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It's time to cut the heel of the neck to change the angle. I can make a pretty good estimate of the amount of material I must remove from the neck by a simple calculation. Remembering that the neck had pulled forward nearly 1/4" I can make a simple calculation:

Essentially, the neck had pivoted at the 12th fret where it joins the body. If I want to pivot the neck backward 1/4" all I have to do is divide the distance from the 12th fret to the nut by the distance from the bottom of the fingerboard to the heel cap. If I then divide 1/4" by that amount I get a result of a little less than 1/16"
of material I must remove at the bottom of the heel to allow the neck to pivot backward the right amount.

I'll then scribe a little line on the heel cap to indicate the depth of cut and I'll place the neck in my special fixture. (I only use this fixture on rare occasions when I'm working on a neck that needs to have a substantial cut made, and which has a delicate finish.)

I can adjust the table of this fixture so that the neck protrudes exactly the amount I need, namely 1/16" at the heel and zero at the fingerboard. I have a flat chisel with an extremely fine acute cutting angle. I use this chisel in a diagonal slicing motion to slice off the end of the neck where it touches the body:

You can see the light area where my chisel sliced off the neck, and the dark where the original undercut surface still remains:

Now, I'll undercut the area from the finished edges to the dovetail, so I won't have to fit too broad a surface when I'm making my final adjustments:

I'll make my final neck angle adjustments by slipping fine sandpaper (180 grit) between the body and neck, holding the neck in place, and drawing out the sandpaper. That way I'll get the neck to fit exactly, even if the body edges are not perfectly straight (they never are.)

After each pull of the sandpaper, I'll check the neck angle and "string line" (side-to-side alignment) with a straightedge.

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