bridge r&r

[Matt Atkinson]

I am working on a friend's 1970's Martin D35.  There are a number of issues including the need for a neck reset, a B string pickguard crack, a very badly place bridge and a concave top.  I have pulled the neck and adjusted for the reset.  The saddle was only 1/16" above the bridge and action was high.  Also the bridge had been shaved down.  I have also removed the bridge and will replace it and fine tune the neck reset to it.  The bridge appears to have compressed the spruce beneath it (?) or perhaps previous work scraped a little too much away?

 

 The scratch line is how far back the new bridge has to go!  Also maybe you can see how deep the pocket is where the old bridge was.

Question:  Can I glue on some top wood fit to the old footprint, level it to the finish, and then glue on a new bridge?

Also the top is slightly concave.  I am humidifying it with sponges inside open baggies and the soundhole covered.  Hoping it comes up at least to flat.  Suggestions if that doesn't work?  

 Thanks for looking.  Advice appreciated.

[Matt Atkinson]

Thanks!

[Christopher Parker]

To answer your most recent question, if the top is concave, then I definitely wouldn't radius the bottom of the bridge.

Regarding your original question, you shouldn't build up the surface and "level it to the finish". The finish must be removed up to the scribe line, so that you are NOT gluing the bridge on top of the finish. If you glue the bridge to the finish, then you're doing it wrong, and the bridge will turn loose eventually.

Regarding the low spot, it just doesn't look that bad to me. Just pin your new bridge in place and scribe through the finish, all the way around the bridge with an xacto blade, and then chisel off all the wood up to your scribe line. The chiseling will bring the gluing surface closer to the level of the low spot. The idea is to create a flat-bottomed pocket for the bridge to sit in, so that the entire bottom of the bridge is contacting bare wood. Small deficits are inconsequential. Just as long as 90% or so of the gluing surface is direct wood to wood contact, the bridge will never come off unless you want it to.  

I attached some pics of a J-45 that I made a bridge for. Notice how the bare wood on the top has scribed and excavated to the exact shape of the bridge. If you do a clean job of it, it looks like the bridge is glued to the top, but the rosewood is in direct contact with spruce all the way around.  

[Mike Kolb]

Hey Christopher, I love seeing that old Conn Stroboscope lurking back there :) Very cool...

[Christopher Parker]

Thanks! The old Conn doesn't get much use these days. The digital Peterson strobe doesn't look nearly as cool, but it requires no calibration, and you don't have to turn a knob to select the note you're tuning to. 

[Hesh Breakstone]

Actually there are more than one way to reglue a bridge and have excellent results.  As such I disagree that bridges need the finish removed to the perimeter of the bridge.

Collings is well known for rabbiting the bridge bottom.  We do that too and have reglued hundreds of bridges through the years with no known failures beyond an unfortunate episode with Fish Glue before we went back to HHG.

We rabbit the bridge bottom the measure and specific thickness of the finish upward and rabbit inset maybe .025".  This exploits very nearly the entire bridge perimeter foot print but has the added benefit of or the .025" overlap sitting on the finish.  It makes for a perfect looking reglue with the guitar looking as it did before the bridge ever lifted.  If the bridge is fitted correctly the loss of 1 - 2% of gluing area over what you do is mouse nuts.

What I don't like about your way and how many folks clear to the perimeter is the next time that the bridge lifts, and bridges lift for all manner of reasons so it's still possible, the next guy who has to reglue it has no margin for error.  Finishes get flakey etc. making the next repair likely an exercise in finish touch up too.

Regarding fitting the bridge we fit the bridge to the top and if this means a slight radius that's what we do.  We want to have the bridge be perfectly down in all locations with only slight finger pressure.

Not trying to argue with you at all here but saying that one method must be done is not accurate since there are lots of ways to do lots of this stuff. 

I'll add too that it's impossible to describe bridge reglue best practices without a shout out to understanding glue, wood, clamping, RH, etc.

[Christopher Parker]

Rabbiting seems like a really slick way of attaching a bridge. But it sounds more appealing in a guitar building scenario than for a reglue or replacement. Rabbiting an existing bridge could change its outside dimensions, because some bridges are tapered, or have angles other than 90 degrees, therefore rabbiting will make certain bridges smaller.

Myself, and the shops that I've worked in have reglued hundreds of bridges by removing the finish to the perimeter of the bridge. They have all been very clean jobs, and I've never heard of one of them coming loose. I'm quite confident that none of the bridges that I've glued will ever come off unless someone wants them to.

That said, I'm intrigued about the rabbiting method, I've got a couple of router tables, so maybe i'm equipped. Is it just a matter of using a pattern bit that's slightly smaller than the cutter,?

[Hesh Breakstone]

The upward rabbit is only cut to finish thickness so we are speaking of often .003 - .006ish and even if the bridge is tapered there is a very small reduction in the perimeter.  If you do the math though it's likely imperceivable.  IME most brides that are taped are not tapered at the bottom edge.

The technique is not new by any means and we know lots of both builders and repair folks who rabbit bridges regardless of if it's new construction or a reglue on an old instrument.

Again my friend not trying to argue with you at all but the notion that any bridge that any of us have ever reglued with our methods will never lift again is something that I would not ever assume.  We can't know if the guitar will be left in a black car in the long-term at LAX for a month in the summer.  Damage from an injury, improper RH management, etc. can all take our seemingly perfect repairs and let that bridge raise it's ugly back side again.  Nonetheless I get your point I just don't agree that perfect workmanship and superior methods are impossible to damage down the road.  Players will be players ya know...:)

You didn't address my argument that removing finish to the perimeter makes the next reglue likely a job with finish repair and scope creep.  

I build my guitars with serviceability very high on the list and for the same reasons the repairs that we do are always serviceable as well because someone else may come after us and where I'm going it will be hot enough without hearing some Luthier in my ears cussing me out for making his/her job harder...:)

The method requires one to make a special, miniature single purpose router based on a tightened up Dremel or it could be done with a lam trimmer but that's over kill in the horse power sense.  The jig has a table that is also slightly radiused to match bridges that are radiused.  The Stew-Mac Dremel router base is what our jig is based on.

I'll try to find a pic and post it for you.

One more benefit of not cutting to the perimeter with your method if you wander from that line while cutting back finish into the visible area we are SOL.  With the rabbiting method you have in our case .025" of margin for error or how ever much inset you want to create.

I'm aware that lots of folks clear to the perimeter which I agree more gluing area is desirable since some manufacturers leave as much as 40% of the gluing area with finish over it....  I've actually measured 40% before being interested in this question perhaps beyond it's worth....  Not good.  But they do this so the finish looks nice and tidy tucked under the bridge.

Rabbiting permits that tidy look but with what you rightly want much more wood-to-wood gluing area.  Combine this approach with fitting the bridge perfectly so that only light finger pressure is required to have it down everywhere and it's pretty bullet proof but again could fail too with abuse.  We still clamp firmly and as uniformly as possible though regardless of the decent fit.

Off to find a pic of the jig.

[Hesh Breakstone]

Found a pic of our bridge rabbiting jig.  It's very fast and easy to use and since we are only removing slivers no decent power is required.

By the way I believe that Taylor is also rabbiting their bridges.  In the guitar manufacturing world reducing potential and future liability from warranty claims is huge.

[Christopher Parker]

I guess it would be naiive to assume that every bridge I've ever glued on will be there forever, but I'll be surprised if one ever comes back in my lifetime. If a guitar gets left in a black car at LAX over the summer, there will be more repairs to do on it that a just bridge reglue! But I get your point. One of the reasons lutherie is a noble profession is that a good luthier will always try to make repairs easier for future generations even though he eventually won't be around to be held accountable if a repair does fail in the distant future.

To be honest, the way I prep and glue bridges is the way I was trained to do it. Therefore I always thought of it as the right way. I'd heard of rabbiting the bridge before, but it never occurred to me that it might be easy, and I hadn't really considered the advantages, but I'm not too narrow minded to learn something new.

Thanks for sharing the photo of the jig. What kind of bit are you using? How do you remove the finish from the top? With a router? A chisel? Any other info on the procedure would be appreciated. Thanks.

[Hesh Breakstone]

Hey Christopher - We all have our own methods and training and my current way of gluing bridges is not at all the only or the "right" way to do it.  All I can say is that it's a bit different from what you do.  No problem.  

We also evolve since we teach Lutherie and our methods morph over time perhaps being refined further or taking a completely different approach.  For a couple of years now instead of  filling open top cracks with spruce or what ever... for certain instruments when appropriate for that specific instrument we may fill with of all things epoxy.  Man did I turn up my nose when I first heard of this until I learned that some of the best and most well known repair guys have been doing this successfully for years.  So.... we tried it and like it a great deal too again when appropriate for the specific instrument.  Would not do it on a pre-war Martin though and yes the epoxy is tinted to match...:)

The Dremel bit is a Dremel brand bit and I believe it's just a standard 1/4" available at any hardware or big box store.

For preparing the bridge patch we carefully score the finish and try to score only the finish not any wood fibers for the rabbit inset, around .025".  The inset can be what ever you like.  If you are an older dude like me and may not see as well as you once did more room from the perimeter might be helpful.  

After the finish is scored we use very sharp chisels to completely remove any old glue or bridge wood fibers.  Loose spruce fibers are left so long as they lay down properly on their own and then in the gluing process, HHG only for us, the loose fibers are glued back down by clamping the bridge in place with enough HHG to have squeeze out all around the bridge.

An added benefit of rabbiting the bridge and not removing finish to the perimeter of the bridge is it creates a "pocket" that the bridge will "snap" into during that time sensitive slap it down and get clamps in place in 15 seconds or so that HHG benefits from.  We also preheat the bridge around 12 seconds in a microwave and that extends clamping time when needed.

The bridge patch is cleaned very well and so to is the bridge bottom and of course the bridge bottom has also been fitted to the top as described prior.  Likely the vast majority of time that I spend on a bridge reglue is cleaning the bridge patch of old crap, etc.  

Removing bridges is a process too with lots of detail for us in that we use a heat lamp, shields, and read the top runout for the proper approach direction with our pallet knives.  The better the bridge is removed in the sense of the least top wood damage the easier the rest of it will go.

So yes a sharp chisel for cleaning up the bridge patch.

Now mind you again this is how we are currently doing things and our methods do change with opportunities to refine or even do it completely differently.  I'd hate to write a book on this stuff because it would be dated in 12 months time...:)

A month ago we had to reglue a bridge that had been reglued previously by one of the top ten or so best known Luthiers.  He did a great job, took it to the perimeter as you do and have, but it lifted because the owner let the guitar dry out.  For us it was harder to reglue because the finish taken to the bridge perimeter was now flaking off....  Anyway this was preventable with the rabbit and that's why currently we like the micro-rabbit method.

[Andrew]

"...saying that one method must be done is not accurate since there are lots of ways to do lots of this stuff. "

This. Theres more than one way to skin a cat... Whatever that means lol.

Thats a nice little jig Hesh. Might have to snaggle that one - Ive been wondering what to do with that little binding routing attachment for quite a while now.

I do still have concerns about facilitating future bridge removals, though. Usually we want them just to stay on, but there are feasibly some situations where a bridge that isn't lifting might need to be removed. Any thoughts, sir?

[Matt Atkinson]

Well right here where the bridge was installed in the wrong place!