Loose Dovetail Struggles

[Philip Secrist]

I have done a few dovetail joints, but I always seem to struggle with getting them to fit snug. I am building a flat top and am currently trying to fit the neck to the dovetail joint (fretboard is glued to the neck, and the body and neck are not finished. The neck angle is good and the center-line is good and the cheeks of the tenon look tight to the sides. The joint is just not tight. I tried to drop the tenon into the joint slowly, but obviously took too much off somewhere in the process. The joint does not draw in like it should and is loose. Since the fretboard is glued onto the neck, I can't look from the top to tell if I lost the angle or what. I also discovered that I had not chiseled the side flush with the mortise cheek on one side well enough (the side sticks into the joint a smidge in a spot or two) so after getting it flush, the joint will be even more loose. I am wondering at this point, if I should glue a couple of mahogany 1/16" thick shims to both sides of the tenon and start over fitting. What do you recommend? Thanks for your help.

[Mark Swanson]

In order to require two 1/16" shims, your joint would have to be REALLY loose. Try some veneer, start with adding one thin veneer piece, on one side only. Don't glue it in, just put it in there and see if it fits better. That may be all you need, and if it is too thick then glue it on one side of the tenon and then trim it back. Adding a thin piece of veneer to one side of the tenon should not throw your center line off enough to matter.

[Alan Carruth]

Chalk fitting is helpful when you can't see the surfaces, or even when you can! Rub some chalk on the inside of the mortise, and it will show you where things are touching. Use a REALLY SHARP chisel to remove the chalk, and just a bit of dust from the wood surface.

This step epitomizes what I call the 'koan of lutherie ' : the slower you work the faster you get done.

[Michael Lewis]

This is where a really sharp chisel is your best friend. Oh, and patience.

[Jeff Pittel]

Two issues can cause loose dovetail joints, one it's bottoming. Thats simple to check, just measure the length of the neck joint and body joint. Two, it's touching the back (it shouldn't), there should be a slight gap. That might be hard to determine, as you've already glued the fingerboard on. The angles on your jigs (for neck and body are the same, right?)

Jeff Pittel

[Todd Stock]

Don't forget to see if the skinny end of the tail is bottoming out in the socket, as well as what was mentioned concerning trimming overall depth of tail...if the added shims are not properly trimmed or the socket was not well-cleaned, it can hang up. Also useful to make up a block with edge cut at appropriate dovetail angle (they vary between Martin, Gibson, Guild, etc. - check the bit you used to fab the joint) and covered with peel-n-stick 120-150 grit paper to remove shim material from tail as indicated by chalk. Chisel to trim, file to smooth, and block to final fit works well, and even average skill level with each will get you to a good solution.

Useful to keep a set of .010 fiber shims handy...used to give you an initial idea of how thick a shim to add to one or both sides of the tail (surprising how many previously reset necks are not actually on centerline or aligned properly)..if it takes two shims per side to stop the extension short of flush, that's just a little less than standard veneer thickness, so a shim cut from maple or mahogany veneer is a good place to start. Note - I never use fiber shims in the joint - just to get an initial idea of how thick the wood shims need to be to get started.

[Patrick Hanna]

Philip, dovetail joints are a struggle for me, too. I've only done two of them, but I launch into them with a bit of a sense of intimidation. That said, I've managed to make them work by running through the steps described by our friends.

These guys have all given excellent advice. Once you're sure you're not bottoming out, follow Mark's advice about dry veneer shims. If you discover that your joint is still loose, add dry shims to either side, one at a time, rather than making thicker shims. Just take it a step at a time from there. This next might sound odd, but wait until you're REALLY in the mood to be patient and analytical about finding the problem. Don't launch into this while you're all frustrated about it.
Just work on something else until you're confident that you can track it all down. Then work patiently. I'm sure you'll be okay.

[Alan Carruth]

Ideally you want the dovetail to bottom out, but only just. The end is not very large, but that little piece of real estate can go a long way toward holding the neck from folding up. Again, IMO, chalk fitting is the best way to go, whether you use sandpaper or a chisel to clean up, but do try to get the bottom of the dovetail to fit.

[Todd Stock]

During dry fit, I want the extension to very near bottom as the dovetail seats against the side surfaces with hand pressure...maybe 1/64th inch to go. I don't care if there is a gap between the bottom of the tail (as opposed to the mating faces) and the bottom of the socket...if the mating faces are in good contact, the neck is not going to go anywhere. The drawback to having a minor gap between the the bottom/toe of the tail and the very bottom of the socket is that, if the luthier is not careful when gluing up, there can be a pretty significant glue blob there, which if Titebond will take a bit to soften during a neck reset...another good reason to glue dovetails with hide.

[Alan Carruth]

Todd Stock wrote:
"..if the mating faces are in good contact, the neck is not going to go anywhere."

Back when I did more repairs the dovetails I saw that moved were the ones that didn't bottom out. I also got used to fitting the bottom in making violins, since it's the joint between the heel and the button that holds the neck. It really is not hard to get that surface to fit, and makes some difference in the stability of the joint, IMO.

[Todd Stock]

I've done fewer than 200 resets, Alan, so I appreciate the insight. The dozen or so guitars that I've reset that actually showed movement prior to disassembly (gapped heel or mobile neck joint) either showed broken tails (sheer failure across the base of the angled faces...usually on Gibson necks with their broad, rounded tail) or extremely thick glue lines on the mating faces...the former condition well outside the designed strength of the joint and the later an overall fitting issue. My own assessment is that that some factory builders are over-reliant on glue for a final fit, and that reliance generates a couple dovetail resets a year and a dozen or so M&T rebuilds on the old Martin economy joint for this shop.

[Joel Nowland]

Philip

I know this doesn't answer your question but something to consider.

About 15 years ago I switched to a bolt-on neck design and am grateful I did. I would never go back. It takes some work to get set for it but very much worth it, IMO.

The bolt-on neck design I settled on is super strong and more stable than a glued in joint. And the best thing is it's adjustable at original set-up and for future adjustments for angle and fingerboard height above the body.

See my web site for example.
http://nowlandguitars.com/designs.php

Joel