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This is only my second post - I spend most of my time lurking around here, absorbing all this useful information... I am planning to get more involved - but it'll definitely be a long time before I have anything useful to say!

Anyway, there's an issue I had regarding fretboards that I'm not able to figure out. I glued an African Ebony fretboard (radiused but not fretted) onto a 'Spanish' cedar neck. Both had been planed flat beforehand (although the neck blank was done several weeks ago). I used a flat beech caul, about 1.5" thick, relieved in the centre to allow for fretboard radius.

After unclamping this, a significant bump had appeared around the 5th fret. I can't figure out how this came about - if anything the surfaces to be glued were ever so slightly hollow... now this bump will have to be scraped/sanded out but it's something I need to avoid in the future.

Any ideas on what might have happened here/how it can be avoided? Perhaps differential clamping pressure along the neck might have distorted it; or maybe the moisture from the glue (titebond) did something...?

Was the fretboard clamped to a flat surface when you planed it flat? I'm wondering if the bottom of the fretboard (the surfaced now glued to the neck) was not flat, even though the top was flat. Just a thought.

It was attached to the workbench with double sided tape - the bench surface in that area is flat to about 4 thou so I'm pretty sure that couldn't be the cause.

I've been thinking about it more; maybe it was something to do with the sanding process (it was taped to a piece of 16mm MDF and radius sanded). I had noticed the nut end being slightly lower during the sanding process but am pretty sure that had been corrected by the end.

Before glueing the board on, I placed it on the bench and checked (the radiused side) for flat with the straightedge - there again, thinking about it, I suspect you can only really verify if a thinnish piece like this is flat if it's firmly taped down onto a flat surface...

It's like detective work trying to figure out where the problem lies with so many variables in the mix!

It could happen during clamping, the weight of the clamps can bend the glue up if you do it right.

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Possible culprit could be your mdf surface ....... if it wasn't flat .......... then you could have actually sanded a top surface level to a new datum while the bottom side wasn't. When you glued the fret board to the neck you now no longer have a flat top side.

My two cents.

God Bless.

Kevin

That would be my bet. Titebond is known to create those kind of issues. Usually the problem is avoided if you keep the clamps long enough, like 24 hours.

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Alain Moisan
Former full time builder of Acoustics, Classicals and Flamencos.
(Now building just for fun!)

Wood can move around when you machine it and then allow it to sit for a few weeks. Did you check the neck for flat before you attached the fretboard?

Yes. It swells the top surface of the neck, causing the back bow. One solution is to use a curved caul. The one I have is 1 1/4" thick oak, with about 1mm of camber planed into it.

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John

What you've just built is a hygrometer. Ebony and cedar have very different coefficients of lengthwise expansion with humidity change and very different lengthwise Young's moduli (on average). Most of the ebonies are notoriously unstable. When the humidity changes the assembly will just bend; higher humidity it will back-bow, lower humidity it will forward-bow, much like a bi-metal strip reacts to temperature changes. That's just how it is. If you want a neck to stay straight, chose woods that have a more similar coefficient of long grain change with humidity. The Cedar/ebony combination has one of the larger differentials.

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Trevor Gore, Luthier. Australian hand made acoustic guitars, classical guitars; custom guitar design and build; guitar design instruction.

http://www.goreguitars.com.au

difficult to get a commission for a classical guitar without an ebony FB, though.

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Thanks for all the replies and suggestions. The neck blank was definitely flat before the glueing took place. One thing is for sure - I did unclamp it early, thinking three or four hours would be enough... I had also read about the relative coefficients of lengthwise expansion in Trevor's design book and didn't follow the advice there (hands up!) The only fretboard material I had around was ebony and I thought I may as well use it...

As a novice, I don't know what to make of the difference in opinion on the materials choice here - two respected but opposing viewpoints..!

Todd - what caul do you use when you glue pre-radiused boards? And am I right in thinking that if using a long sanding beam (the type that StewMac sell), on a cylindrical board (as opposed to conical compound radius) the sanding action has to be parallel to the centreline, rather than strokes converging at the nut end?