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How do you shape the bottom of your bridge to match the top radius on the sound board?
Just curious ...
Thanks
B



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There are so many examples of this in the archives.
Many folk tape a sheet of sandpaper on the top itself and using very little pressure sand the dome of the top on the bridge.
Some have made a convex sanding plate to do the same.


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I use the both, which I learned from the archives here. Took at 6” wide by 8” board and rubbed it in the radius dish to put the shape on it. Then put some 100 grit stick it on it and sand the bottom of the bridge. I also do last minute touch up on the body itself. I like to be able to sit the bridge in place and with very light pressure from one finger see it sit down all the way around the perimeter.


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This is what I do. It works fine. I don't see any real need to build a domed sanding board. In fact, I think it might be counterproductive. You want the bridge to mate with the radius that the top actually has, not what it is supposed to have, based on your radius setup. Wood has a way of surprising you . . .

I place a piece of 3M Gold 150 at the bridge location like many others. A also mark up the underside of the bridge with a white pencil so I can monitor progress and to make sure I've done the entire surface. It can be a bit of a trick for some folks. My last student had a heck of a time keeping bridge flat on the surface. He figured it out eventually.

I start with a convex sanding board and then hand fit from there using a scraper.
One of the challenges for me is holding the bridge securely while scraping. My solution to this is to make a holding fixture out of a plaster cast as shown below.

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Great.
Thanks guys.
Leaning every day


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Thanks for asking the question, SnowMan! I learned a bit from the answers even though I already have a method.

Similar to others here, I use 80 grit on a board with a radius formed using a piece of hardboard bent over a spacer, and then fine-tune using 150 grit on the guitar top. I wonder if I might get a better joint fine-tuning with a scraper.

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Don’t forget to also put the rabbet on the bottom (where is the emoji in Tapatalk?)


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Brad, not sure if ur having fun with this, but I swear I cannot believe anybody can put an effective rabbet on that matches the finish thickness. I tried once or twice and gave up. Never had a bridge pull off. I just run some rough grit paper around edge a few times.

Mike

I do put one on with a jig. It’s just a whisper around the perimeter. I basically stole Colin North’s jig idea to use the SM edge guide for the Dremel. The guide that comes with this kit:

https://www.stewmac.com/Luthier_Tools/T ... e_Set.html

The jig winds up looking a little like this. This is a pic of the one Dave and Hesh use. I took it while I was at their shop. I made a mounting board for mine with a 30’ radius on it and mount the dremel and guide into it (got that tip from Colin).



If it’s good enough for Dave to do it it’s good enough for me.

Mike, you’re going to bring Hesh out of retirement with this!!


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Brad, did not know Hesh retired. And, no, I don't want to stir the pot. I am happy with my way simple process. It matches my technical sensibilities. I recently had to remove one of mine because the owner (a youngish overly enthusiastic strummer) ruined the sound board. I was rather impressed with the difficulty I had at getting it off.

Later, Mike

Just because I happen to be doing this very thing today...

The radius block (rub a block in the top radius dish for a few minutes, easy to make) with some 80 grit on it. I start here because I tend to get impatient and don’t want to be rubbing the bridge on the guitar top while doing the bulk of the work:



Like others, I mark the entire bridge footprint with chalk and rub on the radius block until all the chalk is gone. I flip the bridge an even amount of strokes to keep things equal on both sides.

Once the chalk is fully removed I chalk it again and move to the body to do the final fit:



This is just what works for me. Again, this is because I’m impatient and want to remove the bulk off the body and final touch up on.



Hard to get a good close up there.


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These days no sanding a bridge on the actually top for us.

Instead we position the bridge, press it down lightly with only finger pressure and the read the joint (the bridge fit on the top not what I get from our Michigan pot dispensaries...). I'm looking for gaps.

When I find them I scrape the bridge with a single edged razor blade used as a scraper until I can seat the bridge all the way around with only minimal finger pressure. Fitting by scraping once you learn to read the joint can be very fast, much faster than sanding.

Additionally and Al C. has talked about this for years on this forum the quality of the joint is superior from scraping in terms of joint energy. Sanding leaves microscopic debris on the bridge bottom (or top). Bridges should always be scraped in my view before gluing and clamping and we try to freshly scrape within 15 minutes of gluing.

Try scraping it's way faster and more fun in my view too because you get immediate gratification as the gaps disappear with every trial fit, scrape a bit and trial fit again.

stopped doing this I put them on flat just like martin. No issues

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John, just to confirm...

Flat as in no rabbet, no radius, or both?


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Wait a minute

Hesh shows up out of nowhere and nobody says something?

There is a bat signal over AAG that signals when he's quoted on the OLF.

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We see a surprising number of recent (post-2012) Martin instruments with failing bindings and lifting bridges...when did Martin go to the rabbeted/flat bridge? I have to wonder if there is a correlation between production changes in process and materials for binding and bridge, and the abnormally high number of failures we are seeing?

And I thought the correct male-approved protocol when seeing an old acquaintance unexpectedly materialize after a decade or two of unexplained absence was to grunt and nod, or if in an especially effusive mood, offer a 'yo...' or a 'hey' or similar. No?

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I think top radius is a factor. I'd be more cozy adding a flat bridge to a 50' than a 28' radius. I recall the first maybe dozen guitars I made I used a flat bridge on a 28' radius, and they're fine, but I radius the bridge bottom now.

I radius the bridge and the bridge plate as well to maintain the radius across the top. I doesn't make intuitive sense to me to force the top to be flat between two radiused x-braces. If putting the bridge on flat works in practice that's fine, but I'll stick to fitting the bridge to the top by sanding it on sandpaper on the top.

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That raises another issue - could a flat bridge on radiused top be one of the reasons for the far-to-common whisker cracks originating from the corners of some Martin bridges? While these cracks are only in the finish, and non-structural, they are still unsightly. I want to think that with a sagitta of only 0.013", it would not make much of a difference.

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the lifting bridge had nothing to do with flat ,the old procedure was that they taped off an area for the bridge that allowed a about as much as 1/8 in of finish under the bridge , this is what created that issue. Not exactly sure when they started that . I was out there for training in the late 90's and the used to tape off but the person would scrape the bridge area. Once they stopped opening the pad the bridges started lifting. Today Martin uses technology and they actually rout the top for the bridge. This pocket has stopped the bridge lift issue.
as for the binding , the issue I think is the glue they are using and the plastic. The plastic is shrinking and popping off at the waist. I have done a few repairs on this .
Here is what I do know. When you radius the bridge , most time the top will sink and I have seen a few guitars where people did crown the bridge and the edges causes top cracks. The top is in constant flux to balance with RH conditions so unless you keep the guitar in a totally stable environment you have to allow for some movement

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John Hall
blues creek guitars
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When they rout the top I presume they are only going through the finish and not into the wood.

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they actually go about .020 into the top to assure the bottom of the area is on wood and through the sealer coat Of over 250 guitars the only bridge failure was on the crowned bridges. I don't really think it matters one way or the other so if you feel good doing it don't stop.

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it