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How do you prepare a rosewood bridge bottom prior to gluing to a sitka top? Does anyone use this "toothing Iron" tool on StewMc or do anything similar?

http://www.stewmac.com/shop/Glues,_adhe ... _Iron.html

Thanks,
Ed

nope...

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Peter M.
Cornerstone Guitars
http://www.cornerstoneukes.com

No, I floss my bridges.

The concept actually does not make much sense to me. Most glues in guitar making (epoxy being the exception) are not gap filling. So why would you create a gap and weaken the joint. That tool looks like it reduces the gluing surface by about 50%. A good snug fit and proper application of your preferred glue in my mind is much better.

Shane

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Canada

In the past they had this thing with scratching the bottom of bridges and other large parts being glued. it seems. It should be noted that this is based on hide glue usage. At their small scale, perhaps the scratches work as small reservoirs, storing some more hot glue, or preventing starving? Perhaps someone using this technique could chime in.

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Build log

There's a discussing on the MIMF called "Preparing gluing surfaces with sandpaper" where this was discussed at length, along with the main topic. It's in the acoustic guitars section.

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

I think the concept goes back to hammer veneering. Hammer veneering involves gluing a thin piece of veneer to a substrate, using the 'hammer' as a squeegee. The excess glue is pushed into the voids left by the toothing iron. For that particular purpose it works. For some reason it was thought that this 'keying' of the surface would result in stronger joints. I seem to recall that one of the big companies (3M ?) did a series of tests that proved it actiually resulted in a weaker joint.

Ed;
I've glued hundreds of bridges without one with no failures.

Lets extrapolate here -imagine using one on your top joint.
Or brace joints.

With a good wood to wood joint & modern glues there is no reason to tooth a joint.

gee ! that sounds weird !!
Mike

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Mike Collins

Ed;
I've glued hundreds of bridges without one with no failures.

Lets extrapolate here -imagine using one on your top joint.
Or brace joints.

With a good wood to wood joint & modern glues there is no reason to tooth a joint.

gee ! that sounds weird !!
Mike

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Mike Collins

Self adhesive sand paper placed on the top to make sure its perfectly contoured to the top. Remember tops have a slight radius and the bridge bottom is flat. Needs to be fitted before gluing. I wipe rosewood and other exotic oily woods down with alcohol or acetone before gluing.

I use Hot Hide Glue and prepare bridges the same way MRS does. I see no reason to tooth, even without "modern" glue. ( I don't think HHG would be considered "modern)

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I didn't mean to say it, but I meant what I said.
http://www.brackettinstruments.com/

Old school.....!

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A person who has never made a mistake has never made anything!!!

When you sand the bridge using the top radious, do you also use a jack to raise the top to the same height as when under full string tension which I assume would even increase the top radius more?

If so, how do you measure or determine how high to raise the jack/top? (I see StewMc selling a caliper set-up system for this. Is it worth investing in one?

Ed

I replaced the top on a Giannini classical a year ago, probably built in the '60s or before, and was interested to observe their glue joints. Under the (solid) linings and back braces, the back and sides had been given a coat of some sealer or finish, then raked with some tool having 4 or 5 teeth before the braces/linings had been glued. The linings had shrunk away from the sides in several places, but the back braces held. I note this as a point of interest, but wouldn't advocate the notched approach either.

Kent

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Kent

Facebook Page

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Once the bridge is fitting down with very minimal pressure, the last thing I do prior to glue application is to scrape the bottom clean with a razor blade, exposing a fresh clean surface.

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Tony Karol
www.karol-guitars.com
"let my passion .. fulfill yours"

That wouldn't make sense to me. I sand bridges to the "native radius" of the top before gluing. This should help minimize deformation. Deforming before sanding would make gluing difficult and would probably just result in a bad glue joint and more top deformation at the end of the day since you gave the strings a worse "starting point".

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http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

Like Todd, I use a domed radius disk.

radis disk

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Chris Oliver
Infinity Luthiers
...in the shop.

live every minute...

Filipo Morelli wrote:
"Moisture in the glue is, as I understand, an emulsion."

The 'modern' glues, based on PVA, such as Titebond, are emulsions. HHG is a solution.

If you have a lot of problems with starved joints, the solution is not to make a weaker joint, but to learn to use the glue right. Either you're clamping too hard, or the glue is too thin, or you're warming the joint too much.

It's also possible that the problem isn't joint starvation so much as it is a cold joint. Once the glue has gelled the structure of the glue in the joint is set. If you move it in any way after that you've broken the joint. Thin out the glue to get more working time, or else warm the joint.

I was told that back in WW II the Forest Products did some work on glue joints in laminated wood propellors. They found that the joints between laminations that were sanded to thickness tended to fail, while those that were planed did not. In another study, they found that they got stronger glue joints on pieces glued within 15 minutes of being worked. That's the 'surface energy' thing that Todd Stock mentioned.

When I'm gluing a bridge, I make both the top surface and the under side of the bridge as smooth as I can make them, using scrapers or scraping with a chisel. The clamps are ready, the caul in place, and surfaces are lightly scraped just before the glue is applied. I use locator pins to keep things from shifting.

Andy,
Thanks for the help. However, I do not fully understand what you are saying.

I understand about sanding to the "native radius" of the top with no jack being used. But then in use with strings installed at full tension the top AND bridge would both be deformed together when the strings pull the top and bridge up together. But if the top was deformed by jacking it up and then the bridge was radius-ed to fit nicely (undeformed) and glued while still jacked up, wouldn't the glue joint be just as good a gluing when when down? Then the bridge would not be deformed when up under string tension, correct?

So I do not understand your comment about causing a bad glue joint.

Also, while I think I understand your point about the top being more deformed under tension (it would pull up slight more due to the strings not having to "fight" the bridge as much), the bridge would be less deformed under tension. Is this what you were driving at?

I'm just trying to understand your points so I can learn.

Ed

After re-reading, I was mixing ideas in my head when I wrote that sentence. Let me try and clarify.

Jacking up the top would make it non-spherical making it a bear to get the bridge to fit properly. Even if you glue the sandpaper to the top, it is not of a constant radius so as you move the bridge back and forth you will end up with a joint that has some sort of dish, just not a mate to the dome of the top. That's one of the greatest benefits of a spherically or cylindrically domed top. - no matter how you hold it or where you sand, you still end up with a matching dish & dome.

When gluing the bridge, you'd have to use un-even pressure to close the joint, even with the jack in place. That could create a bad glue joint. Plus having the jack in there would probably complicate things from a logistics point of view.

Does that make sense?



At the end of the day, it would be just as deformed, just geometrically in a different spot than the other scenario.

For example (setting any geometry effects aside which would be minimal or non-existent in this case anyway). Let's say you took a perfectly flat long 1" square stick supported at both ends and hung a weight in the middle so it deflects 1/8". Now you take a 1" square stick with a curve that put the middle 1/8" below level with no weight. Add the weight and now you have a deflection of 1/4". The same would be true for laminates.

Hopefully this is more clear rather than more confusing.

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http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

To glue a flat bottom bridge to a radiused top you would be forcing them together with your clamps. This forcing could actually maybe flatting out the top radius or curving the bridge which would cause problems trying to get your flat bottom saddle to sit well in the bent bridge.The scissor jack that stew mac sells is mainly being promoted for brace and other repairs not bridge gluing on new guitars.