Dovetail joint question

[nkwak]

It feels like I've had to reset the dovetail neck joint on my third build several times over the summer. Here are my issues:

1: It's not 100% tight. It holds firm but there's a slight gap on the treble side that has enough play to easily pop the tenon loose. I notice that my humidity has been fluctuating over the past month or so. Is this a humidity issue or a geometry problem? I've tried shimming and it seems to have only slightly helped.

2: this would probably affect the first question, but I'm noticing that there's a gap between the fretboard and the top on the bass size but not on the treble side. The neck itself does not appear to be twisted when removed from the guitar. I've done the chalk check for contact points and it looks like there is a high point at the lower point of the bass side of the dovetail and the higher point of the treble side of the dovetail. I'm thinking I need to carefully remove material

3: my centerlines for the neck and body are slightly off (less than 3/32") but the axes are parallel and the frets are perpendicular. Is it okay as-is or do I need to file away at the dovetail and shim the other side?

Another question: is it okay to start finishing the body and neck? I'd be doing French polish. These issues seem to be setup items and I feel like having the pores on the sides sealed may actually help. Am I wrong to assume this?

I appreciate any perspective based on experience on this. I'm feeling like I'm chasing my tail. Thanks!

[Skarsaune]

You might have already done this, but John has a series of videos on YouTube that are the best reference I've found on setting a dovetail. I re-watch them nearly every time I have to fit one. Search "Blues Creek Dovetail" and you should find them.

Having said that -
1) Fit should be tight, no wiggle once in place. That's a geometry problem.
2) That's a fit problem also. As you said, rotation/twist of the neck can be done taking some from the top/bottom of opposing sides.
3) 3/32" is more than I would build with - especially when centering it up is straightforward. As you said, shim one side, and it will shift the neck over.

An acronym to remember is ACE - Angle, Center, Elevation. It's the order you want to work in.

Angle - needs to be correct in all 3 ways. This is hard to describe without pics, but here goes.
One is viewed from the side, fretboard angle to top, so the strings hit the bridge where they should.
Second is viewed from the top. Centerline of neck needs to be parallel to centerline of body, not pointing off to a side.
Third is the twist you describe in #2. Viewed from the headstock, fretboard should be parallel to the top.

Center - once angles are correct, get the neck centerline matched up with body centerline. Exact issue you have in #3.

Elevation - once angle and center are correct, remove material evenly to bring the neck down into the pocket so the fretboard surface is even with the top of the guitar.

In practice, as you work through the steps, you're always checking to make sure the changes you make haven't affected something other than what you wanted. So, for example, if you add a shim to shift your centerline over, check your angles and make any corrections to those before working the elevation.

I'm not sure I'm doing a good job with the explanation, so I'll tell you again to watch John's videos. Work slowly and carefully. And you can always glue another shim on and start again.

[nkwak]

Thanks for the reply! I’ve been tabbing through that video on many occasions. The angles are mostly good. The centerline is off by 1/16” toward the treble side and the bass side of the fingerboard has enough of a gap to slide a business card beneath. It’s looking like I’ll have to take about 1/16” of meat off the side of the tenon and build a shim for the treble side. I have off cuts from the side of another build made with mahogany that are about .1” thick that I can work with. Once I have the centerlines aligned then I’ll worry about the elevation variance.

[tippie53]

if you need to feel free to call me

[nkwak]

if you need to feel free to call me

Thanks John. Can I call you Monday? Today and the whole weekend are pretty busy for me. I also probably have too many questions for you and don’t want to waste your time.

[nkwak]

In the mean time a picture can tell 1000 words as they say. Here are some pics:

1: centerline finder jig to measure once

[nkwak]

2: laser level using centerline pencil marks at the nut, bridge and center seam at the tail block:

[nkwak]

3: closeups of the jig for the first measurement:



This tells me the centers are parallel but off by 1/16”

[nkwak]

4: laser level sight points at the frets confirm the misalignment.