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Clamping Pressure for Glue Joints

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Forum topic by Cincinnati2929 posted 07-30-2019 01:08 AM 1171 views 0 times favorited 57 replies Add to Favorites Watch
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Cincinnati2929

41 posts in 386 days


07-30-2019 01:08 AM

After a lengthy and occasionally spirited exchange on clamping solid boards into laminated tabletops, I decided to start a new thread.

According to this article in Wood Magazine, (https://www.woodmagazine.com/woodworking-tips/techniques/skills/take-it-easy-with-clamping-pressure) TighBond said we needed 175-250 psi across the glue surface for laminating hardwoods. That is equivalent to 12.6 to 18 tons per square foot. Now that does seem just a bit excessive for gluing wood.

Today, I spoke with Greg Miller at TightBond Tech Support. He said the old school of thought was to crank down on the clamps with as much force as you could muster. The current recommendation is to use only as much force as required to bring the mating surfaces together, then only as much extra force to get a small amount of squeeze-out along the glue joint. This emphasizes the importance of a properly prepared glue surface, ie: flat, perpendicular to other faces and parallel to the opposite face.

If it takes a massive amount of force to bring the 2 mating surfaces into contact, you would be incorporating a substantial amount of internal stress into the panel, thus increasing the likelihood of the panel moving as the internal stresses are relieved. But if we are properly preparing the faces, just sliding them together gives 100% surface contact. Then only 1 to 2 psi across the joint would give even joint squeeze-out and yet not result in a glue-starved joint. This means I would not need industrial strength clamps like the Bessey Revo, as the lighter duty would easily distribute 1-2 psi at the glue joints

This is a far cry from 175-250 psi specified in the article.


57 replies so far

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ArtMann

1423 posts in 1294 days


#1 posted 07-30-2019 01:30 AM

If I were you, I would do some destructive testing on samples before using such a small amount of clamping force on a real project.

View Aj2's profile

Aj2

2422 posts in 2276 days


#2 posted 07-30-2019 01:58 AM

I don’t think its possible to squeeze out all the glue from two mating surfaces . Just as long as one wets both surfaces.
So the glue starved joints I’ve made were from not wetting both surfaces.
When squeeze out doesn’t matter I’m very generous with glue.
And heavy handed with my clamps.
I also like Artsmanns suggestion about testing.
Good Luck

-- Aj

View Toller's profile

Toller

37 posts in 2078 days


#3 posted 07-30-2019 02:06 AM

The article was odd. Clamps produce pressure. The pressure is then divided by the area to produce PSI. So, to specify the PSI a clamp can produce without specifying the area is impossible.

FWIW, I clamp firmly but not excessively. I have never had a joint fail. Which is not to say that less wouldn’t work just as well.

A couple years ago I helped build a boat with epoxy. We used clamps to keep the wood in contact, but used as little pressure as possible. It worked.

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BuckeyeDennis

38 posts in 176 days


#4 posted 07-30-2019 02:12 AM

The Wood Magazine article isn’t specific as to whether they are talking about pressure at the clamp pad, or pressure at the glue joint. I strongly suspect that the 175-250 psi recommendation is for pressure at the clamp pad. Clamp-pad pressures much higher than the recommended ranges run the risk of crushing the wood, depending on the species. In most cases, of course, the glue-joint area would be much larger than the clamp-pad area, and the resulting glue-joint pressure proportionately lower.

See the USDA Forest Service publication “Mechanical Properties of Wood”, Table 4-3b, for an extensive list of wood strength properties. The one of interest to this discussion is “Compression Perpendicular to Grain”, which is defined on page 4-3.

https://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr113/ch04.pdf

-- Dennis 'We are all faced with a series of great opportunities, brilliantly disguised as impossible situations.' Charles Swindoll

View Cincinnati2929's profile

Cincinnati2929

41 posts in 386 days


#5 posted 07-30-2019 02:40 AM

A problem arises in that it’s very difficult to know how much force a clamp is exerting. Even if you could, you would have to make an approximation of the pressure profile within the “clamping cone”. Bessey Revo has a max clamping force of 1700 pounds and they publish the torque at which that force is reached, but I haven’t seen any data on torque vs other force levels. This is likely just an academic exercise, because woodworkers for centuries have been laminating solid wood tops and I’d surmise there have been very few failures. I would also guess it’s better to use too much pressure than not enough. Here’s a few things I plan to do to assure a laminated panel doesn’t fail.

1. And most importantly is to properly prepare each board before gluing. Meaning, all faces are flat and perpendicular to each other. Using clamping force to bend gluing surfaces together is poor craftsmanship IMHO.

2. TightBond Iii has the longest open time of the TightBond line. It also creates the strongest joint. Mr. Miller said the ideal circumstance would be to apply a 6 mil layer of glue to one glue surface. Immediately bring the adjacent board into contact. I will slide the surfaces back and forth in attempt to uniformly wet the mating dry face. Leaving it in contact, apply the glue to the next board and immediately bring it into contact with the panel. Continue in this manner until all boards have been glued.

3. Apply clamps from the center outward watching for an even bead squeezed out from all the glue joints.

The 1 to 2 psi that I specified seems very low to me. But when you consider that’s equivalent to putting 288 lbs on a
12” square board, this seems like an adequate pressure. However, I’m going to use a curved caul which exerts about 10 psi in my case. And that’s 1440 pounds per sq ft. Seems more than enough. I’m unsure if that’s too much. I hope there’s enough glue left in the joint for strength.

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Cincinnati2929

41 posts in 386 days


#6 posted 07-30-2019 03:01 AM

Dennis, I hadn’t considered that was pressure at the clamping pad. Mr. Miller did not correct me when I talked about it being pressure at the glue joint. Specifying pressure at the glue joint makes sense to me since we’re talking about strong glue joints. Also, I haven’t seen a clamp manufacturer spec pressure across the pad. For example, Bessey Revo is capable of a maximum force of 1700 pounds. The Revo Jr is capable of 950 (I believe). They don’t divide that force over the clamp pad area I would guess because it would change with a board that covers the entire pad vs a board only covering a portion of the pad.

Thank you for the pdf on Mechanical Properties of Wood. It is surprising to me seeing how widely the strengths vary for the different varieties of maple. Now I want to make sure I get Sugar Maple for my workbench top.

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LeeRoyMan

248 posts in 205 days


#7 posted 07-30-2019 03:18 AM

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SMP

1335 posts in 384 days


#8 posted 07-30-2019 03:28 AM

These kinds of things always amuse me when you see the requirements by “science” and then you walk into antique stores and see the 200 + year old furniture that the guy used convex panels nail dogged together or even just rub-jointed.

View Toller's profile

Toller

37 posts in 2078 days


#9 posted 07-30-2019 03:22 PM



3. Apply clamps from the center outward watching for an even bead squeezed out from all the glue joints.

I do just the opposite. I do one side first, getting the top and bottom aligned. Then the other side to get the top and bottom aligned. Then the middle. Never had a problem.

Why is starting in the center better?

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Cincinnati2929

41 posts in 386 days


#10 posted 07-30-2019 04:27 PM

I’m not sure starting in the center is better. But since I’m going to use a caul with a curved surface, it makes sense to me to start from the center and work outward.

I did a search on this before posting, and must not have put the right words in. I see this Wood article has been discussed a few times before. If I had money to burn, I’d buy a few pressure transducers or the pressure indicating tape and test out the scenarios we have discussed. But the fact is, as long as my top doesn’t fail I don’t have an interest in improving the strength of the joint. Who cares if I only achieve a breaking strength of 3000 lbs when it could have reached 4000 lbs as long as I never reach the 3000 lbs stress point.

I was having a hard time letting go of the concept that up to 18 tons per square ft of pressure was needed to adequately glue hardwoods. I know people using vacuum bags to laminate and bend wood and they achieve joints of integrity at around 10 – 12 psi (3/4 ton per sq ft).

I have enjoyed the intellectual banter and really wanted to see who was achieving laminated tops without investing in dozens of Bessey Revo clamps. I have concluded that the better 2 surfaces are machined, the less pressure it takes to get a glue joint that is stronger than the wood itself.

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therealSteveN

3595 posts in 1052 days


#11 posted 07-30-2019 04:28 PM

Well prepped surfaces, meaning you can hold them together and see no light coming through with just hand pressure.

Apply a sufficient amount of glue to cover one surface completely. I think Aj2’s covering both brings to mind James King Fine Woodworking videos, where it looks like a mosh pit around his glue ups. You can have too little, and you can also have way more than needed. I’ll not say too much, that is for you to figure out, but those complaining about the tails of their board not aligning, and HAVING to use cauls on a simple glue up, may want to reconsider their own personal mosh pit.

With an appropriate amount of glue, clamp until you see a squeeze out along the length of your glue up.

This hasn’t failed me in over 50 years of doing this.

More than the glue, or the pressure #1 is by far the most important aspect of a good glue up. Trying to glue ill mating wood is a recipe for disaster.

3. Apply clamps from the center outward watching for an even bead squeezed out from all the glue joints.

I do just the opposite. I do one side first, getting the top and bottom aligned. Then the other side to get the top and bottom aligned. Then the middle. Never had a problem.

Why is starting in the center better?

- Toller

Starting in the center is old school, and likely comes from doing sprung joints, where you had a wallow in the center, that once pulled together pretty much guaranteed your ends would be tight. Think the reverse of a caul. Since more modern days with jointers it is additional work to spring a joint, as the jointer makes a flat plane, or at least that is the desired outcome, and with much improved glues it isn’t necessary.

-- Think safe, be safe

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pottz

5948 posts in 1462 days


#12 posted 07-30-2019 04:56 PM

another glue up debate, seems these keep popping up,didn’t we just go through this recently ? i dont know why so many worry about it coat the pieces with a god even amount and apply even pressure end to end,just make sure there are no gaps.ive been doing this for over 40 years and not had a failure yet.trust what the manufacturer of the glue your using recommends and stop worrying about it,why retest something theve already done.happy glueing-lol.

-- sawdust the bigger the pile the bigger my smile-larry,so cal.

View Cincinnati2929's profile

Cincinnati2929

41 posts in 386 days


#13 posted 07-31-2019 11:49 AM

I can see I’m not the only one that questions even our methods that work. Glue technology changes. Modern Tightbonds are epoxy-like which results in stronger glue joints than the traditional glues of years past. Their tech support says a 6 mil glue film of Tightbond III applied to one of the glue faces will create a 3 mil glue joint with a strength of 4000 lbs. I’m not sure how much soaks into the wood and how much gets squeezed out. A 3/4” board, 3 ft long would only need about 0.1 fluid ounces of glue. If you have. A medical syringe that’s 2.7cc. – that ain’t much!

View Robert's profile

Robert

3516 posts in 1959 days


#14 posted 07-31-2019 02:03 PM

If you’re using cauls it doesn’t matter, but I don’t use cauls.

I start my clamps at one end and work across the panel, flushing the boards as I go across.

I can tell you that I don’t use as much clamping pressure now that I did when I started out. Just enough the get the appropriate squeeze out.

-- Everything is a prototype thats why its one of a kind!!

View SMP's profile

SMP

1335 posts in 384 days


#15 posted 07-31-2019 02:06 PM


Their tech support says a 6 mil glue film of Tightbond III applied to one of the glue faces will create a 3 mil glue joint with a strength of 4000 lbs.
- Cincinnati2929

But you have to learn to look past all the marketing BS and ask “what does it mean in real life?” If you take any decent glue joint made from any cheap wood glue and the wood breaks before the glue joint. Does better glue and “optimum pressure” mean the wood really really breaks before the glue joint?

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