How-to keep a neck dovetail true in neck reset

[Drew L]

Hi all--

The neck joint on my mando (scratch built #1) has opened up for the 2nd time. I was able to take the neck out and the mando looks good overall. However, the joint is a straight dovetail, and it's pretty clear to me that just trying to shim it back together tight again is not a good long term solution. I know it'll loosen up. I'm looking to do something more significant. Essentially, i want to recut and establish a good tapered dovetail. I think there's enough meat on the neck dovetail and I can fill in the neck block.

However, every time I have attempted to get a good fit on such a joint, it's failed. Actually, that's not true-- the joint is tight, but the neck is never pointed in the right direction when all is said and done. The neck is either cocked to either side, not level, or tilted poorly. I use a chalk or contact paper process and carve off the dark spots, but at the end of the day, it doesn't seem to do a good job of keeping things true.

I've searched on this site and the internet and I haven't found a good tutorial on how to step through it or fill me in on the little nugget of wisdom that I'm missing.

Does anyone have any suggestion or link to help out with this?

Sunburst-- I am inspired by your neck repair job posted on your site. That's great looking work.

[billhay4]

I share your pain, Drew. I can't get this right either.
So, any advice will be greatly appreciated.
Bill

[sunburst]

When you set a neck with a tapered dovetail, the neck is "moving" as you remove wood from the dovetail. You want it to move to a place where it is in the mortise to the correct depth, the neck angle correct for your chosen bridge height, and the center line of the neck lined up with the center line of the body, and you want that to happen with a good fit in the dovetail and a good fit between the heel and the rim.
Look at your starting point and see where you are on all of those. If you put the neck in to the depth it will go when you start out, do the center lines line up? Do they line up at the fingerboard surface and at the heel button? Is the height of the fingerboard surface the same amount higher above the riser block as the projection of the surface is above your intended height at the bridge position? Does the neck center line point toward the center of the body at the tailpiece position?
If you don't have methods to check all of that, you need to devise methods, and the "trick" is to figure out which direction the neck needs to move to get it from where it is to where you want it to be. The only way to move the neck is to remove wood from the dovetail and from the heel of the neck, so study the situation and decide where you need to remove wood. Remove a little wood and check your progress. It's easy to make a mistake in figuring out where to remove wood, so go a little at a time and check your progress regularly. If the neck moves the wrong way, then you probably figured wrong so you need to re-think where to remove wood. When you're done, the overstand should be acceptable, the bridge height should be acceptable, the neck should be centered and aimed to the center of the tail piece, the dovetail should fit well on both sides for it's whole length, the heel should fit tight against the rim, and all that needs to happen at exactly the same time!
I think it's a good idea to get things centered early in the process, because the curved surface of the head block (on standard Gibson-inspired construction) means any movement left or right will also affect the direction of the neck center line pointing toward the tailpiece.
So first step, devise methods of measuring and tracking the neck position.

[Dobe]

[QUOTE=sunburst;884612]When you set a neck with a tapered dovetail, the neck is "moving" as you remove wood from the dovetail. You want it to move to a place where it is in the mortise to the correct depth, the neck angle correct for your chosen bridge height, and the center line of the neck lined up with the center line of the body, and you want that to happen with a good fit in the dovetail and a good fit between the heel and the rim.
Look at your starting point and see where you.........QUOTE]

Yeah- perfect is good enough ! Or as the pilots know- it's all about Roll , Pitch & Yaw.

I was just gonna add- "It definately aint easy" , but I would agree with all of the above.
Good luck & try not to get frustrated. Take your time !!

[Michael Lewis]

If you get the cheeks of the heel to fit the body and the neck lined up properly, even with the dovetail loose, you are getting close. The next step is to glue shims onto the tenon that are a bit thicker than you need. When they are dry you can re-fit the joint. Since you have at this point got the curved surfaces to fit at the correct angles, all you need to do is make the dovetail fit down into the mortise. Sometimes carbon paper can be helpful in such cases to show the "high" spots. Just keep shaving off the high spots until the neck drops into place.

Dobe is right on with the advice of " try not to get frustrated. Take your time". Now you know why we get the "big bucks".

[HoGo]

Drew,
I'd suggest having a close look at the joint to see why the problems happened in the first place. If your sides are not perfectly perpendicular to top plane you will have big problems fitting straight dovetail. It needs to slide in snugly from insertion point down to final position. If your sides are not perfect you may find that it binds when pressed down, but removal of material may misalign the neck or it may be loose at heel while it's tight when inserted...
Tapered dovetail has a bit more difficult geometry but the neck goes in pretty loose and tightens as it seats down, this enables use of carbon paper (on straight DT carbon paper may leave traces anywhere along the joint as it should be tight). Also there is no problem with imperfect sides with tapered DT.
I'd suggest tapering the dovetail by removing 1/16" at bottom of the heel and gluing 1/16" shims (or slightly thicker) to block and taper them with flat sanding stick or flat chisel (cut perpendilular to grain). Keep extra wood in the joint and follow advice of John and Michael above.
Good luck.

[Jim Hilburn]

Carbon paper is your friend. I was shown to fold small squares and position them between the neck and body on both sides and press the joint together. This is a very tricky operation and I finally said "screw this' and now I tape off the sides close to the projected edges of the fit and take those little squares and smear the black carbon all over the body side of the joint, on the cheeks and in the dovetail. Then I can just press it in and see where the contact points are.
I use a long piece of plexiglass with a centerline etched in it and lay that along the centerline of the neck to see how it aligns with the end of the body. It flexes downward while maintaining the straigtedge. If it's off to one side of the center joint you correct it by removing wood fromthe dovetail on one side and the cheek contact on the other. I probably do 100 minor adjustments as it slowly lowers into place. If it's getting close and everything isn't aligning or the bridge hieght looks wrong then it's shim time.
It's most important that the heel is very tight. You want everything tight but the heel is where the string tension will exert the most force. The top is being pulled tight to the body.

[sunburst]

Re: carbon paper.
Your dentist may be able to tell you where to get articulating paper. It's like little strips of carbon paper, just the right size for dovetials, that dentists use to check the fit of your teeth when they're working on them. I have some that someone gave me years ago, but I never use it because I prefer to use chalk. I use regular old blackboard chalk (must not be "dustless"!, that contains some kinda grease) or pastels (from an art store).

" Roll, pitch & yaw", I like that.

[Jim Hilburn]

I got a batch of carbon paper that was called "smudgless". I said I want the kind that smudges like crazy.

[rockies]

T mentioned in a thread afew years back how to use one of those cheap laser levels mounted vertically above the workbench for aligning necks or any other shaped object. The laser will draw a straight red line down whatever you are aligning. Put a little piece of masking tape at tail piece , at center of neck block and end of neck and the top of headstock. MarK centers on tape with a dark pen. When sanding the shims it easy to push the neck in and see where to remove wood to bring all those center marks into alignment with the laser beam.
For neck angle I use a fake bridge the right height (about 1/2") or so taped into position and a straight edge to get the angle correct. The last time I used a Siminoff joint and the jig (board with the fake bridge on it), and with the mandolin upside down and all centers marked the laser gave me the alignment for fittiing. Then finally for clamping and gluing on the jig. Turned out perfect. best fitting and easiest joint I've done. Then dowelled for strength. Try the laser trick, much easier than the plexiglass as you can work with the laser mark there and no removing and putting the plexiglass back in place to check alighment. IMHO
Dave

[craigw]

I've been using the laser alignment method as described by Dave and find it quite helpful and efficient. I also use either 80 or 120 grit microfinishing film cut in strips the width of the tenon, inserting between the joint surfaces and pulling the film as much as necessary to mate the joint surfaces. Don't have to use any carbon with this method.

[fscotte]

I would take a gander and guess that the OP's cut lines were not perfectly vertical to the plane.

I cut my first dovetail on the bandsaw, and I spent 1/2 hour adjusting the blade to be sure it was cutting absolutely vertical from the side and from the front.

[Michael Lewis]

Jim, the carbon paper you want is called "pencil carbon paper" as opposed to the more durable stuff used for typewriters. There is also some great stuff from the local yardage store used for transferring patterns to cloth that comes in sheets and has sort of chalk surface on one side. it comes in several colors and white, and shows up much better than regular chalk. The white shows well against mahogany and the blue or red show well against maple.

[Lefty Luthier]

I have found that the chalk pencils that are used by tailors work very well. They are handy and keep your fingers clean.

[sunburst]

Thanks for that, Byron. I didn't know about those!