One should watch the video of Robert O'Brien using the LMI neck-joint jig before trying to understand the presentation below.
https://youtu.be/aWtAy5vS6gA
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.
The Dovetail Neck Joint Jig
Since I started using the LMI jig to cut dovetail neck joints, my control over the process has improved significantly. The parameters that must be controlled, however, are different. Routers are a hand tool, but they are not gracious like scrapers and planes that remove wood ever so slowly. Routers can make a mess before you can bat an eye.
The O'Brien videos are very good. It might take several passes through the video to get all of the details. Also, in the video, O'Brien uses the newest version of the jig, with improvements over Woody's jig that you see pictured here.
December 2019
Cutting the Dovetail
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.
LMI has improved the hold-down clamps on the new jig.
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.
LMI's first neck-joint jig.
The LMI Neck Joint Jig (for starters)
First, ya gotta...
Buy and assemble the LMI kit, relatively simple tools needed for assembly. It took me a couple of days. During assembly, everything must be kept absolutely square and true for the final jig to work as designed. A cabinet builder would be right at home.
I am not an experienced cabinet builder. Still, I bought the first LMI neck-joint kit several years ago. Woody has mastered it (almost). The instructions that came with it were adequate but the methods of execution depend on you and your shop. It would help to have a good table saw when building the cabinet.
The time and techniques required to build the jig will ensure one thing. The builder will know how the jig works before using it.
The rest of this post assumes that you have seen the O'Brien video. I use the same LMI jig that you see in the video but an earlier version. O'Brien has some useful router set-up and operation tips. Pay attention to these tips in the video. Especially router rule #1, #2 and #3. Woody has not repeated them here.
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.
The MORTISE
...Bottom line: The LMI neck-joint jig will get you close but, ordinarily, the final alignment and fitting of the neck joint will be up to you and your sharp tools. That's the way it should be.
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.
SUMMARY
MORTISE
...put the guitar body into the jig.
...SQUARE the guitar to the mortise.
...cut the MORTISE__ "now you're committed".
Dovetail TENON
...mount the neck onto the jig.
...align the neck with centerline.
...set template so that the first cut will be a bit oversize.
...hog down to depth with a straight bit.
...final pass...7 degree dovetail bit.
...Test fit on the guitar. Shorten and narrow dovetail as needed to fit mortise*.
*WORK SCANT! Avoid cutting the tenon too short. On the first cut, intentionally make the dovetail too long and too wide. Width and length of the tenon change together as the template is moved in or out.
I hope to see you back here next month,
Woody
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.
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.
On the old LMI jig, the neck is aligned with the aid of the circular viewport on the swing panel.
Notice the circular viewport on the swing panel.
Woody's
neck angle gauge (mm)
The TENON
Dovetail and heel carved and ready for joining to the body.
Keeping the mortise square to the centerline of the body. Yes, that's a carpenter's square.
3.5 mm
Measuring the neck angle clearance.