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I'm working on my first. I've routed the mortise in the body, I've cut the tenon for the neck. And I'm using the new Cumpiano bolt-on system.

The question is, what kind of tolerances am I looking for in terms of fit? Should it be snug when the tenon's inserted in the mortise? That will, presumably, limit my ability to manage the center-line of the neck lining up to the centerline of the guitar (a la Hesh's toot).

Or is the primary point of contact the edges of the neck that contact the sides? That's how you make sure that everything's lined up and those points of contact are sufficient to transmit all the relevant vibrations.

Does that mean that the fit in the mortise/tenon should be somewhat loose?

As always, thanks in advance for all the help.

Steve

I like a fairly snug fit. You should have to use much pressure to put it in though. I establish the center line after the tenon is in.

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Ken Franklin
clumsy yet persistent
https://www.kenfranklinukulele.com

Well, feel free to slap me down, call me sloppy if you want, but...
I am only on my fourth, and to be honest I like a fairly loose fit to be able to adjust for line-up to centre as I sand in the shoulders where the neck will contact the sides.
After lining up, I glue scraps (about 12 x 15 mm) sized to the gaps each side of the tenon near the top to keep the neck in position for final bolt up.
The way I see it is the shoulder is the point of contact (look at the way the bolts pull) and you could use balsa wood for the scraps on the tenon cheeks without effecting the "vibes"
That's my £ 0.02

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

Sorry, you were asking for clearances - I use about 1.2 mm total (about 0.05")

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

It really needs to be a snug fit on the sides of the M&T joint. As for alignment, if you're going to stay with the bolt on M&T, then you need to take the time to design and make your own jigs for both procedures.
If you would like photos of my jigs, PM your email to me and I'll send them as attachments.
Mine were done on a cnc using nice Baltic birch plywood.
The tenon jig references (centers) on the truss rod slot in the neck blank. The mortise jig references on the center line of the jointed top, so as long as you get the top glued on center, the mortise will follow. Any minor alignment then is usually due to the cheeks of the neck heel not being square/flush to the body. That's addressed by shaving (or using the "pulled sandpaper" trick) on the face of the cheek(s) to bring the neck heel flush and into alignment with the center seam of the top.

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Hank Mauel

This seems to me to be a matter of personal preference.
Can anyone explain why a snug fit is better than a loose fit?

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"An adventure is only an inconvenience rightly considered. An inconvenience is an adventure wrongly considered." G. K. Chesterton.

Unless you are doing something wrong, the forces acting on the neck will be pulling it towards the bridge -- there shouldn't be any forces pushing it to the left or right. The fretboard (either glued or bolt-on) are entirely sufficient to "locate" the neck in terms of alignment. After that, all you really need is one bolt on the heel close to the back. In my opinion, all other concerns are entirely superfluous. In other words, proceed with a method that is easiest for you to achieve good results. One of the most important things is a flawless fitting of the neck to the body. Take care of that and you should be just fine. For the record, I also like a little side-to-side play in the joint since it allows me to achieve perfect neck alignment very easily.

On a bolt-on neck it is the tension of the bolting system and the cheek contact with the rim that bears the load and set the yaw angle. The fit of the tenon sides to the mortise sides can only influence the offset parallelism of the neck to the centerline of the mortise. I my self like a close, somewhat snug but non-interference fit between the tenon sides and mortise sides the odds are there will be some yaw adjustment needed to make the centerline of a neck match up as close as possible with the centerline of the tops center seam ( the visual centerline of the body) If the tenon/mortise has an interference fit the interference will inhibit the yaw adjustability.