I did it last time, but not without some trial which led to error which led to me making a whole new neck. Mahogany neck blanks cost around $35, so I was interested in finding a better, more efficient way.
I found a website that offered a jig that would set the angle straight from the guitar top and then allow you to rout the tenon at the precise angle, but it's made of metal and obviously for professional luthiers who are going to do this a lot. It cost $899. (But today it's on sale for an amazing $799 while supplies last!)
Not going to happen.
But then the owner of the guitar-building forum that I subscribe to, Robbie O'Brien, offered to the forum members plans for building a very similar jig for just $20, with the cost refunded to the first person to build the jig, use it on an actual build, and post the results on the forum.
Well, I sent the $20 in right away, and not because I cared about the refund. I have two necks to build and I want them--and every one after them--done right, and as efficiently as possible.
I had to purchase about $75 worth of hardware, and a half-sheet of cabinet-grade plywood, and it took me parts of two weekends to build the jig.
Before ever using a new jig on an actual guitar, it's imperative to test it on scrap. I spent the better part of Saturday morning testing the mortise side on a piece of 2 x 4.
|Test cuts in 2 x 4|
Once I was sure I had everything dialed in and was getting consistent results, I was ready to move on to the guitars.
|Tim's guitar clamped on the mortise side of the jig|
|Centerlines lined up (ignore the parallax)|
Five minutes later and the mortise is cut in Austin's guitar. (This is why doing two guitars at once is faster than doing two separately: there's only half the setup and cleanup time involved. Set it up, do two guitars, then clean it up. I could do more guitars at once except I can't afford to have that much money tied up in materials.)
|Fine looking mortise|
To achieve the neck angle I want a straight-edge that is flush with the 14th fret (where the neck joins the body) to have a gap of about 3.5 mm at the bridge area of the top. The jig has an aluminum angle bar that extends above the top of the jig that is attached to a pivot board underneath it. This pivot board is attached to the top of the jig with hinges, and the neck is clamped to the pivot board.
The guitar body is stood upside down on the top of the jig, with the guitar soundboard flush against the aluminum angle bar at the centerline.
|Top flush against bar|
|Gap at bridge (which is several inches below the end of the bar)|
|Neck clamped to jig|
|Neck clamped to pivot board, which is slightly off vertical|
Boom! Done, and perfectly! The tenon fits snugly in the mortise, and the neck fits flush onto the top of the guitar sides.
|Snug fit (the gap at the bottom is irrelevant; it's the sides that need to be tight)|
In reality? 3.5 mm.
I love it when things work the way they should.
A task that has been filled with apprehension and frustration I can now face with confidence. The nice thing about this jig is that setup is pretty simple: clamp it to the workbench, put the right bit in the router, adjust the templates for the size mortise and tenon needed, and it's ready to go.
And it didn't cost me $799.