how to determins neck extention thickness

[Greg Martin]

I always wondered how would you determine the correct thickness of the neck extension. .I know Benedetto's book calls out 5/8" thickness,but if you didnt have that gem of info, how would you figure it?
Also why only a small rabbit on the neck extension joint? why not wider rabbit or why not use the same dovetail bit? which would enhance the glueup procedure? just curious.

[Chris Richards]

The thickness is derived by measuring from the fretboard joining face down to the top of the body plus the depth of the recurve, I don't think that you can be "super" accurate initially, as until you have the neck fitted and scraped the recurve you don't know the exact dimension, when building mine the 5/8th was plenty to allow for shaping the extension to the recurve. Getting this correct is quite important as ultimately you are trying to get the string height over the bridge position no less than 3/4 inch, this gives your strings a good break angle which in turn gives good downward pressure on the bridge and top plate. So getting the neck joint correct is very important.

Here's a picture of mine.

[Matt Cushman]

A dovetail for the neck extension is a good idea. In Irving Sloans book there is a picture of D'Angelico cutting a dovetail for the neck extension. I have used this type of attachment on all my F type mandos. this effectivly makes the extension part of the neck. I like to have a lot of space between the extension and the top. It is easier to finish in there with a little gap between them. I like the extender thin but it should be thick enough to mount your fingerrest if you are using one.

[Greg Martin]

Great info. I think also the dovetail extension joint would require less jig ups (Bennedetto book). Allowing simpler clamping. Ill stick with the 5/8" thickness too, but my layouts indicate a finished bridge height of 15/16" to 1-1/8" and that would yield a break angle to the tail of 13-15 degrees. Thats just on paper of course.
Great discussion !

[Matt Cushman]

The extender thickness at the neck body joint will determine neck height. A good fit between the body and the neck ext. underside is important. A good method to mate these surfaces is to minimize the contact area and keep the surfaces flat. The recurve of the top plate ends at the neck block and shouldn't extend beneath the neck. By keeping the mating surfaces flat, a good fit is greatly simplified.

[Patrick Hanna]

Greg, I don't want to be construed as arguing with your correspondents because they all have given good input. But I'd like to clarify something: I believe the neck height is really determined by two different factors. First is the neck to body joint angle. Then, of course, the proposed bridge height (which usually falls within a small range of heights that will be workable. Shooting your string path from nut to bridge (with the neck angle you have chosen--usually about 3.5 degrees) you dry fit the neck into its mortise until your desired string path is achieved. Bingo, now you can measure for the neck extension. I do this early in the process by clamping or taping my fingerboard to the neck while I dry fit the tenon and mortise. I have even done the preliminary neck fit before the back is glued onto the box.

On both of my archtop guitar necks, I've used the dovetail method, cutting the dovetail the full height of the neck heel. Later, I dovetail the neck extension on, too. It's a little bit trickier to fit, but I like the looks. I don't believe you need to glue the extension to the top, unless it's going to touch. You can float it a little bit over above the top. If I were to build a bolt on archtop neck, I would not fasten the extension to the top. A floating neck extension might slightly lessen your sustain on those notes, but how many times are you really going to play that high on the fingerboard? And, if you do, how long are you going to stay up there? It's not a significant issue for my playing.

I've built a couple of mandolins with bolt on necks. These were not true archtops, but they have a pronounced induced arch. On those, my method was considerably different. I actually glued the neck extension to the top, but not to the neck. The neck slides into its tenon and is snugged up to the extension by bolt and string tension. I "tack" glue the end of the fingerboard onto the extension with a couple of drops of glue. This has worked fine for me.

I also built a tenor banjo, which has no neck extension, but which DOES have a very critical neck height at the tension hoop and a critical string path. Mine has neck screws and coordinator rods. Once again, I dry fitted the neck until I had it right, then I marked and drilled the pilot holes for my heck lag screws.

So, to sum up, everything follows from the string path, which is determined by neck angle and bridge height. Neck height at the body is adjust to give the desired string path. Neck extension height follows from all of this.

[Matt Cushman]

Just to be clear, the bridge heighth and position is planned first and all other points are fitted to get the desired string angle and string heighth. When fitting a neck to a body, the angle and heighth are combined to get the desired result. If an area of the extension is to be glued to the top in the Benedetto style then the thickness of the extension effects the heighth. If you remove too much material from the extension when fitting to the neck, it will sit too deep resulting in a low fingerboard position. If you don't remove enough the reverse is true. Leaving a gap and not gluing the extension to the top is fine but the look is different from the usual New York style.

[Michael Lewis]

There have been many ways to get this done, from cutting the extension as part of the neck, fitting the extension to the neck as a unit, and fitting the neck to the body and THEN fitting the extension. I use the latter method and level the surface just before gluing the fingerboard.

You can make this a complicated operation if you like, I prefer to simplify it as much as I can and still have a sturdy structure.

[Chris Richards]

Working to a projected string height either from a drawing or a mock-up is not easy as it still won't give you an accurate bridge height, once you get the strings on the guitar and get them up to tension the top will deflect due to pressure on the bridge and there will probably be some movement in the neck.

Please note though, that I've only built one archtop, and I'm VERY proud of it!.... And if it wasn't for the help I got from this forum I think I may well have given up...The Benedetto book was my "bible" for the build and if you stick to it I don't think you'll go too far astray. It's not a complaint as the book does what it says on the cover but in some places it is sparse on explaining the reasons for certain things, as in this case with the whole neck joint/extension discussion.