What am I talking about, you ask? I'm referring to the open space (gap) between the bottom of the fretboard and the top of the guitar body (between the heel and the soundhole) when the neck is attached to the guitar body if the angle is incorrect. This gap occurs when the angle of the neck does not coincide with the angle of the top.
However, inside the many Google references I spent hours plowing through, I stumbled onto an article which was written by K. M. Cierpilowski, of Kenneth Michael Guitars. This article deals with the subject I had been struggling with for months. If you are a novice guitar builder such as myself, or the confirmed expert, you can learn something from this article at (http://kennethmichaelguitars.com/neckangle).
The drawing below illustrates what can be done to address the neck angle dilemma. But, in a brief explanation, the process goes like this . . .
The top edge of the rim is sanded flat from the tail block to the edge of the gray area referenced in the illustration, then slopes down to form the required complementary angle to match the neck heel, which results in the straight line fingerboard plane.
Instead of raising the soundboard or filling the gap with added material under the fingerboard extension, the neck joint is actually lowered slightly relative to the top edge of the rim. All that is required is to sand a slope on the top edge at an angle from the waist curve to the neck block. In effect, the top plate has a tiny invisible bend starting at the waist curve.
But, the dilemma for an amateur builder (myself) is how to accurately sand the slope on the top. Top line guitar producers use automated large disc sanding equipment that is simply unavailable to me, so I had to determine a simpler way to accomplish the task.
In studying the pattern I use for marking the wood sheets used in producing body sides, I realized a gradual, but definitive, slope exists in the design where the guitar back meets the neck heel.
So, why can't that design be applied to the top? It can!
Instead of sanding a slope on the top, I decided to experiment by incorporating the back slope design logic to the top. After determining the required distance from the waist to the center of the neck joint opening in the body, and determining the degree of neck angle, I returned to the side pattern template I use and applied a two-degree slope from the center of the waist forward.
Yes, it was an experiment. I bent the newly profiled ribs and glued up the heel and tail blocks in the same manner that I've always used, and applied the liner. Then I glued the back to the ribs, and then the top was glued in position. The process was either going to work or not, but my options for addressing the neck angle problem were less than two! If it didn't work, the body would become a new design for a drum. If it did work, I had solved my dilemma. In a cost/benefit analysis, what was there to loose?