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I recently attempted a panel glue up consisting of 7 2"x6"x6' planks of reclaimed yellow pine. The boards were planed flat on one side and edge jointed. All went well it seemed during the glue up (i.e., it was flat and tight along the seems). I glued it up in sections a couple of boards at a time over a couple of days. All went pretty well, then when I came back after the last section (3 boards), there was a pretty good bow running down the middle of the top - a hill on top, a valley on the underside running right down the middle. It was flat when I set up the last glue and clamps. Could the bow be because of over tightening the pipe clamps?

Glue up strategy was pipe clamps alternating on top and bottom every foot. I also used biscuits.

What else could have contributed to the bow?

  • Is it possible that your jointer fence is a smidge out of square? – Benchwerks Apr 15 '17 at 5:32
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Glue up strategy was pipe clamps alternating on top and bottom every foot. I also used biscuits.

Alternating clamps top and bottom is a good way to help achieve a flat glue-up, but if you want to be absolutely sure you need to actually clamp the top flat somehow. You can do this with individual clamp pressure at each joint being glued (using just a C-clamp or with a purpose-made clamping accessory) or do it across the entire span using a caul.

Some related info in these previous Answers:
What is a good way to prevent jointed tabletops from bowing when tightening fasteners or the glue sets?
Not-so-obvious disadvantages of butt joints

FYI biscuits to nothing to help panels stay flat (and they don't add strength). They're merely an alignment aid in this sort of jointing.

Could the bow be because of over tightening the pipe clamps?

I don't think so if you alternated clamps. Definitely a possible cause if you hadn't done this though.

Brief note on clamp pressure: with a joint like this which relies completely on the glue bond for strength you actually do want to clamp hard. The clamp pressure should be sufficient to squeeze out all the excess glue, which in practice means you want to clamp hard enough that you're forced to use scrap blocks to prevent denting of the edges by the clamps. If you've done your edge jointing well this sort of clamping pressure will result in glue joints so thin you can't see them — minimum standard you should aim for is not being able to seem them at arm's length, if you're more discerning they should be completely invisible no matter how closely you look.

What else could have contributed to the bow?

Clamping the glue-up flat with further clamps may not have prevented this from occurring however because I suspect acclimation may have been at least partially the cause.

Wood not ready
Regardless of how long the pine sat in your workshop prior to you using it if you planed it down to final thickness in one go (especially if from one side more than another, or one side only) and then used it quickly that could have directly led to it warping unfortunately.

It's a much better strategy if time allows to plane down partially to thickness, putting the wood aside (stickered, and with heavy weights on top) to acclimate again, and then after a few days or longer planing off the last 1/8" (3mm) or so to get down to final working thickness. This gives the exposed core of a plank, which is generally slightly damper than the wood at the surface, time to get down to equilibrium with the working environment in stages, rather than all in one go.

Not quite square
Another possible cause is an ever-so-slight discrepancy in the squareness of the edges, which after the glue has dried and the clamps are removed makes its presence known. Far less than half a degree is enough for a bow to be generated. So if you're not 100% certain that your edge jointing results in a perfect 90° edge it's a good idea to joint each pair of mating boards in alternate directions, that is, one face in to the fence and the next face out. This way any slight angle difference is cancelled out — see diagram here for clarity. The drawing relates to hand-jointed edges but the same principle applies to edges jointed by machine.

  • I would also be interested in the cut of the wood (most likely plain sliced) and the orientation of the growth rings in adjacent board. There are way to orient and layout the glue up to miniimize warping in panel glue ups. – Jacob Edmond Apr 14 '17 at 14:51
  • @JacobEdmond Yes the ring direction can cause issues but I thought not likely in this case because of the short timeframe. But just by itself this could be the cause and I should have included it. – Graphus Apr 14 '17 at 21:49
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    I think the most likely cause out of everything listed above is edges not perfectly square. I will try the method to cancel the error next time. – mattmar10 Apr 15 '17 at 12:28
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Some things that could have happened:

  • The reclaimed pine could have warped due to sudden moisture changes (ie. moving from damp outside storage to the warm, dry indoors without letting the wood adapt first)

  • The boards may have needed to rest a few days to allow them to settle after planing

  • The clamping down of the boards may have caused you not to notice slight bows in the final planks

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