timber post lamination/glue pressure

[Albert Lee]

I have a job coming up where I need to glue 5 pieces of 19mm thick x 14cm x 180cm timber to form a timber post with cross sectional area of 140mm x 95mm, 1800mm long.

They are all flatsawn so therefore 10kg/cm2 (150psi) should suffice?

14 x 180 = 2520cm2 x 10kg/cm2 = 25200 kg

I have a pneumatic table but the ram can only exert 4000N at 0.8mpa each, thats roughtly 400kg, there are 6 rams, therefore 2400kg.

it seems I have to use my T bar clamps(1500kg capacity) or my Urko clamps (1800kg capacity)

thoughts?

[Jamie Buxton]

The usual woodworking glues (for instance Titebond) do not care how much pressure is applied to them. A blob of Titebond will eventually dry when it is just sitting out in the open. Clamps are used to pull the workpieces together so the glue line is thinner, and to force excess glue to squeeze out the sides. If your timbers are machined flat, you don't need much force from the clamps. My usual practice is to mill the joint properly, spread a thin layer of glue on the joint, and apply clamping pressure until I'm seeing good squeeze-out. If I apply more force, nothing changes. So that extra force isn't needed.

[Bill Space]

Is this post a structural member?

If so, you need to chose a glue rated for structural use. I seem to remember that glues like the Titebond series are not recommended for structural applications.

Just a thought...

[Wayne Lomman]

Just clamp it enough to close the joints and get squeeze out. Also make sure you apply the glue to the entire surface ie brush or scrape it on uniformly. If this is just a post, any decent glue including titebond is Ok. It's just vertical lamination and that works structurally even if just nailed together to code. If it is a beam, then you need a structural glue such as resorcinol or epoxy. Titebonds are not structural as Bill pointed out. Cheers

[Steve Schoene]

There is plenty of evidence that PVA glues such as Titebond give strongest bonds when clamped with substantial pressure. Published research has demonstrated that increasing pressure until just short of deforming the wood increases strength. Basically, you can't over clamp such a glue joint. Glues such as epoxy have different requirements and tolerate thicker glue hunts.

[Mel Fulks]

It's true PVA s are not rated structural ,it's an expensive process. So it's a market positioning thing. But many of us have run our own tests,wearing hard hats and found them sound.

[Albert Lee]

Thanks All.

Its not a "structural" item however its the post for a bunk bed. so in a way it is a structural element.

I think I will stick with 150psi for flatsawn and just PVA.

[Ted Reischl]

There is plenty of evidence that PVA glues such as Titebond give strongest bonds when clamped with substantial pressure. Published research has demonstrated that increasing pressure until just short of deforming the wood increases strength. Basically, you can't over clamp such a glue joint. Glues such as epoxy have different requirements and tolerate thicker glue hunts.

So what is "substantial" ???? And what about that "deforming"??? Is that deforming the wood under the clamp head or the entire glue surface?

If this "research" is published, who published it? Please give a reference.

I am challenging you on this because you made statements without giving any references.

Edit: I have seen an article in FWW with a Dr Rabiej that agrees with what you wrote. But a Google search indicates there are lots and lots of other "experts" that disagree with him.

I would be interested in reading this "research".

[Mel Fulks]

Ted , the tech reps at Titebond say they have never seen a glue joint that was "glue starved" from over clamping . But obviously the project can suffer from damage to the wood from too much pressure.

[Wayne Lomman]

Pva as a generic category of glue is not structural according to our building code. Epoxy and resorcinol are. I tend to go with the code writers on this one. Cheers

[Ted Reischl]

A post for a bunk bed? I would not worry about fussy pressure settings.