Before compensating the nut, and after neck relief, saddle height, and nut height have been set as well as possible, and a buzz check has been done, it’s a good idea to test the amount of intonation error at the nut. Note the cents sharp or flat for each string. This will give you some idea of how much adjustment is going to be needed on each string, before making any alterations. See Measuring the Intonation Difference Between Two Notes.
Nut compensation is done by comparing and adjusting the nut for the open string to be in tune with fret 2 or fret 3. (I don’t use fret 1, because of its proximity to the extra height of the nut.)
(When completed, and if compensation at the saddle is perfect, then the open string note will also be in tune with the 12th fret, but it’s more important that the open string be in tune with the lower frets, than it is with the 12th.)
The amount of compensation needed at the nut depends on string size and action height, but it also depends on the actual position of the nut on any particular guitar - they vary a lot!
Working on several guitars with medium low action, trial and error resulted in offsets of about .05” at the 6th string and about .025 at the 2nd string. Other guitars had slightly less than the calculated width between the nut and fret 1. These needed less compensation for sharpness on some of the strings, while other strings were already slightly overcompensated, and the nut needed to be filed back.
It's possible for a guitar to actually have the first fret width wider than the calculated width, or the nut may be pushed backward from the fretboard by debris or excess glue. That would need even more compensation, or trimming the fretboard.
Before doing any work on the nut, it's best to try to get an idea of how much compensation will be needed. You can do this with fretboard measurements , but it's much quicker and more helpful to measure the actual intonation errors with an accurate tuner. See Tuners and Tuning.
Electric guitars usually have an unwound third string – the unwound string needs more compensation than a wound third - about as much as the 6th. Likewise, classic guitars have a fat unwound third, and the wound strings have a stranded nylon core, making them very flexible, therefore needing less compensation. The third string and sixth require the most compensation and the other strings may require very little.
People are reluctant to do anything that would alter the appearance of a guitar, so that may be an important factor in selection of a compensation method. On the other hand, I have found that a compensated nut may look a bit strange at first, and at close range, but, after a while I don’t even notice it.
When it gets right down to it, I will always choose what makes the guitar sound better, in spite of a small change in appearance. (But then I don’t have a 1930’s Martin, or a 1950’s Les Paul.)
For final string height adjustment at the nut on an acoustic guitar, I use a feeler gauge to set the first and second strings to .013”, and graduating up to .016 on the sixth string on an acoustic guitar.
Electric guitars are typically set at about .010-.012. treble, graduating to 13-15 bass.
Never set the the first-fret string height to under .010, or less than the string height above fret 2 when the string is fretted at 1.
In the methods that follow, I write about filing the nut back when a string is overcompensated by the forward location of the nut. This can be done by filing a facet on the front edge of the nut, similar to how the saddle is often faceted.
The problem on the nut is that there is not much exposed nut height to accommodate a facet. I sometimes use a small Dremel grinding wheel, held horizontally to the front edge of the nut, to make a small hollow grind. This keeps the upper edge of the nut as vertical as possible, while minimizing the height of the cut.
Stephen Delft aptly describes using strips of material to compensate each string separately. This is a good method, especially when experimenting, and when there is high action, for example a setup for playing slide.
This method can be found at: https://www.mimf.com/nutcomp/ (not available at this time). Delft also describes a method of checking out the accuracy of the entire fretboard. Notice that this article was written in 1992; Delft was a pioneer! So, nut compensation is not new - just mostly ignored!
When compensation is needed for all the strings, I prefer to make a shelf-nut, by adding a continuous strip (shelf) to the original nut, rather than adding an individual strip per string.
To make it, start with a piece of bone a little more than 1/16” wide by about 3/16” high by about 2 ¼” long (The oversize facilitates handling). Cut a slight arch in the bottom edge and sand to match the radius of the fretboard by laying sandpaper on the fretboard. Then cut it to a bit over length and maybe take a little more material off the top, still leaving enough height to hold onto while gluing it to the original or new nut.
Now the tricky part: super-glue the shelf to the nut. Coat the end of the fretboard with fretboard oil or mineral oil, to prevent gluing the nut or the shelf to the fretboard. Be sure to use enough superglue on both surfaces, because there is not much common area of the two pieces for the glue to hold (the finished shelf will be very thin). Don’t wait too long to carefully remove the nut from the fretboard. After the superglue is dry, I like to add more superglue to the joint, especially if the original nut has been rounded on the top edge.
Then file or sand the excess material off the shelf to match the height and width of the nut, sloping upward. You can file back some of the width of the shelf that you think you will not need on the smaller-cored strings. With future-past experience* you will know about how much to take off.
Continue the original slots through the new shelf. Considering the added length of the slots, try not to make the angle of the slot too great; my guess is that an angle of about 8° is good.
Install the shelf-nut (but don’t glue it yet). The string height will need to be adjusted downward because of the upslope of the groove in the shelf, however, remember that you will be filing off the width of the shelf for most of the strings, which will also lower the height at the edge of the shelf. You will need to coordinate your final height adjustments with your final shelf-width adjustments (not so difficult, but requires care and patience).
Begin making measurements and adjustments. For each round of trial adjustments, I test the intonation of all the strings and note the error amounts before removing the nut to make more adjustments. That saves frequent loosing and re-tightening of the strings. The strings need to be at full tension for testing, because tension effects intonation. Use care to loosen the strings only the minimum needed to remove the nut - otherwise, strings are inclined to break, from repetition, at the tuning machines.
* Future-past experience: The experience you will have had after you have done one or two of these.
When the width of the first fret space is less than its calculated width, some of the strings will not need a shelf. I make partial shelves as needed. Some strings are already overcompensated by the original nut, likely the first and the smallest wound string, and maybe more. File the original nut back a bit. File at a slope, or use a small Dremel grinder horizontally, to carefully produce a “hollow grind” in order to retain a more vertical edge at the top of the shelf.
It is somewhat easy, especially for builders, to move the nut forward. For electric and classical guitars, the nut could be moved up enough to compensate the third and sixth strings, and then, file back the top front of the nut for the other strings that would be over-compensated. For steel-string acoustic guitars, moving straight forward would be alright, but moving forward at a slight slant, to compensate the second and sixth, would be better. Then, file back the nut, as needed, for overcompensated 5th, 4th, 3rd, and 1st strings.
It should be noted that if the nut is moved forward, without intonating the individual strings, the notes on the lower frets will be less sharp, and on average will be more in tune to the equal temperament scale. However, some of the strings will likely be overcompensated, and others will be undercompensated. The fretted strings will diverge from each other by the same amount as before. The smallest wound (overcompensated) string played against the largest unwound (undercompensated) string will still not sound so good. A closed chord will sound the same as before (only not as sharp). Chords with open strings should sound better than before. These effects can be mitigated with tuning offsets; the Buzz Feiten System is an example of this.On this inexpensive "Les Paul", I moved the nut forward at a slant, to compensate the 2nd and 6th strings, and then I filed the nut back for the overcompensated strings (1st and 4th, and maybe 5th). I would have been finished if this were an acoustic guitar, but it has an unwound 3rd, so I added a small shelf for the 3rd string. This guitar, with upgraded bridge and pickups, plays and sounds better than some guitars costing four times as much!
I made a one-piece shelf nut for my friend’s Peavey. I screwed up the first one, but the second one came out very good. It was way too much work, without having a proper setup, but if you are going to make a lot of them, it would be well worth tooling up for, and it would result in a better product. (With the mottled look of the dark stained bone, you have to look really hard to see that the nut is compensated.)
Hey, I finally did make a setup for making shelf-nut blanks - see below.
Available to fit an assortment of fretboard radii, this oversized bone blank will have a shelf already cut to fit your guitar's fretboard radius. Compensation offsets can be adjusted to any guitar by filing the shelf back the needed amount for each string! Available in bleached or unbleached bone. See a one-piece nut blank fitted and installed on a guitar. Go here for more information and to Order Shelf Nuts. If you have further questions, Please contact me for more information.
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