Yes, it's tricky to accurately create and reproduce the shape of the back of a guitar/bass neck. When you look close, really good feeling necks are more complicated than simple V's, C's, U's etc. Necks aren't just a simple radius, or a part of a cylinder, or even a part of a cone. They are a 3-D shape, with radiuses that change around the neck, and along the length of the neck. And then there's the transitions at the headstock and the heel.
So, the question is how do you define, measure, layout, and repeatably carve to that shape? Without getting into 3-D Laser Scanning, Graphic Modeling, CNC Machining, Nano-Particle Beams, and all that other new-fangled computer stuff?
Those of us who still shape necks the old-fangled way have developed our own methods of layout and measurement. There are many different ways to get there. My favorite is what I call the 5-facet method.
Basically, I layout and shape the neck in a sequence of angled bevels (or facets). I define these bevels, their angle and width, in simple layouts of the cross section at the 1st and 12th positions; sometimes others. The neck starts rectangular, cut to width and depth (plan) measurements. The layout lines for the bevels are measured and drawn as distance off to the side of the centerline on the back, and distance up from the fingerboard glue line on the side. I draw and cut the primary bevels first, then draw and cut the secondary bevels. This leaves the neck as 5 facets, with the center facet flat, right down the middle. The facets taper in width and twist slightly as they go down the neck. At the transitions, they curve and taper into points.
This brings the neck quite close to the final shape. I make sure all the facets are straight (no lumps or waves) the right width and angle, and symmetrical side to side (if it's a symmetrical profile). Only after all that is right, do I carefully round off the small ridges between the facets. This brings the neck to the final shape.
It's a repeatable process. Using this layout method, I can hand shape necks years apart, and have them measure and feel the same. I have notes, little cardboard gauges, and custom aluminum straightedges as my layout guides. It doesn't take as long as it sounds.
I'm sure that my description above is confusing. Here's a link to a set of old pages showing my process for making Series IV Scroll Bass necks. The 5-facet process starts on Page 8. There are a couple of good pictures of how I lay out and cut the bevels.
http://www.xstrange.com/Building/Buildingneck1.html
