As John Hall will attest, I'm the biggest scrounge in the world. Nothing makes me happier than creating something from what otherwise would be scrap. The Guitar Makers Connection at Martin sells reject and overstock necks at various discounted prices depending on the condition and reason for rejection. The most common necks are 14 fret Martin long scale (25.4”). Unfortunately, if you want a 12 fret (or a 12 fret with M&T neck joint), they are more rare and bring a premium price.
This gave me the incentive to work out (with John’s help) a system to simply and reliably shorten 14 fret necks to anything you want.
We start out making a couple of reference marks. Position a fret board of the correct scale for the neck with the 14th fret in the correct position, then mark the neck at the nut position. Don't forget to remove the roll pins Martin uses to position the fret boards.
Then place a fret board of the scale you want to end up with, positioning the 12th fret (or whatever fret you want at the body) at the end of the neck. Mark the neck at the nut position.
Here you can see the 2 marks. These are not the cut marks, only references to measure how much to remove. Measure the distance between the marks.
Lay a straight edge along the back of the head stock, and mark the neck where it intersects the top of the neck.
Using a small square, use the truss rod slot to extend the mark you just made across the neck. This will be one of the cut marks.
Using the measurement from earlier, make another line further down the neck, this will be the other cut line. The first two lines can now be erased or marked out to avoid confusion.
Before we cut the neck, using whatever method you have available, rout two shallow (about ¼ to 3/8 inch) channels for reinforcement and alignment later. These need to be perfectly parallel to the center line of the neck, and should extend about an inch past each cut line.
Martin head stocks are cut at about a 3 ½ deg. angel, so using a wedge cut to match, clamp the head stock to you rip fence with the center line, truss rod slot parallel to the table top.
With the blade fully retracted, position the fence so that the left side of the blade is just shy of the cut mark furthest from the head stock. This will allow you to tighten up the cut after you start to crank up the blade and check out where it begins to cut.
When you have it positioned just right, crank the blade through the neck.
Now you can reposition the fence to make the other cut, referencing from the right side of the blade this time.
This is a good time to make any mods (binding, slot head, etc.) to the head stock while it’s separated and easier to work with.
Now using pieces of hard wood or carbon fiber assemble the pieces taking care to get everything lined up. The truss rod is good to use for alignment.
For aesthetic reasons, I wouldn’t use one of these necks on a high end guitar, but it’s perfectly functional and completely flexible for scale length.
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) and I see a few ways to possibly do this - however -