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  #16  
Old 05-13-2018, 11:03 AM
M Hayden M Hayden is offline
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If you have the ability to mill the slot deeper, chances are good that you could make a new bridge entirely, and that might well be the best solution.
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  #17  
Old 05-20-2018, 01:33 PM
Peegoo Peegoo is offline
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I'm guessing the original wood was loose-grained to begin with. Sometimes you find pieces that have weak grain lines and that causes them to fail like this.

Another repair option would be to rout away the portion of the bridge where the slot is, inlay a nice solid piece of rosewood, ebony, or other hardwood of choice, slot it, and profile it to blend with the original bridge plate.
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  #18  
Old 05-22-2018, 06:47 PM
Placida Placida is offline
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The scope of this project has expanded considerably.

I fabricated a makeshift "saddle" out of a framing nail with the head lopped off, just so I could string it up and check the action, and have it remain in place without rolling forward. This lifted the strings about 1mm off the top of the bridge. Brought the strings up to pitch and at the 12th fret they were about twice as high as they should be. The nut slots are about a mile high and I'll need to cut them deeper or fabricate a new nut, but even when I put a capo on the first fret the action was still super high. (By the way the guitar sounded great!)

Here's a photo of a straight edge with the strings off. The bottom of the straight edge rides about 1.5 mm below the top of the bridge. With strings on tuned to pitch it ran smack into the front of the bridge (about 3mm lower).




We all know what this means - and I intend to do it myself (go ahead and cringe ....). Seriously though, I have spent a good part of my life building, repairing and restoring things made of wood - wooden boat restorations, cabinetry, lots of custom interior trim work, etc., and I have a pretty good skill level working with simple hand tools, making things fit correctly. I am certainly not a luthier but I know I can do this if I am very careful, take my time and follow the very helpful instructions generously provided by guitar pros on You Tube, and particularly the step-by-step photo guide from Bryan Kimsey. I will document this here once I get started. One good thing about this guitar, Goya pre-drilled a hole in the neck block with direct access to the dovetail joint, so no need to pull a fret and drill holes for steaming the neck off the guitar.

Now, back to that broken bridge. Here's a photo of the front showing a perfect cutaway view of the saddle slot. I've roughly illustrated where I intend to deepen by 1/16" the middle section of the slot directly in front of the pin holes, as John Arnold suggested. I am going to do this with a very small, very sharp chisel. When I shape the saddle the bottom will fit perfectly into the recess and the ends will sit firmly in the original depth of the slot. This will give the saddle a little extra support below the repair.




For the repair itself I'll use two part epoxy but before I glue I am going to drill three 1/16" diameter holes through the broken part at an angle into the main part of the bridge. As soon as I fit the epoxied piece on I'll counter sink #2 screws coated with a drop of CA. I haven't figured out what type of clamp I'll be able to use on this but perhaps I can get by with just the screws. Feel free to guide me here ....

Photo of the broken piece in place, no glue of course.




The bridge plate. At some point in the 52 year-old solid magogany guitar's life someone slotted the bridge and the plate - very sloppily. As you can see in the photos above the ramps are not perfectly straight, and the photo below reveals a fairly chewed up bridge plate caused by someone who didn't know what they were doing. I would like some of you professional luthiers to advise me on how I should correct this, or if I should just get by as it is.




A few more photos of this really nice little guitar.




Brazilian?



Solid mahogany neck & plate.
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  #19  
Old 05-26-2018, 01:05 PM
Placida Placida is offline
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I have had too many other things going on to begin working on this guitar until at least a couple more weeks, and regarding the aforementioned neck rest that's probably a good thing. It has always been my thought that a neck reset was needed because the neck joint had worked loose.

Then I found this at Fretnotguitarrepair.com:

Many are under the impression that neck resets are necessary because the neck moves, in actuality that is rarely the case. Neck resets are performed because the top's arch has increased, raising the string height. We often call this "top belly".

I've also read that top belly can be caused by long term string tension, and high humidity. In my search I found this thread:

https://www.acousticguitarforum.com/...e+dehumidifier

In that thread there's lots of discussion on using silica gel in the guitar case to reduce water saturation in the guitar, reduce the belly and to maintain a level that will keep the top from bulging back to where it was. So my question is, if you can bring the belly down and keep it at an acceptable level, a neck reset wouldn't be necessary, correct?

Right now my 00 guitar has a belly that produces a 3/16" (4.7mm) gap on each end of a straight edge that is placed just behind the bridge. I am only 3mm away from having the straight edge ride just atop the bridge when placed on the fretboard. It's sounding like I will be able to avoid a neck reset. The neck joint on this guitar is rock solid, with no sensation of movement when I apply moderate pressure, and there is no change whatsoever in the heel-to-body joint with or without strings.

Educated comments, please!
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  #20  
Old 05-26-2018, 02:20 PM
charles Tauber charles Tauber is offline
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Quote:
Originally Posted by Placida View Post
It has always been my thought that a neck reset was needed because the neck joint had worked loose.
Very rarely is the neck joint have anything to do with the need for a neck reset.

Quote:
Neck resets are performed because the top's arch has increased, raising the string height.
That isn't generally true.

Neck resets are usually necessitated by the string forces trying to fold the guitar in half. Prolonged string tension causes the shape of the body to permanently change geometry, resulting in rotation of the upper bout as if the end of the fingerboard was trying to fold into the sound hole. The upper bout movement takes the neck with it.

Prolongued tension does cause the area between the sound hole and the butt to form an "S" shaped curve, with the area between the sound hole and the bridge sinking and the area between the bridge and the butt rising. This is rarely so large as to necessitate a neck reset unless the guitar top has been way under-braced. The rising of the top can contribute to the need for a neck reset, but is rarely the primary cause.

Most guitar makers design for some rising of the top due to string tension - about 1/16" is common. Trevor Gore has suggested that about 2 degrees of rotation of the bridge (into the "S" shape) is optimal.

Quote:
I've also read that top belly can be caused by long term string tension, and high humidity.
Most "flat top" guitars are intentionally built with a slight curve or dome to them: most brand new guitars do not have flat tops. It is important to distinguish between three things. First, the initial curvature built into the top. Second, how much, if any, the shape of the top has changed due to prolonged string tension. Third, what the current humidity level is compared to the humidity level at which the guitar was made and assembled.

The top and back of a guitar are fixed around its edges, preventing the overall size of the top or back from changing. As ambient humidity levels change the top and back will absorb and desorb moisture until the moisture levels in the top and back are in equilibrium with the moisture content of the surrounding environment. Very little can be done to prevent that.

When a top or back absorb moisture they increase in size. Being prevented from increasing in overall size, they bow or dome. If the top is dead flat, it will form an arch: if the top is arched, it will increase its curvature. As it does so, it will take the bridge with it, increasing string height above the fingerboard.

If the guitar is maintained at, or near, the humidity level at which it was made and assembled, the geometry of the top or back will be, but for changes resulting from string tensions, the same as when it was made. Many manufacturers build at 40 to 45% relative humidity. If the humidity increases well beyond that, one can expect the dome in the top to increase and the string height over the fingerboard to increase.

Start by measuring the actual relative humidity of the guitar's environment: don't guess, don't use outside weather reports and don't start a course of "treatment" without knowing what actually needs to be done - raising or lowering the humidity. If the humidity values are more or less the same as those to which it was built, dehumidifying is not the answer. If you dehumidify too much, you run the risk of cracking woods as they shrink.

Quote:
So my question is, if you can bring the belly down and keep it at an acceptable level, a neck reset wouldn't be necessary, correct?
Humidity levels and neck resets aren't really related.

If you maintain the humidity levels where they should be, any bellying is not caused by changes to humidity. If there is excessive bellying, often there are structural problems of the top and its attached structure. Those are best addressed by other means than a neck reset.

Quote:
Right now my 00 guitar has a belly that produces a 3/16" (4.7mm) gap on each end of a straight edge that is placed just behind the bridge.
That doesn't necessarily mean anything. For example, I build guitars with a side-to-side arch of 3/16" rise / 16" run. What you are describing is almost "brand new" geometry, at least on my instruments.

Quote:
I am only 3mm away from having the straight edge ride just atop the bridge when placed on the fretboard.
Don't get hung-up on the straight edge test. It's just one possible measure.

What ultimately matters is that you have the string height you want at the 12th fret while also having sufficient break angle over the saddle at that height. What necessitates a neck reset on most instruments is that there is little saddle left while the strings are too high off the fingerboard.

What happens as many instruments age is that the permanent rotation of the upper bout effectively raises the string height above the fingerboard. To counter that, on traditional neck attachments, the saddle height is reduced to compensate. As the geometry continues to change, more and more saddle height is eliminated until there is insufficient to maintain a minimum break angle over the saddle. By then, the vertical string height measured from the guitar top to the bottom of the strings at the bridge is reduced, sometimes reducing volume and projection of the instrument.


Quote:
The neck joint on this guitar is rock solid, with no sensation of movement when I apply moderate pressure, and there is no change whatsoever in the heel-to-body joint with or without strings.
As it should be and rarely is otherwise.
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  #21  
Old 05-31-2018, 09:06 PM
Placida Placida is offline
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Charles - thanks so much for the details in helping me to understand what I'm dealing with. I actually had to re-read your post for it all to sink in, and now I pretty well understand the various factors involved in acoustic guitar movement and the effect in the geometry of the guitar.

Right now, with my windows open all day/night in coastal Florida I know the humidity level is high in the house. Though I currently do not have a reliable hygrometer I intend to get several, verify the humidity of the guitar's environment, and go from there. I would at least like to try and get the guitar's moisture level down before I begin repairing the bridge.

Again, thanks for the help. I truly appreciate your sharing your knowledge and experience with me.
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