Fingerboard hump at neck/body joint

[Craig Bumgarner]

Any idea why this occurs? Haven't had the problem on my own, but see it fairly often and have been asked to repair several times. Do people build them this way? I doubt it. Does it happen do to changes in climate some how? Age? Neck and body stresses? I'd be interested in ideas on this.

[sam]

Timely topic, Chris. I'm also interested in the causes and fixes, though will probably look to someone with more experience
to work on my 12 fret instrument.

I'm getting buzzing above the 7th fret, mostly on treble strings. Sighting from the bridge, there is a slight bulge around the neck/body
joint. The truss rod is not currently in tension.

Guitar was previously strung with extra light .047 and is now strung with EJ-16s (.053, I believe). The problem was first noticed
after the change of gauge.

Sam

[sam]

Correction, Craig. My apologies. S

[woodrat]

Trevor Gore explains this really well in "the book" ...basically the area of the sound board where the neck extension glues on (if standard construction) needs to be prepared so that when the neck is attached it forms a continuous line with the area that the fret board extension will be glued to....in other words, the neck and the area of the fret board extension will be in the correct plane tangent to the dome of the top so as to also allow the correct saddle height and string action...

...as I said...Trevor describes it fully and well in "the book" ...section 4.6.12.3

[simso]

The reason it happens is because of a few situations,

The first is you have a hard surface that you glue the fretboard too, from 1 to 14th fret positions, then you have a section of fretboard extension with no strength, just a fretboard really hanging in the air, you are clamping this section of fretboard to a 3mm non ridgid structure, so theres no strength, hence the hump at the 14th

Next point, is if the neck angle wasnt perfect on production, then you will be gluing the extension onto the body at a slight angle (hence the hump)

If the neck block is not suffciently strong enough the body will lightly fold at the front of the sound hole,m you will alsoi get a hump at the 14th

Because the fretboard is not really supported well over the body, when you fret this section you introduce compression which when coupled with the fact there is no under structure of really any strength, means the extension will be expanded by the insertion of the frets, and you get a hump again at the 14th

There are more reasons why this phenomenon occurs, but it is minimised by making an understructure to the fretboard, example taylor necks

[jeffhigh]

Steve summarizes the reasons pretty well
It is important to realize that it may not be a problem that needs fixing.
If you think of it as "Fallaway" past the 14th rather than a hump. As long as the frets from 1st to 14th are level it doesn't really matter if there is a bit of a downwards slope from there.

[sam]

Thanks Steve & Jeff.

My instrument seems to have 'fall-away' on either side of the 12th fret (body joint) and hence the buzz on notes below the 12th fret.
I've been controlling and monitoring humidity, but wonder if natural ageing (shrinking) might be a factor. It
has a traditional dovetail and hide glue construction.

[jeffhigh]

Sam, the first thing you need to do is check the neck relief, do you know how?

[sam]

Jeff,

Clearance at 12th fret is 1.5mm high E and 2mm Bass E. Guitar is primarily used for finger style playing. There is also a few thou clearance at 1st fret on either string when fretted, 3rd fret. I am seeing a slight reverse S bend along the length of the fretboard on treble side. Buzzing appears to occur only on B and E strings when fretted above 7th fret.

S

[jeffhigh]

Sam
Hold the E string lightly to the 1st fret and to the 12th fret
Now measure the clearance from the underside of the string to the fret at the midpoint between 1st and 12th.
This is what is called the relief and is what will tell you whether you need to adjust the truss rod.

[sam]

Jeff,

I'm showing a few thou under .5mm on top and bottom strings at the fret, mid point.

[jeffhigh]

Ok there is the problem
0.5mm or 20 thou is far too much relief
I generally aim for 4 thou maximum
You need to tighten the truss rod to straighten the neck.
After you do this and get the relief set, you will probably find you get buzzing all over the place, do not be deterred
Now shim the saddle or make a new saddle to bring the 12th fret clearance to your requirements, I find 2.0-1.5 a bit low unless you are a really light player.

[sam]

Thank you, Jeff. This has been a good exercise. I'll post when I've made some progress with truss rod and saddle.

Sam