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Some details about cutting the bridge to height

 




After the bridge feet are fit to the top, you need to cut it to the right height. You have a choice of things to do.

If the instrument was strung up when you got it and the bridge was non-adjustable or is still set to the same height, then you can measure the height of the old bridge, and cut the new bridge maybe 1/16" or even 1/8" higher to begin with. Put the strings on (at low tension) and bring a C string and an A string up to about half tension. Measure the height of both strings above the 12th fret. Then proceed as I describe below to adjust the height of the bridge.

If the instrument was not strung up and you have no old bridge for it to use as a pattern, then this step is a bit more complicated. Here is what I did on one instrument:

After the feet are fit, stand the bridge up at the bridge location on the top. Then take a straightedge just long enough to go from the nut to the bridge (a tape measure held sideways is good for this), and lay it along the fingerboard from the nut, extending it out to the middle of the bridge. Now, raise the bridge end of the straightedge until there's about 3/16" of height over the 12th fret. Then mark that location on the bridge, and cut it to that height to begin with. (It will need to come down some more, but this is a starting point.)

Another way to do the same thing (or to check what you've done) is to extend a long straightedge along the tops of all the frets, extending the straightedge over the bridge location as before. Measure the straightedge's height over the top at the bridge location, about 1" left and right of center. Add about 5/16" to this to leave some extra, and mark and cut the bridge.

At every step, leave a little extra height if in doubt. It's easy to take it down later.

Now you're ready to put some strings on the instrument, if you did not leave the old strings on it to start with (which I prefer). Stand the bridge up where it's going to be, and put at least two strings on, the outside 1st and 4th strings. Tighten them up to maybe half-tension. Measure the height of each string above the 12th fret.

I recommend, to start with, that you aim for about 1/8" action on the bass side and 7/64" action on the treble side. See how much higher the two strings are than that measurement, and then double the difference.

For example, if the C string is 3/16" high at the 12th fret, the difference is 1/16", and twice that is 1/8". If the A string is, say, 13/64" high at the 12th fret, then the difference is 6/64" or 3/32". Twice that is 3/16". Let the string tension down and take the bridge off. Now take the two numbers (1/8" on the bass side and 3/16" on the treble side) and put pencil marks that far down from each end of the bridge top. Connect the two marks with a pencil line, and cut the bridge down to it. If the fingerboard is arched, start including a (somewhat wider) arch on the bridge top now.

Now put all the strings on the instrument with the bridge in place, and bring them all up to about half-tension. Go through the height-measurement process again at the two outside strings, and cut the bridge down again as necessary. As I said before, you can use an action as high as 1/8" on the bass side to begin with.

After the bridge is cut close enough to height for a first try, then bring the strings up to tension. Make sure that the bridge keeps standing straight up-- adjust it at about 2/3 tension if necessary. Then find the right location for it.

I recommend that you first place the bridge this way: take the 12th-fret distance from the nut, double it, and add 1/8". You can then adjust it a little up or back as necessary.

Once you have the bridge placed, tune the instrument and try it out. Even at the best bridge location (found by comparing the 12th fret note and chime), you may need to make compensation cuts on the bridge in order for them all to note well up the neck. If you are using a wound A string, then you'd leave those strings without compensation and make a small compensation cut (maybe 1/32") for the D strings, a little more for the G strings, and a little more for the G strings, maybe 3/32".

As the instrument settles in, you may need to adjust the bridge height later on. If this is necessary, it's a good opportunity to do a quick re-fit on the feet as well.


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