The bridge truly is the heart of an acoustic guitar. Typically made from a piece of ebony or rosewood, it is glued to the guitars top where it serves several critical functions for the instrument.
Via the bridge pins, the bridge serves as a firm anchor point for the strings to attach so that they can be wound to tension by the tuning machines at the other end of the instrument. The location of the bridge pin holes also sets the string spacing for your picking hand. Working together with and supporting the saddle, it helps determine the intonation. Lastly, its attachment to the top helps transmit the vibration of the strings to amplify the sound and create the tone that we all love to hear.
When strung to standard tuning, a set of light gauge acoustic guitar strings exerts about 160 lbs worth of tension across the instrument. Since the strings are attached to the bridge, this pulling force is partially transferred to it. Though guitars are designed to resist that force, if there is any weakness in the glue joint connecting the bridge to the top, it can fail.
Though certain labor saving manufacturing techniques can contribute, or indeed be wholly responsible for a bridge pulling, and the need of a bridge reglue, in my experience, improper humidification of the instrument is the often the primary culprit.
When Is a Lifting Bridge A Problem?
Except in rare cases, a bridge does not usually pull completely off of an instrument without warning. Usually they start to lift just a hair along the back edge and creep further over time. While this may not sound like a big deal at the moment, it is potentially affecting the volume and tone of the instrument, and may eventually warp the top or progress to the point of complete bridge failure.
The bridge pictured is from a beautiful Taylor 914ce. As you can see, my business card fits pretty far underneath the bass side edge. It was the same on the treble side.
This is the type of test I usually do to determine if a bridge needs to be repaired. As a rule of thumb, if I can slide a thin piece of receipt paper much further than about 1/8″ under any one part of the bridge, I recommend pulling it. If I cant slide it quite that deep, but it fits under in multiple locations, I would also likely recommend pulling it. Lastly if you can fit a heavy piece of card stock, like the business card pictured, under any portion of the bridge, any amount, its got to be pulled. This one was a no brainer, it needs to be fixed now!
If the bridge is up, it needs to be removed so that the problem can be fixed before it is reglued. I have re-repaired numerous bridges where a well meaning, or overly busy, tech decided that instead of pulling it, they would do a shortcut bridge reglue by simply wicking some glue underneath the lifted portion and clamp it down. In my opinion, this is a temporary fix at best. There is a reason the bridge has lifted. It is best to remove the bridge, fix the problem, mate the bridge to the top, and reglue it.
Preparing To Remove The Bridge
Even a lifting bridge can take a fair amount of effort to remove from the instrument.
There are many different methods of removal, and they all have merit in their own way. Bridges can be shocked off using a hammer and chisel, routed off (if replacing), sliced free with palette knife, or heated via a lamp or a heating blanket and pealed off with a spatula. In my shop, we often use a temperature controller and heating blanket system. When used properly, I find that it makes for the cleanest removal.
To start, I taped off around the bridge to protect the top while I worked. Then I removed the proprietary ES2 Pickup from the bridge so as to avoid damaging it. Next I placed the heating blanket and thermocouple in position and held the in place using a couple of loose sockets that I had in my toolbox. After just a few minutes, the bridge will be ready for removal
Removing a Loose Bridge
Once the glue has been sufficiently softened by heat, removal with a spatula should be rather easy. Even still, experience is key here. If one is not careful, it is easy to gouge and tear pieces of the top of the guitar at this point. If you look at the photo above you will see that very few of the fibers from the top have been affected by the removal. That will make refitting the bridge easier.
Fitting An Acoustic Guitar Bridge
Once the bridge has been removed, the most time consuming portion of the job can begin. That is, fixing whatever problems might exist underneath the bridge and properly mating it to the top. Sometimes that could consist of removing excess finish from underneath the footprint of the bridge to ensure a proper glue joint. Other times it could involve flattening a bellied top, regluing a loose internal brace, or repairing/replacing a damaged bridge plate.
In this case, it appeared to be just a bad glue joint. I scraped off the bulk of the old glue and excess material with a cabinet scraper before doing the final fitting with a piece of sandpaper on the instrument itself.
Regluing and Clamping an Acoustic Guitar Bridge
Once I am sure that the bridge is well mated to the top, it is ready to be glued into position. I typically prefer titebond as the glue of choice because it has a fairly long open time, is fairly easily removed if necessary, and it is easy to apply and cleanup. Hot hide glue is another, more traditional, option. However, it is a bit more work to prep and has a limited open time.
To hold the bridge in position while the glue dries I use specially designed clamps and cauls to reach through the sound hole and support both the top and bottom with fairly even pressure. Once in position, I diligently cleanup the glue squeeze out to ensure a nice clean job. After leaving the clamps on overnight, the instrument was ready for an NT neck reset and setup
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