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I just wrecked a bridge by routing the slot too deep, so I'm starting over. It took me a long time to shape the radius on the bottom to fit the guitar top. What methods are folks using?

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Mike McNerney

Why start over ?? Many guitars have been made that way. I've got a late '60's or early '70's Guild D-55 that was built that way, and it's still fine. If you are starting over, I just use strips of sandpaper on the top itself, starting with 80 grit and then finishing with a scraper, once it's close.

Brent

I made a jig (fixture?) that has the same radius as my tops. It's the size of a full sheet of 100 grit. I just run it back and forth carefully until the radius is sanded in. It's a bit tricky, as you can sand in a bit of twist and tilt, so I flip it around a lot, and go both short ways and long ways in direction (push it back and forth, then side to side). I put witness lines and make sure the last effort takes all the lines out when sanded saideways. When placed on the top it should fit very very close, ending nothing more than gentle finger pressure to close any gaps.

Unless you've gone all the way through your saddle, a carefully fitted plug may salvage the bridge you have...

I just glue them flat to the arched top. Some of my guitars have pronounced radii (or especially some repairs) so then I will use a scraper to get it real close and follow that up with the sand paper trick as mentioned. Since the sandpaper trick puts cross grain sanding marks in it then follow that up with either a scraper or sandpaper with the grain. I like to do that before finishing the top just so I don't end up doing something stupid to a pretty top

Sorry to hear about the bad luck!

I typically keep the area flat through the bridge area...

That said, IDEALLY you radius your braces on a concave dish with a known radius. Then you radius the bridge on a matching convex dish of same radius.

I'm going to guess you're not following the process and the radius is "custom". I had that happen to me once. Then I actually used adhesive backed sandpaper and mounted it directly to the top of the guitar. You have to be gentle and it's a slow process. Before starting, I removed the bulk of the material mechanically (chisels and bench scraper) to get it a close as I could to the desired radius.

That's all I can suggest.

Best of luck with the next one!

Joe

I do pretty much what Ed describes above fut my sanding fixture is not nearly as big (I think I'll make a bigger one). Ed describes a good system for keeping things even, it took me a bit to figure out how important flipping and changing directions can be.

I'll add that after I am done sanding all the witness lines away, I scrape with a razor blade. I'm not sure how much difference that makes in reality.

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Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

I had too deep a slot in a bridge. I carefully fitted a slip of the same Rosewood and glued it in. Seems to have had no adverse effects.

Dave

I'd make a new one too and I'm not a fan of a saddle sitting directly on the top. Depending on saddle thickness it also takes away valuable wood-to-wood gluing area. There is also the issue of UST installation in time where you want something harder than a spruce guitar top under a UST.

Let's please keep in mind that from the moment we string up a guitar and put tension on the strings the bridge is forever.... now trying to come off and escape.... dang thing...

Regarding matching the top radius by the numbers meaning say I radius my top braces in a dish to 25' the top is not a 25' radius too. Instead the flat top that does appear to conform to the braces does not do so completely unless you are bracing with 2 X 4's... Instead they usually reach a compromise and the top's radius will be a bit less than the braces carved in radius.

I used to fit my bridges with 120 on the top and marked where the bridge position is and then sanding the bridge on that spot, approximately. There is a tutorial of this method here that I put here 12 years or so ago.

These days in our commercial shop we reglue many bridges weekly for Martin and others. Flat bridges on radiused tops can be fine for years until the steward lets the thing dry out and then it can lift.

As such when we reglue a bridge we do the following:

1). fit the bridge to the top so that with only minimal finger pressure the bridge is down in all places including the wings. Fitting for us these days is with a belt sander for speed and heavy lifting using the curved end of the platen followed up with scraping with a single edge razor blade.

2). Once the bridge is fit well and the bottom has been freshly scraped within 15 minutes or less of applying the glue we make sure the bridge patch on the top has been cleared of excess finish and old glue. We use a sharp chisel for this and razor blades to score finish for more finish removal. We seek and get a gapless fit all around with minimal finger pressure only.

3). We remove finish not to the perimeter, we think that looks lousy and it can show AND it complicates future repairs if the bridge has to be reglued again. Instead we clear finish to maybe around .020 - .030" from the perimeter.

4). With both surfaces ready for gluing I heat my HHG in the pot monitoring temp to not exceed 145F for more than a few minutes as it heats up.

5). Dave's breakfast burrito from last week is removed from the microwave.

6). The bridge and all clamps are put in place in a dry run and then the major clamp is taped in place in a special set-up that we do that permits the clamps to be all in place and snugged down in less than 15 seconds when the HHG is in play.

7). With the bridge prepped and fitted, the top prepped and finish cleared, wax paper on the caul under the bridge plate so HHG snot does not glue the caul to the guitar or drip in the back of the guitar we are ready.

8). I heat the bridge in a microwave for 10 - 12 seconds. Next I slather on the heated HHG and slap the bridge in place, snug down the clamps, double check my fit all the way around and wait two minutes.

9). Two minutes later I remove excess squeeze out with a toothpick, paper towel and hot water from the glue pot.

10) The guitar is hung for a day, over night with clamps in place. Clamps can be removed in say four hours too, no biggie.

11). The next day the clamps are removed and excess glue is cleaned up with hot water and a paper towel. BTW We've checked with a microscope Bounty paper towels won't scratch guitar finishes....

12). The pin holes are drilled out again to clear glue and the pins are fitted with a reamer but left out of the guitar in so much as some of the glue in the pin holes can be tacky right after drilling and reaming and could glue a pin in place.

That's it this is how we fit and reglue bridges.

Also I've left out rabbiting the bridge ledge to fit the "pocket" in the finish that we've cleared. We do this as a matter of course but it requires special tools and adds more complexity so you don't have to do it and most guitars were not made this way. Collings does it, Taylor too and a number of the top, well known Luthiers we know do this too.

Hope this helps

it's a 12 string so I decided to no let the saddle sit directly on the spruce. I prepared the blank (oversize). I alternated between my belt sander (gently) and with a 80grit clamped on the top (finishing with 150) and completed it in 15 mins and successfully routed the saddle slot. Thanks all

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Mike McNerney

I do them like Martin
Never bothered to put a radius on a bridge. Did it 2 times both tops cracked at the ends of the bridge.
Martin has done millions that way. I don't think it is that big a deal , if you like to put a radius on so be it.

I would never allow a saddle to sit on the spruce.

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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

What is your top radius John? I feel that makes a difference...

Martin has made over 1000000 and they never radiused the bridge, I use a top radius of 28 feet . From an engineering stand point there is no advantage. I used to work where they had a a stress lab. We ran a test on one of my tops that shows the stress lines like isobars.
as you have changes of rh the corners showed a higher stress level. also do you radius your bridge plate? Yes you can do it but I don't see any advantage or disadvantage.

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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

Just curious here

If you don't radius the bottom of the bridge, then the top comes a bit flatter and the bridge takes on a slight curve to accomodate everything. The bottom of the saddle slot now has a slight curve to it. How does this effect sound transmission from the flat bottom of the saddle to the curved slot?

Ed

the bridge is in constant flux. It also moves so but it doesn't flex near like you may think.. The bridge , top ,and plate work together. The amount that moves is insignificant , what the bridge does do is roll forward. Did you actually measure the bridge for curving? In the stress machine at work we saw a lot of movement but little in curve more in torque.
Spruce can move a good bit but you have cross grain glued the to with
A braces
B plate
C bridge
D top
this structure is pretty stiff. as an experiment make this structure and watch how much it warps. It isn't very much.

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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

John, I'm finding this part of the discussion interesting. I want to clarify that my braces and bridge plate were sanded in a 25' dish & glued up in a 25' dish. When glued to the body it is curved, probably not 25' exactly but something? When you lay a 'flat' bridge on the top there is approx.020 gap at each end. This seems obvious to me I needed to shape the underside. However I shaped it on the top and not on a 25' convex form. Does Martin build or assemble their tops flat or curved?

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Mike McNerney

Like Hesh mentioned above we all aim for a particular radius but when the box is built up it's very often if not usually the case that it's not that radius any more.

So then for a 28' top and a 6in bridge you would expect to see .04in at the tips of the wing.

I radius the gluing surface of the bridgeplate in the dish like I do the other braces and the gluing side of the bridge with a sanding fixture fitted to the radius dish. Technically speaking, the radius of these two surfaces are should not be the same because they are offset by the thickness of the top. I don't get too hung up on this. As Hesh points out, when you glue the braces to the top the top doesn't perfectly conform to that radius. Further, when you constrict the rim by attaching the top, every humidity shift will alter the radius.

When I go to glue the bridge sanded on my fixture, it fits very well. Certainly well enough that everything can clamp together nicely. John has a lot of experience with Martin guitars and methods. I don't doubt that gluing the bridge flat (especially if the bridge plate is flat) allow for effective clamping and a lasting joint. The top should conform more to the plate and bridge than they will to the top. Once the clamps are removed and RH changes, everything will be moving anyway. It will never be perfect.

I'm thinking about two extremes in my head knowing they both can work fine and wondering what other differences there may be. On one side you could have a perfectly flat bridge bottom an bridge plate glued to a radiused top all clamped down properly so the parts conform to each other as they will. on the other side you could (theoretically) have a bridge bottom and bridge plate that conform absolutely perfectly to the top and its radius. The stesses on these structures will be different and both will change over time. I wonder what these differences mean for tone and geometry (subtle i'm sure just doing a thought experiment). I also wonder if one would "play in" faster than the other. I'm not really sure what is happening during this break in period but suspect it has a lot to do with the stesses built into the guitar and applied by the strings settling out a bit.

Just some thought, I could be all wet.

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Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

Hesh thanks for the professional view on this. Just because a big manufacturer has done it one way doesn't make it right! After all we are trying, as individual makers to do things as well as they can be done.

Dave

I disagree. The way some large producers of guitars who have been in business over long periods of time do certain things that are known to fail at times in the future such as gluing on a bridge may and in the case of some of these manufacturers be best practices that have resulted from actual failure data from warranty claims.

When you have paperwork and a database of say hundreds of bridge lifts on fairly new instruments you may reevaluate your technique and production methods. I won't name names but I know of two manufacturers that did exactly this. In one case the review based on failure reports led to the addition of additional parts.

When I was learning to build I would spend my lunch hours when I was not on business travel in the moldy humidifier room of a Guitar C*nter. I believed that there was a lot to be learned for better or worse from the people who produce thousands of guitars and more. I still do.

Production techniques are a valuable source of information for a small builder who is keen to provide the highest value possible with their own instruments.

How we reglue AND glue a bridge on a new guitar since we do that too at times with ghost set-ups....is our current take, it morphs too on what will work best over the longest period of time AND be 100% serviceable in the future. It has elements of some best practices of commercial concerns and some techniques that are all our own.

We are currently regluing a bridge on a 1937 g*bson owned by a famous movie star and this is the first time the bridge has lifted. It has not really even lifted he's just concerned that we can get a feeler gauge in there. This glue joint is still decent and after 80 some years too with the guitar in use the entire time. It's a great example of a good job of gluing on a bridge based on the positive results. The guitar also has a sunburst finish leaving no room for error or mistakes when we successfully removed it with no finish damage.

Regardless of how many bridge reglues we do we still learn from them and we still learn from what others did and do currently and in the past. So to us it's not a matter of what's right or wrong, that's not what we see it's simply information that we have access to to evaluate on our own for our own uses to see if that golf club in our bag would improve our game AND the value that we provide for our clients.

Seems to me that what others who produce far greater quantities of instruments than an individual builder ever could do and how they do it is exactly what folks would be interested in on a guitar building forum. I know that I was and still am.

You're right that just because a big producer does it this way it does not make it right. But it does make in valuable to know about and know why for better or worse it works or fails. I'll add that the percent of Lutherie knowledge that one learns from building a single acoustic guitar is just the beginning of what there is to know and learn. Even the guys, my friends who have been in the trenches for decades like our own Chris Pile will be the first to tell you that with all they now know they still see and learn new things all of the time. That's why what any producer does is usually of great interest to people who are Luthiers and work in the trade.

I know you are trying to do things as well as you can and good for you. For me doing things as well as I can means learning all I can about how everyone does it... and what works and doesn't.

By the way if anyone wants a producer to take a very close and personal look at how they do EVERYTHING... check out Collings. I believe these are very well made production guitars that can rival what a Luthier produced guitar "should" be in terms of build quality, set-up, design, materials, overall concept and how successful it all is. My new Collings I-35 is the nicest production instrument I've ever played or owned and I can't put it down. Ultimately that's what we would want a guitar to be for the client, something that thrills them to no end.

there is 2 different schools here.
A a totally radiused up
B a partial radius top.
I do not totally radius so my bridge area is flat.
If you are using a 25 foot radius and totally radiusing the top then your bridge area has changed from the martin design
a point about Hesh's statement
Martin does do them flat. Martin over the years never radiused them and yes I also am a martin repair center. Most of the issues of bridges coming up were the methodology in the finish. They allowed a varying degree of finish under the bridge and this left the tops separate from the bridge, Once the bridge comes off the area of the bridge has a dip from the finish and what ever wood came up with the bridge.
Here I will rout them. and reglue.

so in a way we are flattening the area in the routing process.

Today Martin uses a CNC machine to inlet the bridge onto the top. If you ever get to see martins factory you will see just how flat the martin top is in the bridge area.

so this just proves there are more than one way to do this. there is one point we are not discussing and that is RH control. There is a good bit of movement from RH so that also have to be taken into discussion. Hesh and I have to differing opinions and that is fine. we may still disagree but we don't ever have to be disagreeable. It is good to know there are other techniques out there.

Of all the bridges I see that come up I think most come off from stress of RH swings or too much finish under the bridge. Understand the variables of a process. Sharing information is good enjoy

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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

So let's see; Martin makes the top flat, and glues on a bridge that is flat on the bottom? Seems to me that they're matching the bridge top the top. I radius my top, and match the bridge to that. I may be doing it 'wrong': maybe the flat top is 'better' in some ways, but I can't see any good reason to make the bridge and top different.

John Hall, please accept my apology for misunderstanding and misrepresenting your position. I thought you were saying you glue a flat bridge to a radiused top. I did not get that you (and Martin) were doing a partially radiused top with the bridge area flat.

All, my above post can be largely disregarded. I misunderstood what John was saying and based on my respect for his experience, I accepted what I thought he was saying.

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Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

Thanks John

You know too as we do that when Martins come in with lifting bridges as you said it's often RH abuse that helped get there. Just feeling for fret sprout tells the tale usually and it's a good thing to show the client. Folks usually get that wood shrinks, metal frets don't if you can feel sharp fret ends on a Martin it's been dried out. For everyone else we are discussing if the warranty kicks in or not and it doesn't if you let your Martin dry out.

Just want to be clear on one other point. I'm not disagreeing with flat bridges on domed tops. It's simply not what I did as a builder or what we do now as a commercial, busy shop. I completely agree there are lots of ways to do things so this really does not need to even be said for my consideration, I already agree and know. If there weren't lots of ways to do these things Luthiers like us would not learn new colorful words to exclaim when we see each other's work . Kidding of course.

For everyone else seems to me that an important part of discussing if you radius tops and bridges is why we radius tops in the first place and the value that this provides. It's another topic I know but I don't want it eluding a beginner reading this leaving the impression why not build all things flat. We radius tops so that they are stronger and more forgiving with differential expansion and contraction caused by RH swings and abuse. Or, in other words, the stinkin top will likely crack less if it's radiused. But that's another topic, an important one for sure.

And I do this too I match my bridges to the top.

As for one reason why production houses (f*ctories) do it differently is exactly why they never cut nut slots well either, it's cheaper and easier. Bridges can be mass produced and don't require individual fitting and the semi-skilled labor to do it.

For the most part they can stay on for a very long time anyway and we will never know who's method, radiusing or not is more forgiving to RH abuse. OTOH lifting bridges keep my GAS well funded.

I have to apologize. I did a good bit of thinking. I had a Paradigm shift. What others are saying is correct. I am matching my bridge but in an all together different way.
I am dealing with a flat top bridge area. While I use a radius X brace all my braces are flat .
I also watch my RH and take that to 40% before I glue the bridge on and this is doing what everyone else is but in a different way.

please accept my apology. There comes a time when we all get a little hard headed.
so my advice is when your debating a subject and your one of the few on the other side you have to look at the angles of your process. I misinterpreted my process and now I see that now.

thank you for you understanding

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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