Tuesday, June 5, 2012

Archtop Guitar Build - The Big Glue-Up

I have to admit that this is the part that I've been dreading, the final glue-up of the neck to the body.  For me the choice of glues was easy to make, hide glue.  This glue is very strong, acoustically dense and can be reactivated with heat and steam so if the neck should ever need to be re-set, it can be steamed off fairly easily.  The problem is that the dove tail joint is very tight.  You have to use both thumbs and bulge your eyes out to get it to move from the seated position once the neck is in place.  This is great because it should lead to a very strong joint.  The problem is that hide glue is mostly water and water makes wood swell and lots of water makes wood swell a lot.  If the joint is tight dry, it will be uber tight wet and it is possible that once the glue is in place, the joint will get stuck half way or even worse about 90% of the way in.  To make matters even worse, hide glue gets really sticky as it cools so from the moment the glue is applied it is cooling and swelling the wood, both very bad in terms of getting the joint together.  Hide glue is known to have very little "creep" to it.  Creep is what you get when you put two flat pieces of wood together in a butt joint and put wood glue between them.  When you first bring the pieces together and put pressure on them, they want to skate around on top of each other because the glue initially acts as a kind of lubricant in the joint.  Hide glue is very tacky as it sets so once you put the pieces together, they don't want to move so you had better be accurate with your initial set up of the joint.

The solution to this is to heat the wood to be joined to increase the time the hide glue stays hot.  Also, you must be sure that the hide glue has been in the glue pot long enough to get uniformly hot.  I heated both sides of the joint with a hair drier to get the wood warm and also made sure that all of the clamps and cauls were set and ready to go.  I applied the glue to the joint and to my surprise the whole glue up was very anti-climatic.  The joint slid together easier than when dry and seated easily.  The clamps were applied and I wondered why that went so well?  I think the answer is that when hot and wet, hide glue feels a lot like, well um snot.  That is because it is mostly animal protein and long chain lipids sort of like, well you know...  I must have kept the joint hot enough so that no setting took place before the joint was seated so the whole thing went together easily.  Here is a photo of the glue up.

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