Keeping the mortise square to the centerline of the body. Yes, that's a carpenter's square.
The demonstration mortise and dovetail in the first two photos below are only to introduce the shapes that we are trying to build. On the actual guitar, the mortise must be cut into the heel block after the body is completely assembled. Likewise, the dovetail tenon must be cut into the heel of a rough (but square) neck assembly. The mortise is cut first and the dovetail tenon is then cut-to-fit the finished mortise.
By the way...
Woody Strings has no affiliation with LMI except as a long-term customer. The tools and experiences mentioned here are my own. Yours might be different.
The objetive when cutting the mortise is simply to make a slot, square and true to the center line of the body, that will accept the dovetail tenon on the neck. The LMI video explains the routing sequences quite well.
The dovetail is subsequently cut to fit the mortise and lock the arm of the guitar (the neck w/o the fretboard) into place with the proper neck angle and centered on the extended center line of the body.
Notice the circular viewport on the swing panel.
Dovetail and heel carved and ready for joining to the body.
LMI's first neck-joint jig.
Test the mortise/dovetail fit by inserting the dovetail onto the mortise on the guitar. With a straight edge, check your neck angle clearance at the saddle (3.5 mm). If the dovetail is too big for the mortise, that's good. Put the neck back onto the jig and remove just a 'skosh' more by sliding the template back. Keep doing this until the dovetail fits completely into the mortise and the top of the neck (arm) is flush with the top of the soundboard when the dovetail is tight.
NOTE: Woody works the dovetail down until the arm of the neck is just above the surface of the soundboard (maybe 2mm). Then if the dovetail needs adjustment to the neck angle and/or the center-line alignment, Woody will work those out by hand with a sharp chisel.
Woody screws the jig onto the end of (2) 2x4s screwed to the workbench. This helps access both sides of the jig.
One should watch the video of Robert O'Brien using the LMI neck-joint jig before trying to understand the presentation below.
O'Brien's video and details of the LMI Jig are very good. The methods Woody uses in cutting the dovetail joint are similar to the video but with version 1.0 of the LMI jig. Details are explained below.
Without pins to align the neck, one can adjust the neck alignment through this portal if the truss rod slot has not been routed exactly on the center line of the neck, or if you're using a truss rod that is not exactly 1/4" wide.
The neck stock is attached to the jig in the manner shown in the video. That is, it is placed over the 1/4" pins which slip into the truss rod slot. This assumes that the truss rod slot is exactly 1/4" wide and has been routed perfectly straight down the center of the arm.
In the video, the target neck angle is achieved by adjusting the swing panel until a straight line (the aluminum bar) intersects with the location of the saddle at 3.5 mm above the soundboard. The mortise is not impacted by this neck angle adjustment, only the dovetail tenon and the angle of the neck are impacted by the adjusted angle of the swing panel.
Measuring the neck angle clearance.
LMI has improved the hold-down clamps on the new jig.
On the old LMI jig, the neck is aligned with the aid of the circular viewport on the swing panel.