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Gluing X Brace Question...
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Author:  Rob Lak [ Tue Jan 20, 2009 1:23 pm ]
Post subject:  Gluing X Brace Question...
When gluing two pieces of wood together, it is often said that the glue joint is stronger than the wood itself. Does this hold for gluing end grain?

I came across this abstract but didn't read the whole paper...

Quote:
Abstract: To evaluate the effect of grain orientation on the adhesive bond strength, three-layered Norway spruce wood specimens were tested in shear. The two axial-oriented outer layers were jointed with the middle layer using three typical glues for load bearing constructions, I.e. one-component polyurethane (PUR), melamine–urea–formaldehyde (MUF) and phenol–resorcinol–formaldehyde (PRF). The grain orientation of the middle layers was varied from 0° (parallel to grain of the surface layer) to 90° (perpendicular to grain of the surface layer) in incremental steps of 10°. Samples with middle layers oriented parallel to the outer layers showed shear values in the range of solid spruce wood. Decreased shear strength values were expected for increased grain angles of the middle layer. However, no explicit tendency was observed for the shear strength in dependence of the grain direction. In general, MUF-bonded samples showed slightly higher shear strength values compared with PUR- and PRF-bonded specimens


granted that these are not glues used in building guitars but it leads to these questions:

Does the abstract hold water to you when talking about HHG or PVA glues?

If it is true... (hold on to your hats!) then does that mean the whole theory about capping the X brace is bahooey? The way I'd read this is, if you make the final x brace as tall as an x- brace with a cap then the resulting strength is the same. The glue joint is stronger than the wood and can be dismissed as a source of failure.

Sorry, but got to keep you guys on your toes... :lol:
Author:  Barry Daniels [ Tue Jan 20, 2009 1:33 pm ]
Post subject:  Re: Gluing X Brace Question...
This does not apply to end grain joints. Cap it.
Author:  Rob Lak [ Tue Jan 20, 2009 1:50 pm ]
Post subject:  Re: Gluing X Brace Question...
Thanks!

Is that abstract just regarding separation of plywood layers then and rotating the grain orientation? Can't say i completely get the terminology.

I was seaching the web on gluing surface strength and grain orientation and hadn't found much.
Author:  gozierdt [ Tue Jan 20, 2009 2:01 pm ]
Post subject:  Re: Gluing X Brace Question...
I agree this sounds like an evaluation of how the strength of a 3 layer wood sandwich
varies with the angle of the middle ply. I don't agree that their assumption that the shear
strength will be less as the angle increases is intutively obvious. And none of the glue
joints joined end grain, so it doesn't apply at all to the X-brace joint.
Author:  Brad T [ Tue Jan 20, 2009 2:47 pm ]
Post subject:  Re: Gluing X Brace Question...
Looks like they were comparing different glues along with grain orientation, parallel to perpendicular, but end grain had nothing to do with it.

Look up Duracore (I think it is) that is used to build lightweight wooden boats and see if there is any info on gluing properties during its construction. IIRC Duracore is made of these little blocks of balsa, put together in butcherblock fashion. The end grain is oriented to the top and bottom of the "butcher block" and then a veneer is glued to the end grain of the balsa. I think they use a resorscinol type glue.

In my experience with building solid wood furniture, end grain to end grain glue up is something to be avoided, unless there is a way to shore up the joint, as in a cap on an x-brace. Most literature that you can read on furniture building tells the same thing. Of course, something else to be avoided is gluing a long grain piece to a cross grain piece, as in a moulding across the top of a cabinet side. We do this all the time in guitar building, when we brace a top or back, though, go figure! :shock: At least arching the plates allows for some movement!

Capping the brace can't hurt, and it takes all of 3 minutes to do, so I do it. I'm a belt and suspenders kinda guy anyhow :D
Author:  Bob Garrish [ Tue Jan 20, 2009 3:02 pm ]
Post subject:  Re: Gluing X Brace Question...
End-grain balsa is a very common material for all kinds of composite panels. I think the reason for the orientation is more to stop it from soaking up moisture or shearing than because of ease of gluing, though. They usually seal the end-grain face of the panels before they glue them up, and the glue line isn't generally stressed that highly on a per-unit-area basis like it is in structural woodworking.
Author:  Brad T [ Tue Jan 20, 2009 4:23 pm ]
Post subject:  Re: Gluing X Brace Question...
Bob,

That is exactly the reason according to a book that was printed on the Duracore...that it is less prone to water soaking/resultant decay. I didn't think that it had anything to do with gluing properties, but did not know whether or not there was anything published on the issue of end grain balsa glued to the veneer. Obviously it works, though.
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