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| Shaping bridge’s bottom profile? http://w-ww.luthiersforum.com/forum/viewtopic.php?f=10102&t=2147 |
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| Author: | L. Presnall [ Fri Jun 10, 2005 5:56 am ] |
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I'm ready to glue bridges on a couple of guitars  ...however, I've always been able to just place them on the guitar top, and they've fit fine...these two don't...the wings are almost 1/16" above the top surface. So, do I need to concave the bridge bottom (what's the best way?), or just rely on clamping to get the wings glued down. I have a vacuum clamp and the LMI bridge clamp. I think the LMI will do it, but I just want to make sure that's okay to do.
Thanks, Larry |
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| Author: | John How [ Fri Jun 10, 2005 6:13 am ] |
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Some people don't shape the bottom of the bridge but just clamp it down. I have a 4x4 about 2 feet long with my 25 ft radius on one side and covered with a sanding belt. Just a few passes across that and the bridge fits just right. I made a handle that has pins to engage the string pin holes to make the job easier. |
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| Author: | L. Presnall [ Fri Jun 10, 2005 6:29 am ] |
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John, Whew! I thought you were going to say you used that 4x4 as a clamping caul!  I will probably dry clamp it to see how it looks and shape the bottom if I have to. Thanks!
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| Author: | Wade Sylvester [ Fri Jun 10, 2005 6:30 am ] |
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My teacher told me as long as it is good and flat, and you glue and clamp it properly, it shouldn't matter. but I always tape a square of sand paper centered on the bridge place of the guitar it is going to be glued to and sand it flush to that couture. You need to do this before prepping the top for gluing the bridge. It doesn’t seem to take too long and it makes a difference in the seating of the bridge. |
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| Author: | Don Williams [ Fri Jun 10, 2005 6:39 am ] |
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I use a chain saw to rough out a curve on the bottom. That usually gets it close enough. |
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| Author: | Don Williams [ Fri Jun 10, 2005 6:42 am ] |
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Ok, the truth is I sand a radius into the bottom of the bridge, starting with a file and eventually to scraper and sandpaper. Eventually I'm going to do what John How does (How does John?) and make a convex form from my sanding dish. I've also thought of a couple different ways, but this seems the easiest. |
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| Author: | crazymanmichael [ Fri Jun 10, 2005 6:50 am ] |
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making the bridge bottom contour match the top is just one more step in removing stresses which inhibit the top from performing its function. and it also means that the bridge has less stress trying to make it come off. i can't think of, nor have i ever heard anyone argue positively against doing it. |
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| Author: | GCote [ Fri Jun 10, 2005 7:36 am ] |
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I made a domed sanding disc by using bondo and molding it to the raidus dish I use for my tops. I then cut it into a circle and mounted it to a small faceplate and use it on my lathe. I made a holding device to hold the bridge to it while I sand the raidus to the bottom of the bridge. Takes about 30 seconds to sand it. Gary |
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| Author: | Dave Rector [ Fri Jun 10, 2005 8:05 am ] |
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I do mine the same way John does. I just like for things to fit without having to force them with clamps. Seems as if it should be better that way. I have no data to back that up though. |
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| Author: | Mike Mahar [ Fri Jun 10, 2005 8:11 am ] |
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[QUOTE=Don Williams] I use a chain saw to rough out a curve on the bottom. That usually gets it close enough.[/QUOTE] Chain saws are old fashoned. You should try Semtex or C4 if your supplier looks the other way on the paper work. |
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| Author: | crazymanmichael [ Fri Jun 10, 2005 8:15 am ] |
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c4 and semtex are old tech. tres crude! go for the moderne technique witn mini shaped cutting charges with sequential detonation! |
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| Author: | Pwoolson [ Fri Jun 10, 2005 8:17 am ] |
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I would have to say that if you try to force the wings down you are asking for trouble. I just took a piece of MDF (maybe 3" x 12") and ground it to a 25" raduis on my dish. then put PSA sandpaper on that, chuck it in my vise and back-and-forth it 'till all my witness lines are gone. It takes me longer than 30 seconds but it's very much worth it. |
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| Author: | Terry Stowell [ Fri Jun 10, 2005 8:35 am ] |
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[QUOTE=crazymanmichael] c4 and semtex are old tech. tres crude! go for the moderne technique witn mini shaped cutting charges with sequential detonation![/QUOTE] GROUP BUY! GROUP BUY! 
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| Author: | L. Presnall [ Fri Jun 10, 2005 9:13 am ] |
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I used the last of my C4 on my mother-in-law's head cold...(what, she was a little congested  ), so I guess I'll do the MDF grinding that Paul speaks of...one day I'm going to motorize my sanding dishes and all my problems will be solved! (I know, I know!)
Thanks guys! Larry |
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| Author: | Don Williams [ Fri Jun 10, 2005 9:25 am ] |
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[QUOTE=L. Presnall] I used the last of my C4 on my mother-in-law's head cold...(what?! she was a little congested...) [/QUOTE]
ROFLMBO  |
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| Author: | crazymanmichael [ Fri Jun 10, 2005 10:14 am ] |
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hope wife has sense of humor!!! |
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| Author: | L. Presnall [ Fri Jun 10, 2005 10:15 am ] |
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By the by Paul, I just got in from the shop making the MDF thingie, and sanding the bridge bottom...perfect! Thanks for the info...10 minutes to make the caul, and 10 to sand!
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| Author: | L. Presnall [ Fri Jun 10, 2005 10:16 am ] |
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Michael, my wife doesn't read this forum!
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| Author: | crazymanmichael [ Fri Jun 10, 2005 10:22 am ] |
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ain't you lucky!!! |
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| Author: | Dennis Leahy [ Fri Jun 10, 2005 11:20 am ] |
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Visualizing (as a woodworker, not yet as a luthier) even a very wide bridge, it seems to me that going to the trouble of creating a convex dome for sanding would be overkill. I would think a 2-D convex curved surface (as per John How), and sanding along the length of the bridge, would be more than adequate. I guess I become more and more wrong as the bridge becomes wider and wider (Kasha-esque.) I'm saving my supply of C-4 for soundboard voicing. Dennis |
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| Author: | Tim McKnight [ Fri Jun 10, 2005 11:24 am ] |
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I use my motorized sanding dish sander. I made a convex form by lining my concave dish with wax paper, mixed up some Bondo and put it on the wax paper. Roughed up a piece of 1" plywood with 36 grit on a grinder and then pressed the plywood into the Bondo. Allowed it to cure, cut it in a 14" circle, covered it with 100 grit PSA and chucked it on my dish sander. |
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| Author: | Mattia Valente [ Fri Jun 10, 2005 11:52 am ] |
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I did what Paul did, minus the motorization. You've got a radiused dish with sandpaper (one assumes), you've probably got some scrap MDF, so make yerself a 'positive' arched/radiussed surface to sand the top contour onto the bottom of the bridge. Honestly, I think making an MDF 3-d arched surface is faster than any 2-d arch ever. MDF sands very, very quicly, and there's not a whole lot of material to remove to reach 25' or 28' or whatever other radius is in use. Quick and painless, and you get a nice tight fit. |
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| Author: | L. Presnall [ Fri Jun 10, 2005 12:17 pm ] |
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Mattia, 'zactly what I did, per Paul's suggestion...worked great, and so easy that anything else would take longer to build than all 7 bridges are worth, time-wise! Larry |
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| Author: | Pwoolson [ Fri Jun 10, 2005 1:27 pm ] |
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I keep trying to think of some fancy way to do this. Motorized, all jigged up, etc. But alas I grab my stick of MDF and spend a few minutes of sanding. Sometimes less is more. Larry, glad it worked for you. BTW patent pending on that design. :^) |
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| Author: | Robbie O'Brien [ Sat Jun 11, 2005 12:08 am ] |
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[QUOTE=crazymanmichael] making the bridge bottom contour match the top is just one more step in removing stresses which inhibit the top from performing its function. and it also means that the bridge has less stress trying to make it come off. i can't think of, nor have i ever heard anyone argue positively against doing it.[/QUOTE] YEAH!! 
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