I French polish my classical guitars and I had always been frustrated with the difficulties of finishing at the neck joint and the fingerboard/top interface when building in the traditional Spanish method. Also, handling a complete guitar during French polishing is awkward, and the job of pore filling the body and the neck with different methods is made more complicated than I thought it ought to be.
For these reasons, I now build and finish the bodies, necks+fingerboard, and the bridge separately. The body to neck joint features a pair of mortices in the neck heel and the neck block, a pair of splines connecting the two, and a threaded brass insert in the neck heel that allow for dry fitting using a bolt through a hole in the neck block.
This blog illustrates how I go about creating the joint for a new guitar,
Creating the neck block:
A normal heel stack is created just as with any other traditional method. I've prepared a digital image file of the profile of the guitar at the neck junction and it's modified to show the centerline of the neck and appropriate registration marks. The file is printed and the print is affixed to the neck with double stick tape. The neck is placed on a carrier and the neck block is carefully sawn off following the contour line. If care is taken to cut this joint very precisely, then neck alignment and neck block gluing are foregone successes.
The nascent neck block. Normal breathing has resumed.
A 3/8' hole is drilled through the neck block and 1/2" into the neck heel to allow passage of a 1/4" cap screw that will eventually be used to temporarily draw the joint together while the neck is being glued to the body. The holes are offset by the thickness of the top.
The mating pieces. After the mortice is cut, the neck block then goes off to be shaped and glued to the sides using the exterior mould.
Creating the mortices:
For this and previous guitars, I've used a router method for cutting the mortices in the neck block and heel. The bit is a spiral upcut round ended 1/4" bit. Centering and alignment of these cuts is critical to the body/neck fit. Doing the cut on a table saw and jig using a 1/4" dado head is a good alternative to this method. The mortice on the neck heel is done the same way.
The cut in progress.
A 1/2" long 3/8" brass insert that accepts a 1/4" cap screw is installed in the neck block and the fit is stablilized by saturating the mating surfaces with regular CA glue. Getting it in straight can be a challenge. Rockler, and I'm sure others, sell a hand drill bit/device made for this and, with a shot of liquid courage, it goes in with less fuss than you could imagine
The rough joint:
A pair of 1/4" thick mahogany splines are prepped and fitted to the neck mortice, but not yet glued. This is the essence of the joint.
The butt of the neck heel is relieved 3-4mm inside the expected profile of the neck so that later fitting is easier. It's usually at this point in the process that I leave a little DNA behind for authentication.
The heel countour is roughed out so that the fit of the neck to the body can be assessed more easily.
This shows a dry fit of the rough neck to the unbound body. At the end of the finishing process, the neck with fingerboard will be glued to the body via the splines. I use Zpoxy supplemented with chopped glass fiber.
One further advantage of this whole method is that there are no surprises with regard to action when the guitar is finally glued up. There is ample opportunity all along the way to assess and make adjustments to neck fit and action.