be for breadboard ends? I can't add too much to the length, its custom fit for a banquette-type space in my kitchen/breakfast area. But, I could probably add five or six inches overall if necessary. And I think with a Domino machine it's a pretty easy task.

- I'll attach a pic of the design and the base - is that what you mean, by "caul-type"? The top would be held down at the ends, and possibly along the side and middle stretchers if
necessary. Was going to use figure 8's; but threaded insets may be better if I go this route given this issue.


Here's the design - maybe it's sufficient on its own to hold it down flat?
508902

[Kyle Brookshire]

And some pics of the "before" to give ya'll an idea

508903508904

[Russell Hayes]

- I don't have a jointer, but I do have a track saw: do ya'll think that would give me a good enough glue line down the middle if I went that route? This would only cost me an 1/8 inch of
width

No, if you just put the track on it and cut again it will duplicate your problem. If you can't beg, borrow, or steal time on a jointer, split it with the track saw. Then take one half and make another cut (just a kerf wide), then flip the other half and make another new fresh cut. That way if your track saw is off a little it will equal out when you reglue it.

[Andrew Hughes]

I would like to take my turn at guessing why the your top cupped. When creating edges square to the faces the faces need to be flat. What we don’t want is to add 2 or 3 degrees to each edge something hard to detect.
Could it be that several joints in the middle are slightly coopered ?
I also think your base might be enough to hold it flat it looks robust.
Good Luck

[Michael Burnside]

- I don't have a jointer, but I do have a track saw: do ya'll think that would give me a good enough glue line down the middle if I went that route? This would only cost me an 1/8 inch of
width and some time. I do think too much clamping pressure probably caused this - trying to close up a less than perfect joint.


It's more than 1/8 of an inch it will cost you. You'll have to rip one side up and another side down in at LEAST two places, maybe a couple more. If you just rip once with the track saw you'll be back to the same problem just 1/8 narrower table :)

I'd suggest going with your current proposal and see what happens.

Years ago I built a few table tops with just a tracksaw, before I got my bigger tools, like a jointer. The trick is to rip each edge using the "up-down" method. Basically align your boards how you want and then mark each mating edge, one with U and one with D where they come together. Then rip those edges with your tracksaw as indicated. Up is facing you and the track when you rip and down is facing down to your bench. This way you'll create complementary angles. Even if your tracksaw is 89.5 degrees vertical, you'll get a perfect glueline. You can do the same thing on a tablesaw. Similarly with a Jointer but facing in to the fence or away (a lot of people call this the "in-out" method and I use it for EVERY glue-up). Jointers generally just provide a much smoother finish.

Why this happened, who knows. Far too often people clamp way too hard to overcome something they should have fixed in the build process. Maybe that's what happed here or maybe you just got unlucky. Sometimes, well, wood moves...

[Mike Cutler]

Kyle

We've all been there. Everyone that has ever tried to glue two boards together ends up with a cup every now and then. Even if you do everything "right".
I do not believe that you can "pull" that glueup into flat. I've tried, and in the end it was an exercise in futility. I'm sorry, I know that is not the answer you're looking for.
Can you "joint" and edge with a tracksaw? Yes, you can, and if you prepare for it well enough it will be seamless.This is how it is done.
-Rip the seam the was the last glueup right down the glue line.
- Push the two edges back together on a flat surface. There will be a gap, but don't worry about it.
- Clamp both halves flat to the bench, with the seam butted. There should still be a gap. Don't clamp them to each other, or it won't work at all.
- Run your track saw right down the middle of this "gapped" seam.
- When you get done, if the gap is less than the width of a blade, you will have two sides that have complimentary angles and will butt up against each other perfectly. If you still have a slight gap, do it again.
- It's critical than the two halves cannot move apart during this process.
I did a 9' long, 2" thick, 40" wide, padauk glueup, this way.

[Thomas McCurnin]

No jointer? No problem.

For under $150, buy a jointer plane, yes a long number 7 or 8, the longer the better, hand plane. These things are obsolete in the minds of many people, and are single use tools, so I've found that you can get one for $50 or so in a flea market or craigslist.

[mike calabrese]

if you saw cut the top you can do a lot to straighten the cut with a 100 grit sandpaper double sided taped to the longest flat board or metal flat or 2 foot or longer level to dress the edges. Check the match as you go so you don't sand any longer than you need to.
Also you can stack the cut pieces top to top or bottom to bottom depending on the bow. clamp the edges as even as you can and go to the sanding block.
any angular mismatch will be complementary when set back edge to edge.
calabrese55

[Gordon Stump]

Breadboards need to slide so..... I do not know about a domino machine but if the top cannot move in the breadboard it will split the top. So maybe not the breadboard idea. I think you could pull that top down some with your base design with 3 lag screws or bolts in threaded inserts in each cross member. Elongated holes at the ends.

[Kyle Brookshire]

Thanks for the candid and honest responses, guys. I suspect some of y’all who suggested that the jointed sides aren’t completely square along the whole length are likely correct. That, and over clamping are probably the biggest culprits, here.

I’m using a Sawstop JSS, but the throat plate is kind of “wobbly”, I’m not sure how else to describe it. This movement isn’t that big a problem on shorter cuts, but on a seven foot rip it adds up. I need to look into a fix, or if it’s an inherent deficiency, an upgrade. My recent search history definitely includes the PCS, but my shop space is limited. (Same reason any more than a bench top jointer is probably in the cards)

For now I’m going to try and work with what I’ve got by adding three pieces of C channel, and hoping the base design is enough to hold it flat, so we’ll see. It’s still in the clamps until the c channel arrives, moisture readings have leveled out, and it seems like I can almost push out the tiny bit of warp that’s left by hand; so the c channel may hold it. If not, I’ll re-rip everything, seek out a proper jointer and add a board to re-gain the loss in width.

Thank you all for the advice. This forum is a great learning resource for me; I try and contribute where I can, but I usually have usually more questions than answers.