If I had the choice I prefer to rout the neck for the truss rod after I remove the old fretboard but before the neck is removed from the body. In this case, the neck on this old Regal was already dangling by a thread so my 2nd choice for routing the truss rod channel if a neck is dangling is described herein. Keep in mind that there are many different approaches to routing a channel in the neck to accept a truss rod. The approach I use works well for me.
I've previously worked on a rough version of vertical and horizontal alignment of the neck to the body. This step introduced a threaded insert that's been epoxied into the dovetail and which allows me to attach the neck to the body.
With the neck firmly attached to the body I can remove the tape holding fretboard onto the neck, which leaves me with the exposed neck surface ready for the truss rod channel. I determine the centerline down the middle of the neck then mark the width of the truss rod itself so I can be sure I'm routing straight with proper start- and end-points. I extend the centerline to the end of the guitar body to make sure my rough neck alignment isn't too rough. If the alignment looks good then I'm ready to fire up the router. Photo 1.
I use a Bosch "COLT" brand router. I don't do much routing besides the truss rod channel so this little "COLT" model is a perfect size and strength for what I need. My routing "jig" consist of a long section of aluminum box stock and two ratchet clamps. The aluminum "bar" serves as a fence to guide the router up and down the neck. To determine the position of the "bar" I retract the router bit so that the router itself can sit flush on the neck and position the bit as close to the centerline as I can see. I eyeball the position of the aluminum bar offset from the centerline and with some trial and error, get to a point where the "fence" is parallel to the centerline but also offset to accommodate the router base so it can rout the channel where I want it to. When I'm certain everything is accurate then I clamp the bar into position through the sound hole. Photo 1.
Before I dig into routing the channel too deep, I take at least one pass with the router turned on but with no bit showing. I move the router up and down the neck until I'm 100% sure everything's ready for the bit to be lowered. My first pass for real has the bit lowered only enough where I can see where the channel will be. This pass is only paper-thin deep but it gives me one last chance to make adjustments before go whole hog and make a long, deep mistake in my innocent, vintage guitar neck. Photo 2.
Things I keep in mind as I progress through the routing process:
- take many passes as required but with as little material removed as practical each time;
- always check alignment before the next pass;
- measure depth of the channel as you go;
- remember that the truss rod adjusting mechanism is both wider and deeper than the channel will be;
- once the channel is finished and special attention is given to accommodate the truss rod adjusting mechanism, test fit your truss rod in the channel;
- make sure the truss rod itself is "flat" (no bend) when test fitting in the channel.
The final inspection should show the truss rod surface and the flat neck surface to be just a hair shy of flush. Photo 3.
I pay special attention to what's happening at the neck block because that's the spot where a little tweaking might be needed to keep that end of the truss rod from poking up above the flat plane of the neck and fretboard extension. Once the truss rod passes the "test fit" stage, I try out the truss rod wrench to make sure everything fits proper and works well. Photo 4.
This approach to routing a truss rod channel takes lots of practice and lots of patience. Be sure to checkout other approaches so that you can make an educated decision on which approach is most comfortable for you.