I am using a book and drawings as the basis for building this mandolin. The book has pictures of an elaborate jig used to cut the opening in the top of the body to accept the neck. Since the neck attaches at a 6 degree angle, and the “V” is cut at 13.5 degrees off vertical along each leg of the “V” it does take some precision to accomplish. I spent the better part of an afternoon building the recommended jig. As I was building it directly from the drawings, I kept thinking, this will never work. There was no way to adjust the overall width of the “V” cuts to match the width of the neck. When I had completed the jig, sure enough, it did not allow enough movement to cut a wide enough “V.” So, I came up with a jig of my own to accomplish the task. Mine was a bit simpler – I created a platform to hold the body of the mandolin within an outline and used precisely cut wedges to tilt the platform at 6 degrees. I then built a second platform tilted at 13.5 degrees which goes under the 6 degree platform and sits perpendicular. This platform can be reversed from side to side so that the left and right side angles of the “V” can be cut in separate steps.
After cutting the groove slightly undersize, the final sizing was done the old fashioned way with a chisel.
The neck was then attached and the 6 degree angle was double checked with some basic geometry – at the location of the bridge, the body should be 7/16 of an inch away from the plane of the neck extended (without the fretboard attached) After gluing in the neck, two holes are drilled from the back straight down through the edges of the “V” effectively locking the the two pieces mechanically. This should make for a good strong joint.
Here’s the progress to this point: