Clamping Pressure for Panel Glue ups?

[Erich Weidner]

Wow, my head is spinning. I started reading tonight trying to figure out why my panel glue ups sometimes cup pretty badly...

Just came from reading various articles on clamp pressure for gluing up panels.
A FWW article states you will have trouble getting enough force with bar clamps... https://www.finewoodworking.com/2010/05/11/how-to-glue-up-joints-the-right-number-of-clamps

This one says, you are probably using too much pressure: https://www.woodmagazine.com/woodworking-tips/techniques/skills/take-it-easy-with-clamping-pressure

Articles in a C. Schwarz book show a single clamp for small glue ups.

Several posts here on SMC have advice that clamping too tight causes warping.

So... which is it? :)

[Doug Dawson]

Wow, my head is spinning. I started reading tonight trying to figure out why my panel glue ups sometimes cup pretty badly...

Just came from reading various articles on clamp pressure for gluing up panels.
A FWW article states you will have trouble getting enough force with bar clamps... https://www.finewoodworking.com/2010/05/11/how-to-glue-up-joints-the-right-number-of-clamps

This one says, you are probably using too much pressure: https://www.woodmagazine.com/woodworking-tips/techniques/skills/take-it-easy-with-clamping-pressure

Articles in a C. Schwarz book show a single clamp for small glue ups.

Several posts here on SMC have advice that clamping too tight causes warping.

So... which is it? :)

If the edges are prepared properly, you would need just enough pressure to maintain firm contact across the length of the joint. What is "firm contact" is to be judged from experience.

If you look at the FWW article, they appear to be referencing Hoadley's book "Understanding Wood". If you have that book (and I do,) the extreme pressure requirements referenced in the article derive from Hoadley's supposition that somebody did a really crappy job of preparing the surfaces to be joined, or spreading the glue out on them.

[David Buchhauser]

I would imagine that in addition to the clamps to squeeze the boards together for gluing, some other clamping arrangement (top and bottom) would be helpful to keep the boards laying flat until the glue dries.

[Lee Schierer]

For thin panels you have to reduce clamping pressure or the panel may cup or even pop right out of the clamps. By alternating clamps top and bottom you can minimize this tendency. You can get good joints just pressing the boards together by hand, but it can get pretty boring holding the boards for 10-15 minutes. You cannot over pressure a glue up where you starve the joint for glue, providing you put on enough glue in the first place. I routinely use pipe clamps and have had no joint failures in over 40 years.

[Robert Engel]

I'll have to disagree with that, with the exception of thin panels as mentioned. With respect to clamping, as mentioned, alternating clamps is very important especially with pipe clamps, but even with parallel clamps.

IMO panel cupping is due to several factors.

Joints not congruent to each other, ie. either 90° or complimentary. A jointing technique I use is to alternate faces against the jointer fence. This cancels out any discrepancy off 90°. (IOW if the fence is 2° off, one edge will be 88°, the other 92°, thereby canelling out the error).

The second relates to the wood itself. Wider boards in a panel are more subject to cupping. Also the type of cut, ie. in order of stability, flat sawn > rift saw > quarter sawn. Allowing the boards to acclimate prior to glue up is very important.

Third relates to environmental factors and how the panel is handled after glue up. For example, maintaining equal air flow on both sides of the boards prior to glue up, and the panel after glue up. NEVER leave a panel laying on a table overnight.

Years of experience has taught me that most of the issues with panel warping occur AFTER the clamp up. The highest risk will come with wide, flat saw boards that are not totally acclimated.

For this reason I clamp the panel in cauls to keep it flat as it acclimates further right up to when I'm ready for use.

Once all the factors are understood, panel glue ups will be much less frustrating.

[Dennis Peacock]

+1 ^^^

What Robert Engel just said.
I've done a LOT of panel glue ups, some very large, but the advice Robert offered there is what I go by most often. I've been guilty of using far too much clamping pressure and sometimes that causes a glue starved joint.

[Mike Cutler]

As Robert stated, Uniform clamping pressure, and cauls will go a long way to help you. Like Robert, I also keep them clamped flat after the glue up is complete.
If I am doing edge joints, I also alternate the boards through the jointer. I rarely do these any longer though. I use a reverse glue joint, or a T&G joint, cutter on the shaper. These make it much easier for a single person to align a panel. The joint is no stronger than a butted edge joint, it's just easier for me alone.

[Brian Holcombe]

I like to tune the edges by hand before clamping first to create a square edge, then to stack the adjoining board onto it and ensure that they are flat across when checked with a straight edge.

I also prefer spring joints, hollowing the joint lightly and using a fairly large amount of pressure across multiple clamps alternated for sides.

[Erich Weidner]

Well... wish I'd have taken the time to revisit this thread right before glue up. But day job ran super long (12 hours) and my helper is not the most patient soul.

Also someone moved the clamps and I couldn't lay hands on several initially. (And the dog ate my homework) Forgot to go back and add clamps on top (alternate), everything was on the bottom. Clamp pressure was pretty tight (using parallel bar clamps). But no more so than on the other glue ups. there was some bow on one of the boards which we coaxed to be more flush with a dead blow mallet and selective clamp tightening.

Will see what happens. Helper will be taking the glued up 12' x 32" panel to the lumber place to have it flattened tomorrow.
I hope it doesn't cup. I have to be at work before the lumber place opens so I won't see it until after it is processed.

[Erich Weidner]

If you look at the FWW article, they appear to be referencing Hoadley's book "Understanding Wood". If you have that book (and I do,) the extreme pressure requirements referenced in the article derive from Hoadley's supposition that somebody did a really crappy job of preparing the surfaces to be joined, or spreading the glue out on them.

I do have that book, though it's been years since I read it. Did he really state that was the conditions he outlined for? Worst case it sounds like?
If so, doesn't sound like a useful bit of science applied to woodworking as a craft.

[Doug Dawson]

I do have that book, though it's been years since I read it. Did he really state that was the conditions he outlined for? Worst case it sounds like?
If so, doesn't sound like a useful bit of science applied to woodworking as a craft.

I have the first edition, so the current one may be a bit different, but that about sums it up. The quoted pressures of course increase with the denser exotic woods, because it's harder to moosh them together if the surfaces are rough or insufficiently matched. That should say something. :^)

Otherwise it's a very good book, along with the "Encyclopedia of Wood" (dead tree edition.) Some would say they are weighted towards industrial manufacture, but you can read what you want out of them.

[David Buchhauser]

I do have that book, though it's been years since I read it. Did he really state that was the conditions he outlined for? Worst case it sounds like?
If so, doesn't sound like a useful bit of science applied to woodworking as a craft.
So Erich,
What are you making? If you don't know what these panels are for, then that's OK too.
David

[Brian Holcombe]

In a basic sense a tight glue line is a good indication of a strong glue line, wood glues don't bridge gaps while retaining their strength.

[Ron Citerone]

I'll have to disagree with that, with the exception of thin panels as mentioned. With respect to clamping, as mentioned, alternating clamps is very important especially with pipe clamps, but even with parallel clamps.

IMO panel cupping is due to several factors.

Joints not congruent to each other, ie. either 90° or complimentary. A jointing technique I use is to alternate faces against the jointer fence. This cancels out any discrepancy off 90°. (IOW if the fence is 2° off, one edge will be 88°, the other 92°, thereby canelling out the error).

The second relates to the wood itself. Wider boards in a panel are more subject to cupping. Also the type of cut, ie. in order of stability, flat sawn > rift saw > quarter sawn. Allowing the boards to acclimate prior to glue up is very important.

Third relates to environmental factors and how the panel is handled after glue up. For example, maintaining equal air flow on both sides of the boards prior to glue up, and the panel after glue up. NEVER leave a panel laying on a table overnight.

Years of experience has taught me that most of the issues with panel warping occur AFTER the clamp up. The highest risk will come with wide, flat saw boards that are not totally acclimated.

For this reason I clamp the panel in cauls to keep it flat as it acclimates further right up to when I'm ready for use.

Once all the factors are understood, panel glue ups will be much less frustrating.

I agree completely. A lot of good advice in that post! Any beginners shoulr reread that a few times.

[Keith Weber]

Wider boards in a panel are more subject to cupping. Also the type of cut, ie. in order of stability, flat sawn > rift saw > quarter sawn. Allowing the boards to acclimate prior to glue up is very important.

Just a quick post to clarify something I perceived as misleading. The above comment is talking about stability, and use of >(greater than) signs might suggest that flat sawn has greater stability than rift or quarter sawn. I'm sure that Robert knows the correct order, but the use of >(greater than) signs might cause some confusion with some people (especially math nerds like me.)

To clarify: Quarter sawn lumber is the most stable and will reduce cupping the most. Flat sawn lumber is the least stable and is the most prone to cupping. Rift sawn lumber falls between these two.