I had a problem with the bridge lifting on the latest guitar that I made. I used Titebond, so i've decide to use hide glue the next time. I read an article in fine Woodworking where they recommend Milligan & Higgins. I see that thet have several strengths. I'm assuming that I should use 315gm strength for any guitar construction. Is this true?

The 192 gram strength is a good general purpose glue

The 251 gram strength glue is traditionally the most appropriate for regular cabinetwork.

The 192 gram strength "high clarity" is more expensive than its cousins because it is especially refined for maximum transparency.

The 315 gram strength is a special purpose glue for very high stress applications. It is favored primarily by instrument makers for situations where a joint will be under constant force. Of the glues the 315 has the shortest open time.

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First, M&H supplies glue up to 512 gram strength, and, of course it has the shortest open time.

(Gram strength is measured at a 12.5% solids solution in water, chilled to ten degrees (C) as how much force it takes to press a plunger into the gel a specific distance.)

That said, If you read the old literature you'll find references to hide glue over 250 gram strength as being "too strong" for woodworking. After thinking about that for a while, I think I figured out what that means.

Now, when you mix the glue to working consistency it can take twice as much water for the really high gram glues to make them spreadable. SO, when the glue in the clamped joint dries, there's half as much glue left in there, basically starving the joint. With that reasoning, the high gram glue would produce a significantly weaker joint. Rubbed and very lightly clamped joints may not suffer as much in that regard.

So, I got some of the 512 gram glue that I use specifically for handheld reinforcement patches, and it works great for that use.
I've used titebond and hot hide glue(from Stew Mac), both, for gluing bridges and never had a failure yet. And I've never been concerned about the gram strength of the the hot hide glue I purchased from Stew Mac, plus I thin it down good, for most gluing jobs.

If you are having glue failure, I'd look at how well the bridge contact is to the top before you glue it down.
The way I check this is with a .003" feeler guage. Before I glue a bridge I clamp it in position without glue, then I try to work the feeler guage in along the edge of the bridge, if it can be worked in anywhere, then you need to correct the fit. Basically the bottom of the bridge needs to be flat, and also the top of the guitar, where the bridge goes, needs to be flat.

I think you problem has more to do with the contact surface rather than glue strength.

For most work, the 192 strength works well for me. Jim Bancroft is right when he speaks of making sure the dry fit is tight before tossing the glue-brush around. Unlike Titebond, there's no real gap-filling ability with hide glue.

It's also (again, unlike Titebond) counterproductive to do any roughing of the surfaces to increase the "acreage" of glue-to-glue fitting. Hide glue prefers a smoother, tighter fit and works better with it.

Additionally, make sure the hide glue and water mix can sit overnight before heating it up. Good luck.
Thanks for your reply. Just intuitively I imagined that hide glue was somewhat gap-filling. Interesting that it likes smooth surfaces. I'm beginning to think that Jim B. is right, that there was not as good a fit between the parts as there should have been.
For years the prevailing wisdom was to "rough-up" the gluing surfaces to provide more surface area for adhesion. Many years ago Fine Woodworking ran a series of tests and determined that modern adhesives work better with really smooth and tight and clean gluing surfaces. I've used Titebond on bridges for years, never had a failure. But I make sure the gluing surfaces are clean, de-greased, and smooth as glass.
I did rough up the spruce a little. The bridge was ebony, and pretty smooth. I also left a little finish line under the bridge (about 1/32") which I'll never do again.
Yep, I tend to avoid discussions on hide v Titebond as I have no experience with Hide glue and know that many enjoy using it - but we have had no failures with Titebond over the past 10 years or so and I have also talked directly with their technical department about the suitability of this glue for general luthiery work.
They are quick to point out that the glue is most suitable for general luthiery applications but in high stress structural applications other glues will perform better. Note: guitar neck joints are not high stress structural application in this context and creep factors are not in the equation unless you like to cook your guitars in the back of your car.

I am old fashioned when it comes to keying surfaces and use a toothing iron and a lick of acetone to prepare surfaces such as bridges and fingerboards. Our mahogany and similar is shear planed with helical head cutters (Powermatics) and gets no further treatment before glue-up. Whereas, the older thicknessers and jointers tended to case harden and beat the surface which closed and deformed the grain and this required a bit of planing to expose new wood to the glue.

I recall the Titebond site mentions clean and conformal surfaces as a prerequisite and that keying the surface is ok. Hope some of this helps, Rusty.
I made a new ebony bridge with a much greater footprint than the original one. That should do the trick. After reading your responses, I'll stick with Titebond.

No personal knowledge but quite a while ago I saw a Lynn Dudenbostel piece on the web about guitar building. I can't recall where it was. But I do remember that he glued the bridge with Luthier's Mercantile luthier's glue, which is a liquid glue that resembles Titebond but may be stronger or less flexible. I queried him about using it over hide glue and he said he never had a problem with it.



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