After gluing up wood for guitars more than 30 years I have never had a glue joint fail. With a well prepped joint surface the glue line will always be stronger than the surrounding wood. While it may take hours for big blobs of squeezed out glue to dry, the super thin layer in the glue joint dries much faster. If you can feel a skin on top of the big globs or see that a thin smear of glue has dried the glue in the joint is probably dried.
Music Files will be posted when it's up and ringing. Thanks guys, this is all great stuff. And Silvio, I never thought about the heat generated when sanding bridge blanks. No heat on this neck repair but there have been more than a few times when I'm sanding bridges that those little suckers get so hot, especially the caps, they almost burn my fingers.
I just unclamped that neck I am working on and am in the process of staining and refinishing the repair. I don't plan to put it back together and string it up until tomorrow. I'm with Mike on using rubber bands for cheap clamps. When I glue each of my bridge blanks together, I use 3 large we call them rubber bands down here in Texas Mike ones, each twisted and wrapped times around the assembly. I figure they must exert somewhere on the order of at least a ton of clamping pressure psi.
The glue doesn't separate. Also surprising, at least to me, is the fact that it looks like Mike is building a banjo that actually looks like a banjo. Mike does that mean you are changing your eccentric ways and coming a round? Jerry "The only security you will ever have is in your ability to perform. My rule of thumb Give it double time recommended at the given temperature.. Look at it.. Put it back down and go fishing for a week.. In other words, DON'T be in a hurry..
I know its hard, but you better give it time!! Glue manufacturers have engineers and chemists to determine properties and practical uses of their products. Follow instructions for product selection, surface prep, temperature, clamping pressure and time and your joint will be as good as the manufacturer says it will be. Silvio pointed out what should be the most important aspect of this discussion -- that the glue joint is only as good as the underlying joinery.
Poorly cut wood joints will never be made sound and lasting with any amount of time or clamps. Brian "The choice between the banjo shop and a firearm is largely dependent on the season and the weather.
When I taught dendrology in a past life, the students smashed a lot of 2x4 in the lab to examine mechanical properties of wood. Each year the students built poor word on the destructive results made before. All I can recall is that there seemed to be an optimim glue line thickness for shearing strength. As in above posts, this makes sense with the fit and finish of the wood pieces to begin with. In an object as small as a banjo bridge, I have no suggestions except for this: old-fashioned hide glues have actual skin collagen fibers in it and that's tough stuff.
We do not know where we are going. Nor do most of us care. For us, it is enough that we are on our way. Le Matelot. Just remember that what works for one glue does not necessarily hold for another.
I still recommend following a manufacturers recommendations as closely as you can for any specific glue. I would not trust rubber bands to provide required clamping pressure with all glues, read the instructions. At the same time you have to use common sense, using recommended clamp pressure to glue the top and back on a guitar with Titebond would probably crush the sides.
Brian's right: read the destructions! Clamping pressure suggests a way to get the optimun glue line thickness in the void for the job. Clamping time allows the little molecules to settle down and hook up with eachother and with the wood. Once the glue cures, there's no advantage to extended clamping time, unless you leave the clamp on forever and it becomes part of the structure.
Too-tight clamping results in excessive squeeze-out and can result in less than optimum adhesion. Fine Woodworking published a test years ago where they compared smooth glue joints to "roughened" glue joints. For years, people said that you should rough-up the mating surfaces to provide better adhesion.
FW found out that perfectly flat and smooth mating faces provide greater joint strength. Sivio--good point about the heat when sanding. The blanks that had poor joints all failed when I sanded them on a belt sander I've also used their cabitmakers glue, and their hide glue, but it takes a bit long to set I think its used with veneers. I use plastic clamps from the Dollar Store.
The exert enough pressure to push excess glue out of the joint, but don't compress the wood. This thread has some great info. Thanks Helix Larry for the info on red cap and white cap TItebond - I was always puzzled recently about the dark glue joints from Titebond - now I know why - I bought blue cap last time. I sometimes "cheat" and work on assemblies that have been glued and clamped only for three hours or so, but for the most part, I try to leave my glue-ups alone for 24 hours.
The cure time is dependent on humidity and temperature AND the thickness of the glue joint. I am also with the smoother-is-better school of thought on glue joints - even if you use grit paper, your wooden surface under a microscope is many miniature hills and valleys If you apply Titebond to a very smooth joint, and hold it with finger pressure for only a few minutes, you will be amazed at the holding power - if you try it with a rough joint, you will be less amazed.
It's always a mystery to me how tightly to clamp an assembly - if you coat both surfaces before assembly, I do not believe you can have so much squeeze out that you harm the joint. On the other hand, you do not want to crush any wood fibers, either. So, my compromise is to clamp-tighten joints with cauls protecting the clamped surfaces, and stop before crushing the fiber.
But I have noticed that some masters, as Silvio points out, use no clamps - Dan Pennington also has pictures on his homepage showing a layer for a rim glue up without clamps forcing the sections together. So I am still learning. Guys, Scott Z. However, I think I heard from Scott that he started using CA instead of Titebond after talking about this subject with me, but I'm not sure about that I've seen rubber bands being used in traditional luthiery all the time. There are situations in which they are the only "clamping" device that one can use.
A few months ago I had to repair a tater bug mandolin which had its top cracked in places AND unglued from the ribs: I was able to use Klemmsia clamps to fix the top cracks, using the lightest pressure, but when it came to gluing the top to the ribs all I could do was wrap the whole body with a large and long rubber band which looked dangerously like a bike tyre tube to keep the whole thing together long enough for the glue to settle. I used the white glue that they sell at LMI, as it binds fast, crystallizes completely, and could be reversible with hot water need it be done.
I still don't see any need for clamping bridge tops though This allows the glue to achieve nearly full strength. Quick Tip! Mark the time you glued up each assembly with chalk to know when it's safe to remove the clamps. You can then reuse those clamps in another glue-up. Higher temperature, drier wood, and lower humidity speed drying time; while lower temperature, wetter wood, and higher humidity slow it. Joints made using white and yellow wood glues are most easily taken apart by applying heat.
Use a heat gun photo below on the joint line to soften the glue, and the joint will open with slow, steady pressure. Wood has good insulative properties, and it may take awhile, so be careful not to damage projects with the intense heat.
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