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I want to try nut compensation on my current build a 24 in scale length Parlor.
My question is: will nut compensation significantly change the saddle position.
I will glue the bridge before the setup with the usual compensation (0.6% of the scale length), the saddle is 1/8".
This is usually OK for intonating the saddle correctly for fret 0 and 12.
I do not want to be obliged to fill the saddle slot and reroute it a few mm back or forth.
What is your experience with that?

Thanks

The simple answer is that the nut compensation reduces the required saddle compensation by a similar amount.
You do need to look at it string by string when deciding saddle position and thickness.

24" is a fairly short scale, and will require more compensation, on both ends, than the usual 24.9" or 25.4". Without knowing things like what strings you're going to use and your usual action height it's hard to say offhand what the numbers should be. OTOH, almost any nut compensation will be better than none.

Usually when you use nut compensation less compensation is required on the bridge. It's possible to get reasonably good results by simply splitting the difference. If you'd normally use, say, 4mm of compensation at the bridge with the nut in the 'theoretical' position, moving the nut up by 2mm, and the bridge back by the same amount, could turn out pretty well.

If you use a wide nut and saddle you can find the offsets experimentally. Start with the nut about 2-3mm closer to the first fret than the 'theoretical' position, and make a flat topped saddle. Slip a short section of string crosswise under one of the strings where it goes over the saddle to make a break point, and move it back and forth to get the 12th fret note (NOT the harmonic) to be an exact octave above the open string pitch. Now fret the string at the first fret, and see how sharp or flat it is relative to the note it's supposed to be making. If it's flat that means you've got the nut too close to the first fret (relatively speaking, the fret has been moved back too far). Remembering that there are 100 cents to a semitone allows you to figure out how much. If the first fret note is 5 cents flat, then you need to move the break point at the nut back away from the first fret by 5% of the theoretical distance from the nut to the first fret. For a 24" scale the first fret is supposed to be 34.21mm from the nut, so 5% of that would be 1.7 mm. If you started with the nut offset by 3mm then you'd notch it back by 3-1.7=1.3 mm, and that would make the fretted note come out right. But that's going to throw out the saddle offset, so you'd need to re-do that. A good way to proceed would be to use the 'rule of half': make half the change at the nut you think you'll want, re-do the saddle, and check again.

At some point, after a few rounds you'll either get it so close that it doesn't matter, or get tired of messing around and declare victory. You can, of you want, put a tuner on a piece of two by, stretch a string on that across a nut and saddle, and put in a frets where the first, 11th, and 12th frets are supposed to be. Then you can juggle things on that rig until you're happy (or crazy) and transfer the numbers to the guitar. This is generally a safer way to go.

Thanks for your input. I had made some years ago, a 2x4 test bench like you described. Today, I added the frets for a 24 in scale at 1 and 12 and tried it. I cannot say the results are conclusive. Probably the thing is too crude (action too high), but I am getting a compensation of 2.4 mm at high e and 5 mm at low E. As for as nut compensation I get too sharp or too flat at fret 1 depending how and where I push the string.
I realise that this should be dialed in once the guitar is properly setup , relief,action, nut height, etc. So back to my question. If I glue the bridge in place, will I have enough "adjustment" in a 1/8 saddle to make nut compensation.

If you know how much compensation is normally added to a saddle-only compensated guitar of the same type and set-up, you're off to a good start. For each string, divide the amount of saddle compensation by two and put half on the nut and half on the saddle, moving the nut break point towards the first fret and the saddle break point in the usual direction. That should allow you to figure out where to put the saddle slot and how wide to make it. (I use a straight saddle, 5mm wide).

It works pretty well and to do much better you have to get a lot more sophisticated about it. I guess you know where to find that stuff.

_________________
Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

Thank you Trevor, this give me a good start.

Action too high on your test bed will definitely give you wrong results.
Both the 12th fret and 1st fret string to fret clearances MUST be what you expect to have in a finished and setup guitar.
A 1/8th saddle will work but only if it is in the right spot to accommodate the required compensation on each string.

I like to mask the footage of the bridge before finishing so it is easier to remove the finish under the bridge.
But on this one since it is a new scale length for me and I want to make a compensated nut, I will play it safe and finish without masking.
I will try to rig a floating bridge and play with saddle and nut position.

Not to be contrarian, but can your ear even detect a few cents at the first fret? E.g.: Assuming 440 tuning and equal temperament an F in 440 will be a few cents out if you are playing in the key of D no mater what your compensation, right?


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I'm sure Trevor will chime in with a more complete answer if needed, but I'll give it a go. The basic issue is that the gutiar is, by default, set up in Equal Temperament. It's the only scheme where all the semitones are the same size, so you can use straight frets. Some intervals in ET are already significantly 'off' from pure (beatless) frequency ratios. Any issues with intonation are very likely to make them even worse. Thus it's worthwhile to try to get things as close as practicable, given the limitations of the guitar.