Determine the Neck Angle

 

Now the body can be assembled and the fretboard glued onto the neck. Once this is done (but before frets are installed) you can assemble the neck to the body and, using a straight edge resting along the (flat) fretboard, measure the distance in height between the straightedge and where the saddle will go on the body (Important: no bridge should be in place at this time. Do not place the bridge until you have the backset angle where you want it. Otherwise you risk poor intonation).

From this measurement (the height of the straightedge over the body where the bridge will go) add the fret height and subtract the bridge height. The result should be no less than 1/64 and no greater than 1/16 on an inch (these are Cumpiano’s suggested figures). This is the distance in height between a theoretical straight line drawn from the tops of the frets on a perfectly flat fretboard, on the one hand, and the top surface of the bridge on the other hand. I will call this distance the Action Determining Distance or ADD for short.

If the ADD falls outside these limits, you need to change the backset angle of the knife edges of the tenon shoulders. This involves trimming them with a chisel. Calculate the adjustment needed. Mark that adjustment on one end of the shoulder knife edge. You want a line starting at that mark and ending at the other end, so that the edge at the other end is not trimmed at all. Carefully trim along that line with your chisel. Do this on both tenon shoulders.

The amount you cut off one end of the knife edge should be 1/3 of amount by which your ADD fell outside the theoretical limits given in the previous paragraph.  Think through carefully which end of the heel-block knife edge you need to remove material from.  Removing material from the bottom of the heel-knife edge will pull the far end of the neck down, increasing the ADD. Removing material from the top of the heel knife edge will pull the far end of the neck up, decreasing (lowering) the ADD. This gets very confusing, so be sure to think it through several times until you can visualize it.

Now continue to build your guitar until it is ready to be strung up. When you attach the neck, remember that you have a bit of vertical adjustment possible. Raise or lower the entire neck ­­ (we’re talking a fraction of a millimeter) so that the fretboard extension touches the body solidly but is not forced upwards by its contact with the body - no 14th fret hump. Tighten up the bolts and string the guitar up.

Now you can measure the action. Chances are it’s pretty good. If not, check to see how much the neck has bent from the tension on the strings and make cautious truss rod adjustments until you get the relief right. Make sure your nut height is where you want it. If the action at the 12 fret is still not what you want (I shoot for 7/64 inch between the bottom of the low E string and the top of the 12th fret, 5/64 for the high E string) and if to get it right would require taking more than say 1/32 inch off the saddle height, then you need to take the neck off to make further modifications to the tenon knife edges, using the method given above.

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