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shape bottom of bridge to fit soundboard?
http://www-.luthiersforum.com/forum/viewtopic.php?f=10102&t=6753		 Page 1 of 2
Author:  ecklesweb [ Thu May 18, 2006 12:36 pm ]
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I suspect I know the answer to this question, but I'll ask it anyway...

Should I take some action to make the (flat) bottom of the bridge fit the
curvature of the (not flat) soundboard? Unless I missed a page, Cumpiano
doesn't seem to address it -- though it is entirely possible that I did
miss that page as I use it more as a reference and not a cover-to-cover
read. Kinkead, however, is very up-front about it.

Assuming the bottom of the bridge should be curved, what technique do
you use to achieve that? Only thing I know to do is tape sandpaper to the
soundboard around where the bridge will be placed and sand the bottom
of the bridge. With an ebony bridge, it feels like it will take roughly six
and a half years . Kinkead suggests a scraper, but I <gasp>
don't have a scraper .

Thanks in advance.
Author:  rich altieri [ Thu May 18, 2006 12:39 pm ]
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Jay, I picked up a small dish made for this on eBay. Dont have the name anymore but if you search "luthier" on ebay and browse through you will find it. Others may have different advise or possibly a source. Was pretty inexpensive.
Author:  Pwoolson [ Thu May 18, 2006 12:41 pm ]
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Yes, raduis the bottom of the bridge.
If you have dished forms, you can use them to make a dome in a piece of plywood, then use that to cover with sandpaper and raduis your bridge. If not, afix sandpaper to the top and go at it. Don't push very hard or you are going to change the dome of the top and it won't be accurate.
Author:  Tom Harbin [ Thu May 18, 2006 12:44 pm ]
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One of our sponsors, Luthier Suppliers makes them to any radius you need. I purchased one and it works great! tom
Author:  Serge Poirier [ Thu May 18, 2006 3:03 pm ]
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Thanks for the tip Paul!

Serge
Author:  Mattia Valente [ Thu May 18, 2006 3:16 pm ]
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I arch the bottom. Like Paul, made my own 'sanding dome'. Scrap of MDF, about 3 minutes with my sanding dish, voila, perfect match. This is one of those things I see absolutely no reason to buy as an 'extra'. It's not like you have to jig up or anything...
Author:  Dennis Leahy [ Thu May 18, 2006 3:24 pm ]
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As Paul said, use a sandpaper covered radius dish (if you have one) the same radius as your top, to make a convex sanding block. However when I made one, the radius did not match the top perfectly, and it was the opposite of what I would have figured: the top's actual radius was greater than the sanding block. So, I had to go back to the sanding dish with my block, and exaggerate the sanding and rocking motion to make a sanding block that followed my top.

You might want to sand the bottom of the bridge with a medium-high grit, like 320, so you are creating a pretty smooth gluing surface.

As you sand the bottom of the bridge, keep checking ("dry fitting") the bridge on the guitar top by trying to slip a piece of paper under the (back, front, and ends of the) bridge, while you hold the bridge down gently.

Hope this helps.

Dennis
Author:  Andy Zimmerman [ Thu May 18, 2006 4:18 pm ]
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i use a mini convex dish that Tracy L sells. (Luthiers Suppliers)
Attached some sticky sandpaper and rub away.
Works great.
As a sponsor he has several radius available.
I have a 25 to match
Author:  Martin Turner [ Thu May 18, 2006 8:18 pm ]
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I work the underside of my bridges with a cabinet scraper.....more trial
and error involved in getting a good fit but it works for me. If the brogde
is rosewood dont forget to give the gluing surface of the bridge a wipe
with acetone/meths before gluing up.

Campiano puts an arch in his tops by putting a curve in the brace
immediately south of the sound hole. My current classical has been built
following Campianos book and around the bridge area theres not much
curvature at all on the top and minimal shaping of the bridge required.

Cheers Martin
Author:  Colin S [ Thu May 18, 2006 8:29 pm ]
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I've always used the top of the guitar as the pattern for shaping the underside of the bridge, cling film (to protect the top then 120 grit sandpaper at the bridge position, work the bridge on the sandpaper until a good fit. Just because you started off building in a 25" dish doesn't mean that the top ends up exactly 25" radius.

Colin
Author:  Serge Poirier [ Thu May 18, 2006 9:50 pm ]
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Very cool tips guys, thanks!

Serge
Author:  Dave White [ Thu May 18, 2006 10:23 pm ]
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I use the sandpaper taped to the top in the bridge area method that others have described as my tops don't have perfect spherical domes (by choice plus what the top wants to do). I mark the bottom of the bridge with white chalk and keep going slowly until all of the chalk is gone. The biggest chore is having to regularly stop to remove the dust so that it doesn't get onto the top. One of those chores where you go into "automatic mode".
Author:  Dave Rector [ Thu May 18, 2006 11:57 pm ]
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[QUOTE=Dave White] The biggest chore is having to regularly stop to remove the dust so that it doesn't get onto the top. [/QUOTE]

Seems like Colin's idea of covering the top with cling wrap before laying down the sandpaper would help a lot with this part.

Thanks Colin!
Author:  ken walton [ Fri May 19, 2006 1:47 am ]
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There is also the long version..but it works very well.Locate the bridge in the exact spot,and clamp it just enough to hold it firmly,but light enough to slip a two inch wide,long piece of 180 plus grit of sand paper under the bridge,then star sliding the paper back and forth until you match the tops radius,the pain in the butt is,you do have to check it periodically, but eventually you will get a perfect match.(old school stuff).Using cling wrap is a great solution to keeping the top clean.

"If you build it, they will BUY..
Author:  Todd Rose [ Fri May 19, 2006 2:33 am ]
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Rather than trying to fit the bridge to the curvature that the top happens to be at on the day I'm going to glue the bridge on, I think of the bridge as another brace that helps hold the top in the curvature that I want it to be at. After all, the top is domed by the braces and the rim, which have all been shaped according to my predetermined dome. The bridge is just another part of that doming structure, so I want it to be shaped to the same predetermined dome. Am I making sense? When the bridge is clamped and being glued on, the top will flex to conform to the concave dome on the underside of the bridge. The bridge and everything else can flex some, too, so the actual dome I end up with will be a little different than the planned dome, and will continue to change from day to day as humidity changes.

I use a convex sanding board that matches the concave sanding dish I shaped the braces and rim with (I have one I got from Tracey, and I made one of my own, too). I put witness lines on the bottom of the bridge before I begin sanding, so I can see my sanding progress and make sure it's even. When I've sanded almost to the ends of the wings with very coarse paper, I start running through progressively finer grits till it's a very smooth surface. This completes the sanding of the curvature all the way to the ends of the wings, and gives me the best gluing surface. Then I go glue it on right away (freshly prepared surface: best glue joint).

BTW, I use a vacuum clamp to glue it on.
Author:  tl507362 [ Fri May 19, 2006 2:57 am ]
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I like the way Todd is doing it. I think you want to make the radius the same as you did with your braces, then glue it down. This will help bring back the radius that you made your braces if it has sprung back from installing the bridge patch. All methods will work, and in fact, I know of several well respected builders who glue their bridges on flat. If you have a radius dish, the sanding of a 2x4 in the dish to get the dome is an easy enough way to make this, or you can buy one for me for $25, or you can buy one of my radius gauges for $18 and draw the domed radius on a 2x4 if you don't have a dish. Or you can order a radius dish from us and we'll add a 10" round dome on the back of it for $15. BTW, I just used my little dome jig the other day for fitting my Bouzouki bridge, and it took about 3 minutes of sanding with 3m gold stickit paper @ 80 grit. It was a perfect fit also!
Tracy tl50736238856.4990625
Author:  Dave Anderson [ Fri May 19, 2006 8:08 am ]
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For those of you sanding your bridge on your soundboard I would reccomend putting a small machinists jack or some type of support inside.Just a little extra support for your tops. It works for me.
Author:  Alan Carruth [ Fri May 19, 2006 12:17 pm ]
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Some classical builders actually make the top more or less flat, and then pull some dome into it by shaping the bottom of the bridge. I'm of the camp that says make it to the shape you want the top to be, and use it as a brace. Everytnig is going to change anyway with the least difference in humidity.

I'm not a big fan of sanding glue surfaces. Unless you use pretty fine paper (say, around 220) the surface will look like a plowed field, and will be full of wood dust and sand. How well is that going to stick down? Also, I always seem to round off the edges. I'm afraid you're just going to have to get a scraper, or at least a broken piece of glass. I like to make the underside of the bridge just noticably convex from front to back, just to be sure that the edges are down.

Gluing a freshly cut surface does indeed reslt in stronger joints, due to the higher 'surface energy'.

Last, but far from least: use hot hide glue. Accept no substitutes.   
Author:  Martin Turner [ Fri May 19, 2006 12:34 pm ]
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[QUOTE=Todd Rose] Rather than trying to fit the bridge to the curvature
that the top happens to be at on the day I'm going to glue the bridge on, I
think of the bridge as another brace that helps hold the top in the
curvature that I want it to be at. [/QUOTE]

Todd, a couple of comments:

1. if youre putting curvature into the bridge then will you not also be
putting a curve into the saddle slot and thus run risk of saddle not
seating in bottom of slot?

2. Wont flexing the bridge increase the chances of the glue joint failing?

Cheers Martin

Author:  Martin Turner [ Fri May 19, 2006 12:41 pm ]
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Todd cancel last post, didnt read your post properly.

Martin
Author:  ecklesweb [ Fri May 19, 2006 1:02 pm ]
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[QUOTE=Alan Carruth]Last, but far from least: use hot hide glue. Accept
no substitutes.   [/QUOTE]

I'm very hesitant to disagree with this statement, but I'm going to, so that
if I'm wrong I'll have the chance to be set straight.

The reason I'm hesitant is that I don't have experience doing this as many
of you do. The reason I'm going to go ahead and disagree is because my
understanding is that a modern PVA glue and a good fitting joint produce
a far stronger bond than traditional hot hide glue (and an equally good
fitting joint). A modern glue produces a joint stronger than the wood
around it.

IF you take that statement to be true, then coupled with the fact
that bridges "want" to fail by lifting up off the top, doesn't it stand to
reason that it would be better to use a modern PVA glue rather than a hot
hide glue?

The few arguments I can anticipate in favor of HHG are about the acoustic
properties of the joint and/or the ability to remove the bridge.

Like I said - based on my inexperience in lutherie, I accept the fact that I
may be dead wrong about this, so I'm very open to differing opinions.

In fact, I'm slipping into my Nomex suit now.
Author:  Serge Poirier [ Fri May 19, 2006 2:53 pm ]
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Good point Jay, i'd also like to know all the advantages of Hide glue for that critical joint.

Serge
Author:  CarltonM [ Fri May 19, 2006 3:24 pm ]
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Here's the deal, as I understand it. PVA glue "creeps"--that is, if being stressed sideways to the joint, as bridges are, it can tend to slowly move in the direction it's being pulled. When it does that to failure, it pulls wood fiber with it and can tear off a nasty section of your top. Hide glue, on the other hand, dries hard and doesn't creep. If the joint fails, it's more likely to pop off cleanly, making a re-glue as easy as the original job was.

That being said, a lot of bridges have been glued with PVA, and remain solid. It's your call.
Author:  Serge Poirier [ Fri May 19, 2006 4:17 pm ]
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Thanks Carlton, it makes a lot of sense, i also agree by the fact that the HHG i use gives instant bondage, if clamped properly, then there should not be any problems using it for this application especially. Thanks again.

Serge.

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