Laminated Tops: "Under Stress" or Not?

J_SAMa · 1 Sep 2013

Hi all,

I always used to clamp all my glue joints for at least 12 hours. But today (out of boredom) I read the manufacturer's instructions on the glue bottle, it said that the clamping time should be 4 to 15 min :shock: Did a little search on the web, the answers are actually not too different from my glue's instructions. They are all pretty consistent: 15 to 30 min for "non-stressed" joints and 12 to 24 hours for "stressed" joints

Now a question arises: what's a "stressed" joint and what's not? A table/chairs M&Ts sure are, but what of their laminated tops? I mean they don't have to keep the whole thing from wobbling like M&Ts do but they do take (very heavy) weights on them (although supported by legs).

Whatever the answer is, how long would YOU personally clamp laminated table/bench/chair top? I'd really like to hear this as I don't have many clamps, so can't clamp multiple projects at the same time, and appreciate to save some time by clamping for as little time as possible.

Also, bookshelf dadoes. They are stressed, but not quite in the way an M&T is.

BTW, what's with all this "at least 200 psi clamping pressure when gluing up" hype in magazine articles? Heck I don't even see the point of clamping laminations since even a rubbed joint with PVA is stronger than wood (I tried this with spruce).

Sam

P.S. , my glue is Bison's PVA "Super Fast D2" (3 min open time #-o and they don't sell the normal version here... :evil

, probably why it said 4 to 15 minutes of clamping time. Clamping for other types of glues would be much different, right :roll:

ColeyS1 · 1 Sep 2013

I always use to leave stuff glued up for at least 16 hours. Then, I only ever use to use normal pva and cascamite

If I've got a job now that requires using the glued piece earlier, then I've started pushing the boundaries. Any of the titebond glues I usually leave a couple hours ( longer if possible ) and having never had any problems. Not much help I know :roll:

Sent from my GT-I9300

Chrispy · 1 Sep 2013

Hi Sam,
I think the stressed joint refers to how well the joint fits before the glue has set, so if your edge joints are a very good fit then five minutes is fine but if they need the cramps to hold them in place while the glue sets then leave then in place untill the glue has fully cured.
I'm gluing up 54 chairs at the moment and the mortice and tenon joints get as long as it takes to spread the glue on the next one, but saying that the joints are very good with well fitting shoulders and stay pulled up when the cramp is removed.

CHJ · 1 Sep 2013

I read 'Stressed Joints' in relation to gluing to refer to applied loads immediately after assembly.

Something that is going to be laid to one side without immediate further load applying work to be safe after short clamping.

Something that is going to be subjected to use or further work where significant loads are applied to the joint should remain clamped and not disturbed longer to ensure glue has cured without the wood relaxing and opening the glue joint.

The glue may well grab enough to hold in such as a rubbed joint, but may well fly apart if you tried to plane, machine etc.

Stuff I glue up for turning using any adhesive other than CA needs at least several hours to cure before it is safe to be subjected to machining loads. (Cascamite states 6 hrs + dependant on temp) But I know for a fact that the joints can't be opened by hand after just a few seconds due to atmospheric pressure.

Corneel · 1 Sep 2013

J_SAMa":qe26vr1t said:
P.S. , my glue is Bison's PVA "Super Fast D2" (3 min open time #-o and they don't sell the normal version here... :evil, probably why it said 4 to 15 minutes of clamping time. Clamping for other types of glues would be much different, right :roll:

You'll have to shop around a bit more Sam! Usually the Praxis sells all the variations of the Bizon PVA glue. The classic, the extra D3 and the superfast.

With stressed or not stresses they indeed talk about the stress during drying. For example an edge joint with a bit of spring in the joint is a stressed joint, while a mortice and tenon isn't.

J_SAMa · 1 Sep 2013

Corneel":1nlzoqwk said:
J_SAMa":1nlzoqwk said:

P.S. , my glue is Bison's PVA "Super Fast D2" (3 min open time #-o and they don't sell the normal version here... :evil, probably why it said 4 to 15 minutes of clamping time. Clamping for other types of glues would be much different, right :roll:

Click to expand...

You'll have to shop around a bit more Sam! Usually the Praxis sells all the variations of the Bizon PVA glue. The classic, the extra D3 and the superfast.

With stressed or not stresses they indeed talk about the stress during drying. For example an edge joint with a bit of spring in the joint is a stressed joint, while a mortice and tenon isn't.

The closest Praxis is the closest shop to me is still 10 km away and their timber selection isn't that good, not really worth it going there just for glue... The Hornbach nearby my house only sells superfast D2 and hide glue :|
-----------------------------------------------------------------------------------------------------------------
Aha, now that means "stressed" and "non-stressed" have nothing to do with structural stress, but are about stress during dry time 8)