Short glue test

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Forum topic by jklingel posted 02-28-2021 06:52 AM 374 views 0 times favorited 9 replies Add to Favorites Watch
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200 posts in 2212 days

02-28-2021 06:52 AM

Just an FYI, and it proves nothing definitively, but I found it interesting. I generally leave stuff glued overnight, just to be sure, but a wood shop owner told me to allow 1 hour/inch of thickness. That is in his warm shop, low humidity. Data of relevance: 60-62 degrees in my shop, humidity probably 20-25%, as my house has a hard time getting above 20% in winter. TiteBond 3 glue, applied to both pieces of wood. Setup: All wood was machine planed. I screwed a 2×4 to my bench, and glued/clamped six 2×2’s to it, spread apart about 2” and perpendicular to the 2×4. At 3, 6, and 12 hrs I hung a weight on each 2×2, having two to test at each time interval. I hung the weight close to the 2×4 and then moved it out 1” at a time, allowing a few seconds for the weight to dead-hang. I moved the weight until failure. I don’t have the numbers right here, but all 2×2’s broke in the 20-25” range, with no clear pattern. I expected longer distances from at least the 3 hr to the 12 hr times, but nothing was obvious. I don’t know how to post a picture of the 2×4 after testing, but I’d say most of the failure was the wood failing, with perhaps a tad more 2×2 wood adhering to the 2×4 in the 12 hr test. Anyway, FWIW…. j

9 replies so far

View 18wheelznwood's profile


130 posts in 85 days

#1 posted 02-28-2021 07:00 AM

As long as the glue joint didn’t fail it looks like extensive clamping time isn’t that necessary.

View Madmark2's profile


2506 posts in 1643 days

#2 posted 02-28-2021 08:28 AM

PVA glue dries as the water in the glue is absorbed into the wood. It does not out-gas or depend on in or out flow from the joint. Thus the drying time is the same over the entire surface. Most strength variations are due to coverage gaps more than anything else.

Glue starvation by over clamping is a myth. Again, these failures are generally coverage related. Clamping pressure should close the gaps but not crush. Use blocks to spread the force and cauls to keep everything flat. If you have to use pipe clamps to close the gaps your joinery is sloppy.

Under clamping can lead to weakness as not all glues are “gap filling”. This is why “rub joints” aren’t usually as strong as a clamped joint of the same dimensions. Rub joints are sufficient for lots of non-load members, trim, etc. applications.

Clamping pressure fans out at a 45° angle from the clamp location. If the joint is in the middle, the clamps should start 1/2 the width in from the ends and be spaced about the glueup width apart.

Other glues may harden differently. For example RTV adhesive out-gasses acetic acid (vinegar) fumes as it cures. Some epoxies out-gas who knows what.

Direct comparisons are difficult esp. if the manufacturers instructions aren’t followed to a “T”.

-- The hump with the stump and the pump!

View tvrgeek's profile


1769 posts in 2704 days

#3 posted 02-28-2021 11:16 AM

There are extensive objective glue tests on the WEB. Thanks to this forum and my research, I have changed almost every go-to glue I used.

Reminds me, my new sled fences are in clamps overnight from the powered resin glue I used. Long cure time, but I want the fences to stay strait, so a rigid glue instead of PVA.

Clamping time is only until the glue is no longer allowing movement. Clamping longer has no effect. Longer time can greatly effect the strength of both material and bond as some glues have chemical reactions based on time. Epoxy for instance, may set in 5 minuets, but target strength is over 24 hours.

View MikeJ70's profile


84 posts in 1002 days

#4 posted 02-28-2021 04:42 PM

I just follow the directions on the bottle. Titebond II says clamp minimum 30 minutes and don’t stress the joint for 24 hours. I usually clamp for about an hour and then unclamp and let it sit over night. That’s what works for me.

-- MikeJ

View jklingel's profile


200 posts in 2212 days

#5 posted 02-28-2021 06:07 PM

Good info. An engineer told me when he was designing my foundation that gravel bears load at 45 degrees, so I am surprised that wood does, too. It seems it would be a wider angle. j

View Madmark2's profile


2506 posts in 1643 days

#6 posted 02-28-2021 07:01 PM

The 45° has nothing (or very little) to do with material and more with geometry. When you press forward (or down with gravity) the maximum force is aligned with the force axis. As you move off axis the force drops off by cos(x), reaching 0 at +-90°. At the same time the lateral force in increasing from zero by sin(x) to max at 90°. At 45° the forward and lateral forces are equal and the primary force axis shifts to the side once the angle exceeds 45°.

-- The hump with the stump and the pump!

View jklingel's profile


200 posts in 2212 days

#7 posted 02-28-2021 09:54 PM

Mad: Very interesting and got me quite curious. Materials do weird and wonderful things sometimes. On poking around, it seems that when wood is compressed perpendicular to the grain the forces spread through it at about 19 degrees, if I did my trig right. I did not read the entire article, but did see the 1:3 ratio listed a few pages in, and the text seems to support what I took from a brief read. If interested, give it a read and let me know what your conclusion is. (PM if you want, so as to not hog up this post….). Cheers. j

View CWWoodworking's profile (online now)


1664 posts in 1234 days

#8 posted 03-01-2021 12:58 AM

I think people are asking for trouble waiting just 1 hour and applying any force. The joint won’t fail, but other problems may occur in finishing.

Made a dining table top once and didn’t leave it in the clamps long enough. Ended up with a tiny line that Telegraphed through the lacquer. Couldn’t see it in white sanding or stain, only on top coat. Ended up making a new top.

For thinner tops, no force for at least 3-4 hr. For thicker tops, I wait over night.

View jklingel's profile


200 posts in 2212 days

#9 posted 03-01-2021 05:54 AM

CWW: I don’t think I have the grapes to take stuff out of the clamps in less than 6-8 hrs, and since I don’t glue at 0700 or work until late at night, that pretty much means to let it dry overnight. Maybe when I make unstressed things, like bowls, for us I’ll push the time. When I called TiteBond about it, the agent said 8 hrs should be plenty (warm and dry environment) but to not sand for 2-3 days. He said the wood may change dimensions a tad but the glue would not, and would require more sanding anyway. I have had problems with my bowls, glued one day, sanded and oiled the next. I have made 128 in bowls in the last year and maybe I’ve been lucky. Too, they are pretty much 5/16-3/8” thick except for the bottom (3/4-1 1/4”) so they can dry pretty quickly.

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