I've started my nylon string crossover guitar

[Kevin in California]

So is the typical classical bridge not as tall as the steel string?
I am planning on setting the neck up in the same fashion as I do the steel strings, with a bolt on neck. The geometry I was going to duplicate this way rather than what I've read about planing the slope into the fretboard, which I assume is necessary with the spanish heel set up.

[John Parchem]

They are about the same height my classical is about 8mm my SS almost 9 mm. My target height at the saddle is 10 - 11 mm off the top and for a steel string 14 mm off of the top.

For the classical on the bass side start at -2 and add 5mm for the fret board (I have the bass side a mm thinner than the treble) 1 mm for the fret and (3.5 * 2) for the action = 11 mm

With the 14 mm target for the SS, The -2 in the top is about + 2 off the top so

+2 + 6 mm fret board + 1 mm fret + (2.5mm * 2) for the action = 14 mm.

[John Parchem]

... which I assume is necessary with the spanish heel set up.

It is not the Spanish heel, it could be setup to tilt in either direction. From what you do with a steel string at least take into consideration the 2 mm difference at the saddle to account for the higher action. Nylon strings will buzz at a typical SS action. If you only take that into consideration you can build with a flat neck angle. I would not tilt it back like a SS.

[Kevin in California]

Oh really, no tilt? What about a slight taper thinner toward the soundboard?

[John Parchem]

Oh really, no tilt? What about a slight taper thinner toward the soundboard?

Some classical guitar builder do build with no neck angle, a lot easier and they taper the fret board. The taper loses about a mm by the 12th fret. So by the saddle you end up with my -2 mm again. This is easier as there is no funky angle between the neck and the upper bout which is usually built flat on a classical. I have built both ways, flat and tilting the neck forward.

Actually on my second classical I did accidentally have the neck at a slight backwards angle, I fixed it sanding the fret board at a slope until I had the correct geometry. Hard to reset a Spanish style neck joint.

[Kevin in California]

Okay, splain it to me! :>)
What is going to happen if I set the neck angled like I do on a steel string.
Not enough clearance over the frets? Can't I compensate this at the saddle?

[John Parchem]

Okay, splain it to me! :>)
What is going to happen if I set the neck angled like I do on a steel string.
Not enough clearance over the frets? Can't I compensate this at the saddle?

You will have a choice: not enough clearance over the frets or with the correct action for nylon (compensate at the saddle) , the height of the strings over the top at the saddle will be 2 mm higher than your typical SS. A nylon strung guitar really does not want that height as the nylon strings do not have the same energy as the steel strings.

If it was my guitar and I built it with the same geometry as my steel string guitars the strings would be 16 mm at the saddle with proper action. That is too high. The target should be 10-11 mm.

So with nylon I want lower strings to the top and higher action relative to a SS. Both drive the neck angle in the same direction.

You can get some of that back sloping the fret board. So maybe you can keep the angled look. You just need to work out the geometry.