Fretboard extension.to glue or not to glue

[Darryl Young]

I didn't glue mine down on my first build.......or at least not yet. Since it was my first, I figured I would have to take the neck off a few times and I ended up doing just that. So I plan to glue it one day but haven't yet got around to it. No buzzing. I did need to do as Ken suggested and apply pressure on the fretboard at the joint to push the bottom of the fretboard down flat against the body while tightening the neck bolts (due to the extension being curved down slightly). As long as I did that I didn't have issues.

Can't comment as to good or bad affects of this over the long term.

Edit to add: This is a bolt-on mortise in tenon joint that isn't glued (Martin bolt-ons are typically glued but this design isn't the Martin design).

[tippie53]

For short term it will be fine and in a few exceptions it may be acceptable . After having built over 150 of these things and tried many of the techniques described , in the end you will see what does and doesn't work. A fret plane is very critical to the guitars playing . Eventually the weakness of your designs will show up and that is how you make true discoveries.

[kencierp]

Now it should be mentioned again that we are taking about mortise and tenon neck joints --- with a dove tail the fingerboard extension glue surface acts as a cap, its part of the joint and helps prevent the "V" from moving upward --- a fraction of an inch displacement toward the sound-board results in the failure of the joint.

[kencierp]

[kencierp]

Tony --Are you confusing the dove tail conversation with the MT concept? With the MT bolt on the compression washers hold every thing in place and if they do move you just push the neck down and re-tighten. The dove tail is a different animal -- the two "V,s" have to be in perfect contact top to bottom any up ward movement means the joint failed -- so if you glue the FBX to the sound board the neck is less likely to move upward and disrupt the glue joint -- make sense? Or am I missing your point?

[kencierp]

And yes the McPhearson image points out that this is all preference, not a universal law of guitar making.

[tippie53]

The picture uses a floating extension not a free extension. there is a difference . The spring back from fretting is what can cause an issue . Having 2 degrees in engineering Mechanical and Industrial I think I know how to check a process for repeatability.
Having a variable that is not controlled ( fret cpmpression ) will not make a process repeatable. A design flaw may not present itself right away. These are where you have the best opportunity for learning. Stress risers that may be overlooked can often be the scourge of building .
Is there a perfect design ? I doubt it. There are many designs of floating extensions , Martin used it on the Millennium, Tom Humphrey's design and it worked but the fretboard was technically glued to the neck as the McPherson design. Archtops have been built like this since the 20's so it isn't a new design and is a proven concept.
Maybe what we have here is a miscommunication on nomenclature .

[kencierp]

The McPhearson image shows that they feel the neck joint is not compromised, also as mentioned, many different instruments in the "string family" even have dove tail joints and by design, the FB extensions are not secured to the sound-board.

[Darryl Young]

I must admit, glueing the fretboard extension seems would help the stability of the neck even on a bolt-on M&T joint. It would allow the fretboard to help resist bowing from string tension over time (speculating). With that said, when I glue I plan to use very little glue and use Elmers or some glue that fails easily under shear so it's less likely the fretboard extension will crack the top from contraction under low humidity conditions (would prefer the glue joint fails before the top splits).

[kencierp]

Theoretically -- the pull of the strings is pushing the extension against the top adding even more compression? Yeah -- the miss match hard and soft grain woods expansion and contraction has been a nemesis causing cracks in the sound board since the very beginning. The detailed Brune drawing of the 1937 Hauser Classical points this out and shows how the crack problems were addressed.