MIMF

A friend of mine has asked if I could convert an existing guitar's fretboard to just or pure intonation for his own use in plain and Gregorian chant. He is a priest in an urban parish in a blighted neighborhood, so this will be a freebie.

I remember listening to John Schneider's Lou Harrison album back in the 90's, and there is a pic of the Mattingly guitar used with interchangeable fretboards. These western ears could never get used to the intonation, but my mind could change in a chant setting.

Questions are:

1) Where can I find templates or calculations to measure out the frets?

2) Presuming the initial try will be down and dirty on a cheap instrument; removing frets and filling the existing slots with epoxy and the cutting new slots, how should I cut the slots? My own method for the past several years is as Paul Jacobson explained it to me: cutting the slots> widening the slots with dental bits in a Dremel w/router base> then gluing the frets in with epoxy. The partial frets would rule out cutting the whole slot, and I'm afraid the Dremel with a dental bit is not strong enough to actually cut the slot.

Any thoughts are appreciated.

I think you'd have to pick a key, and even then, you couldn't have continuous, single frets at each location. Our standard frets only line up at each string because of equal temperament (more or less!).

Yes, you need to pick a key.
See:
http://jsnow.bootlegether.net/cbg/justintonation.html

Yup, you'll need to pick a key and tuning first, and unless each strong is tuned to the same note, you'll need offset frets. Beyond that, the calculations will just have to be whole ratios (plenty of resources online to find natural tuning ratios.

This will be a labor of love, and I would advise doing to an instrument that you can afford to mess up. In practical terms, I would suggest to your friend that a lap dulcimer may be much more appropriate for this application. Root - 5th - octave tuning, five whole step and two half step intervals (not much need to worry about a full chromatic scale for this use). A lot of old dulcimers are actually already spaced closer to natural than equal tempered, so you may be able to find one that doesn't need much modification.

http://youtu.be/ERF1Kk5kcR8

I might make a flat fretboard, slice it into six tapered strips so I had one skinny fretboard for each string, slot and fret each strip for frets in the spacing you decide for the key you are going to put it in, reassemble the fretboard gluing the strips back together is some kind of jig to keep it all level and square. I would probably do most of the fret dressing before I glued it back together so I just had a light level to do after it was all put together and on the neck. Before I did that I would point out the natural just intonation of a lap dobro or lap steel if you tune it that way and only play in one key ( and never play other chords than the I chord I guess). It would be less work to make a lap steel from scratch than to make a just intonated fret board.

If you want to try it out cheap, how about just putting on a fretless finger board with no slots and using tied gut frets, a la early violas da gamba?

Or combine Jim and David's ideas and make a tied fret stick dulcimer (strumstick).

Ken,
Here's a good start: http://whatmusicreallyis.com/papers/sac ... scale.html

Tied frets won't work, since they can't make the necessary 'kinked' pattern. That's one of the things that pushed the use of Equal Temperament: the lute and gamba players couldn't play in tune with the keyboards set up for mean tone or just scales, let alone the fretless fiddles and singers. There's just no easy way to do this. Probably the simplest thing would be to use 'staple' frets, as was done sometimes on early mountain dulcimers. They used baling wire, but you could get brass rod stock and bend it to shape. Predrill holes to make sure it's located properly.

Tied frets could work for a stick dulcimer, with the fifth always used as a drone or tuned D d dd (or a variation there of) The advantage to the tied fret would be the ability to change from one key to another (between songs) but still keep things "justly" tuned.

If being able to play in different keys is an issue, you could consider to use a 31-tone divisions of the octave.
This will give a meantone kind of tuning, so somewhere in between 12- tet and just tuning.
There's an excellent article on Wikipedia about 31-tet.
There is a Dutch guitarist who uses a 31-tone guitar with partial frets.
Searching YouTube on 31-tet music / 31-tet guitar will gives some hits.

Thanks to all for the responses. I will digest it all and report back on the results....someday.

Paul Beier, a lutenist, and apparently a software developer, wrote a string and fret calculator that can calculate fret positions for several temperaments. Google "Beier String and Fret Calculator" and you'll find a few places where you can download it for free. I have used it in the past and it worked fine.

Why not you ask your friend for exact measurement as he will better guide you that how your convert your guitar.