Comments on: How Much Glue?

By: Zach

Zach — Wed, 08 Jul 2009 10:06:27 +0000

The bagel analogy makes perfect sense.

By: Jeremy Kriewaldt

Jeremy Kriewaldt — Tue, 07 Jul 2009 23:27:07 +0000

One issue that rarely seems to be raised is one that I only discovered recently when reading J E Gordon’s two books The New Science of Strong Materials and Structures (both of which I would highly recommend – although they deal with technical engineering science stuff, Gordon writes in a simple way that explains his concepts succinctly and clearly; and with minimal mathematics!). That is that the bulk of the sticking that an adhesive line does is performed at the edges of the joint – in fact the glue in the middle of the joint is only a passenger. Under tension and shear the edges are where the stresses are greates and if the edge join breaks then the joint will fail totally because the glue in the centre of the joint will then be under a shearing strain which no glues are good at withstanding.

So the lesson from this is that the important thing is to ensure that the parts that will become edges of the joint need to have enough glue to make a strong adhesive bond. That’s why a little squeezeout all along the joint is a pretty good thing – it shows that the glue will make an adhesive bond all along the edge of the joint. Blobby squeeze out shows that there may not be a consistent bond along the whole edge of the joint – ie the glue line at the edge may be like this – - – - rather than this ____________.

The ideal way to achieve this would be a fine and constant line of glue laid down in a line by a syringe on the parts that will form the edge joints. There will be minimal squeeze out and any ‘surplus glue’ will go inside the joint where it won’t be seen (it won’t help there but it won’t hurt either. Since we can’t do that, a coat that goes all the way to the edge is the best practicalway of achieving the strongest bond.

So spread glue like butter, but make sure that whatever you do, the butter goes right out to the crust. The worst thing to do is to put a blob of glue like cream cheese right in the middle of the bagel!

By: Rob Bois

Rob Bois — Tue, 07 Jul 2009 19:59:12 +0000

I think it takes a certain number of projects with major squeeze out before you truly begin to appreciate the “proper” amount of glue required for a joint. It is true that glue molecules bond better to wood molecules as oppose to other glue molecules. So in a perfect world, your glue joint would be one molecule thick, evenly across the joint. However, that’s not practical since it is impossible to cut joints that are perfectly flat and mated with no voids or gaps. But that little fact does change your perspective – the best way to minimize glue (and subsequent squeeze out) is to cut good joinery.

You also want to consider where the joints strength comes from and focus on those parts. For example it turns out most of the strength from a mortise and tenon joint comes from the tenon face cheeks, not the edge cheek nor the shoulders. So if you are brave enough, and your tenons fit well enough, you can just glue the faces (and mating parts of the mortise) and your joint will be just as strong. Since the end of the tenon adds no strength either, chopping your mortises slightly deeper creates a nice glue reservoir. The Schwarz wrote a great article on the science of the M&T in the latest Woodworking magazine that’s definitely worth a read.