Official Luthiers Forum!

If I were CNCless, here's what I'd do:

Make a delrin template the same profile as the fretboard and use it as a pattern to route the inside edge with a die grinder on a vertical base (like John Hall sells). Clamp down some straight pieces, with removable spacers for alignment, as guides to get the outside of the channel. You can 'bounce' between them to get the interior or just clear it with a little chisel.

You can route right through the existing shell.

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Bob Garrish
Former Canonized Purveyor of Fine CNC Luthier Services

hmm...never did one of those. I might use the "drill press router" technique Robbie O'Brien shows on youtube. Just use the fingerboard as the template
Otherwise I would trace the fingerboard on the top and clamp on straight edge guides for the router where needed to follow the line.
Good luck.

I believe that our own John Hall has a DVD on the subject - you might drop him a line.

This technic is good to cut something like a peghead, but won't leave a smooth surface for inlay.

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Francis Richer, Montréal
Les Guitares F&M Guitars

Filippo,
The first thing is to know that you will have to do the neck set NOW in order to know precisely where the fretboard extension will fall at the rosette and its position side to side.
Once you are set with this you can scribe or pencil outline precisely to the fretboard edge.
Remember, the first part of this inlay is a wider black border which extends under the fret board a bit and only shows as a thinner black line as it emerges.
After marking everything out i always cut these with an Xacto knife and a straightedge using a small chisel to remove the wood to purfling depth.
Go ahead and route/bind the top leaving the pearl filler strip in place. The surround is cut with the binding in and it's a simple matter to cut the miters on the purfling as you install it.
Pop the pearl out of the rosette where the cut intersects it. You will be preparing special pieces at both the miters and the points at which the rosette pearl intersect the surround.

That is some nice looking work.

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"Preoccupation with an effect gives it power and enhances the error"
from "Your Owner's Manual" by Burt Hotchkiss.

Filippo,
I would always work up the neck and fit prior to actually cutting the ledges and binding because if there were any slight modifications needed to the mating surface where the upper bout meets the neck heel, it would be done before any routing and would be taken into account.
Also, as I usually did a full 45 style, I needed to have the side heel profile determined early on as well.
Sure, you can mark where you want to cut any way you like just remember that the entire pattern width is wider that the perimeter purfling as it includes the innermost black border which extends under the fingerboard.
You'll get the cleanest cut with a knife but be aware of grain lines as the blade will tend to follow them instead of the straightedge. Cut in the direction so that they merge with the straightedge rather than veer off.