Official Luthiers Forum!

Other than keeping the extension stable is there any structural benefit to gluing it to the body? Given that raised FB extensions are common on achtops I'm thinkin' not.

Peter

There are some structural differences to gluing the extension. Whether they are advantages is up for debate.

You gain some lateral stability in the neck but when I was making prototypes for my raised fb design, I did some very scientific testing. I held on to the guitar body with one hand and gave the nut end of the neck a couple dozen hard blows from all directions with the other hand. I also leaned it against the wall and knocked it over a few times. No problems whatsoever. That was with a small (but reinforced) heel with a butt joint and 2 bolts.

You also spread some of the neck load to the top when you glue the extension. The string tension is pulling up on the neck which wants to rotate the top of the heel toward the butt and the fingerboard down. Gluing the extension to the top spreads some of that torque to the top in a different way that an unglued extension. The extension spreads that downward force to the top and onto the upper transverse brace and toward the rims. The top and tranverse brace then provides some structural resistence to that force as well as adding mass and stiffness to the fingerboard extension which keeps you from losing energy when you play fretted notes over the body.

With a floating extension, you have to account for the loss of that structure. I add carbon fiber rods from the heel block to the waist to resist the torque. To keep the frets over the body stiff, I continue a thin extension of the neck wood under the fb extension and reinforce it with carbon fiber bars. (by the way, I didn't innovate any of this and don't really know who to credit because I've seen various aspects of it in many places)

As for sound, there's no A/B comparing them. If you built a traditionally braced guitar and just didn't glue down the extension, I think you would lose something (and it just wouldn't make a lot of sense). Connecting the extension to the top does help the sound, from what I've seen. But with a floating fb, you can eliminate the transverse brace and in genereal, I find that I can get a lot more activity in the top in the upper bout than with traditional bracing. But they are different beasts and I wouldn't say one is inherently better or worse.

_________________
http://www.chassonguitars.com

ANd one more thing (as if that wasn't enough).

Most traditional heel designs flex way too much to keep the neck stable without gluing the extension. If you float the extension, you need to reinforce the heel. Otherwise it will flex and the neck will pull upward.

_________________
http://www.chassonguitars.com

Consider bolting down the FB extension with a mortise in the neck block extension and a tenon on the FB. I've done this on my last 2 guitars and will never glue again.

_________________
JJ
Napa, CA
http://www.DonohueGuitars.com

As Is well known here I do not glue the fingerboard down but use an extension on the neck that fits in a mortice on the heel block. I only use the normal two bolts to hold the neck in place (butt joint) there is no need for the extension to be bolted down. I've got six steel strings with 100+ pounds of tension pulling it together, why bolt?

This just fits in to:



This.



Colin

_________________
I don't believe in anything, I simply make use of a set of reasonable working hypotheses.

Colin or other who do this. Do you have the area where the transverse brace usually goes radiuses like the rest of the top or flattened out like some do to get a better fit on the FB extension. How do you shape the block that the mortice is in where it glues to the top? Care to show a few shots of your block before you put the top on.

Thanks Kent, all of that makes perfect sense, certainly some things that hadn't entered my thought process

I'm going to try this on my next guitar. I like the idea of bolting down the FB ext with a bolt on neck.

_________________
"After forty-nine years of violin building, I have decided that the search for a varnish is similar to the fox hunt. The fun is in the hunt."
Jack Batts Maker and Repairer of Fine Violins

I have never glued or bolted down any of my fingerboard extensions, nor have I reinforced them. I have 15 guitars behind me and I have not had any problems whatsoever.

It started out that I didn't glue them down because I wanted to be able to quickly remove the neck in case there was trouble on my first couple of builds. I figured if there was a need I could always easily glue the extension down later if I needed to. Turns out I never never needed to. My thinking is that if it fits well, laying flat on the soundboard before the guitar is strung up, then when it is strung up the string tension is going to apply a little downward force which keeps it in place, provided the wood is somewhat stable.

I have taken some heat for this... I have gotten some 'wtf is wrong with you' looks from other builders and have heard predictions that there might be trouble down the road, but so far so good. I have only used very stable well quartered woods for my fingerboards, save for one which I made very thin and didn't season as long I should have. That one, an experimental guitar built to test some completely different ideas, gave me some problems early on but stabilized after a while, and definitely was not something I would build for a customer. For what it's worth I laminate my sides and use an extended spanish-ish heel which I believe makes the overall structure much stiffer than your normal guitar.

Like I said, so far so good. Maybe I'll have to eat crow if one comes back and I'll have to spend 10 minutes gluing it down. Time will tell, but I believe my thinking is solid. I think it more likely that I'll be thrilled that I don't have to unglue anything when the time comes for a neck reset.

http://mcphersonguitars.com/activea.asp?CompID=23&btnSubmit=ByFileCategoryID&cboApplicationID=321&cboFileCategoryID=1068
I believe that our John Mayes works for McPherson.
Interesting design!

So whats the advantage to bolting the FB extension? is it just to make neck resetting easier?
If so, it seems to me it would be less work and less time involved to remove a glued FB extension than to manufacture the bolt on joint
For the amount of years needed between neck resets, bolting the extension seems like overkill to me.
Lex, how do the fretted notes on the extension sound?

Peter

With out doubting Lex here, though I have never tried it. It would seem to me tht unless the fit to top of a "common" extension type was a very tight interference fit or tied into the top by means of gluing or bolting or else a free floating cantilever design or some other way to isolate the extension, vibration of the upper bout would lead to very disturbing buzz or rattle. Glueing a common extention joint ties the extention to the top and would eliminate athe chance of buz or rattle in this joint.

It would sure be a lot easier to fit an ARCHTOP neck to the top plate if it didn't need to be glued.
It's time consuming to shape the underside of the neck extension to fit the arched contour.
It probably would place more demand on the heel to body joint as mentioned above.
I'm not advocating anything, just thinking about how this relates to archtop construction.

Michael, as you know I don't glue my extension down, nor do I bolt the neck extension that the FB is glued to onto the guitar body either. The neck extension simply fits into the pocket in the heel block and the butt jointed neck is bolted on with the usual two bolts. The geometry of a 1.5deg neck angle and a 25' top means that with just a smidgeon of sanding the FB extension fits snugly against the top, I can't get a piece of typing paper under it anywhere. I've done this on nearly 30 guitars now and never had a single issue with buzzing or rattling of any sort. I have never seen the logic of having a bolted on neck, then gluing the FB down. My FB/neck is one unit, the body a second, I can take the neck on and off as often as I like with no adjustments to make, it just bolts on. And, with the FB glued to the neck extension, no chance of a 12/14 fret hump at the neck/body junction. I can't for the life of me understand why anyone still does it the old way.

Colin

_________________
I don't believe in anything, I simply make use of a set of reasonable working hypotheses.

I am assuming that the upper "L" flange that the mortise is cut in is acts as a natural dampener of any vcibration of the upper bout top in the area of your design. So your design makes good sence to me. my comments were in referance to narrowish neck block some what like Stewmac's bolt-on neck block where your extention pass passes beyound the neck block for some distance and on to an active portion of the upper bout. Even with a reasonably tight fit I would suspect that a rattel could be present due to the unsecured contact of the extention to the active part of the top.

?y the way i have moved to an "L" shaped neck block some time ago in preperation to a M&T joint for my extentions. I have not finalized my plans for this new joint as of yet but like your joint. how thick is the flanged "L" on your neck block?

Could some of you who do this show a few more pics of your blocks and extensions.

They sound just fine to me.

Like I said, the tension of the strings holds the extension against the top. I've never had a buzz/rattle.

I thought a lot about this Colin after the other discussion where this came up. People are gluing theirs down (or bolting) because with a similar system as yours it won't stay solid. I think the factors contributing are that you have much more meat under the extension than other people and your heel also looks much beefier than some people are making. What radius are you using for the back also? After you showed me your neck block and neck system those were the three things I figured differed the most between otherwise similar joints yet mine was distorting and yours wasn't. I agree completely that if something is working there is no reason to fix it but I also think that for some systems either gluing or better yet bolting the extension to the top adds quite a bit of strength to the joint.

_________________
Burton
http://www.legeytinstruments.com
Brookline, MA.

Mine:




Before the heel tenon was cut.

_________________
John Mayes
http://www.mayesluthier.com

When I was contemplating fastening my necks by bolting or pinning them, the thought occurred to me that evern though it would be easy to separate the body and neck, the glued fingerboard extension would be quite a bit of work to undo. In years of building furniture and turning pieces to be separated into half-splats or quarter columns, it was common practice to glue the turning pieces together using simple newsprint (newspaper) in the joints of the stock. These did not come apart as they were turned, and to separate them we just took a chisel to the unturned part, and the pieces came apart as the newsprint fibers separated. A little scraping or sanding and the pieces were ready to glue into place. When I started with guitar building I thought of trying this, but, did not because I was not using a pinned or bolt-on neck joint. Still, even with a conventional compound dovetail joint, the neck would come apart more easily than heating and separating. Popping the fingerboard extension loose with a chisel, much the same as with bridge removal, should work fine. I've not thought of this idea for years, until today, and might be worth trying. As someone earlier had mentioned, if it didn't work, it could be taken apart, and after scraping, glued in the traditional fashion.

_________________
Bear Acker
Amherst, Massachusetts






"If you knew what you needed to know, you'd know, ya know"

I've sort of an experience going on. My second guitar, which is my regular player and receives about 1 hour of play/day doesn't have the fb extension glued down. Actually, there's absolutely nothing holding it. I don't have an extended neck block either. It's a standard block and the neck is held with two bolts (LMI inserts).

The fit is air tight and so far, after about 1.5 years of regular use, there is absolutely no buzz. None...zilch...nada...

If I need to take off the neck, I simply undo the two bolts and voila! Will I ever glue down a fb extension... No way.

_________________
I'd like to be able to prove, just for once, that money wouldn't make me happy...

John, do you make the cut out on the face of the guitar slightly deeper than the height of the block glued on the back of the FB allowing the fingerboard to be pulled against the top?

The Larson brothers screwed their extensions - maybe into braces. Click look at http://patimg2.uspto.gov/.piw?Docid=017 ... http%3A%2F% to see the drawings from one of their patent applications. It's patent 1,768,261 if you want the verbiage.

Zach etc look at these threads to see my neck/neck block system.

http://luthiersforum.com/forum/viewtopic.php?f=10101&t=19290&p=271512&hilit=neck+extension#p271512

http://luthiersforum.com/forum/viewtopic.php?f=10101&t=19044&hilit=Cuban+mahogany

Colin

_________________
I don't believe in anything, I simply make use of a set of reasonable working hypotheses.