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Hi everyone,

I'm nearing the end of a jumbo build and I've encountered a bit of a problem.

Having completed the finishing process, it was time to assemble the neck to the body again (mortise and tenon, bolt-on), check all the angles and glue down the fretboard extension.

I got the neck bolted on and all angles seemed good - on centreline of the instrument, correct height over the temporarily-positioned bridge when checked with a straightedge.

However, I noticed a small gap between the fingerboard extension and soundboard (from frets 15 to 17). This gap does not run the entire length of the fingerboard extension - nice contact is made just after the body join (between frets 14 and 15) and again from frets 18 to the end of the extension. At its largest, it is around 0.030"/0.75mm.

Hopefully this picture can help demonstrate the gap:



Now, I've had a gap under the entire fretboard extension on an earlier instrument, and I interpreted that as being caused by a failing on my part to achieve the correct geometry in terms of the dome of the sounboard and neck angle. However in this case the overall angles seem correct - and the contact made by the extension at frets 15 & 18/19/20 would seem to bear this out.

A quick check with a straightedge directly on the soundboard shows that there is indeed a slight dip or sunken spot in the top, between the top-edge of the rosette and the location of the neck block. The top is still as stiff/rigid as I would expect in this area.

Clamping the extension down to the soundboard with medium pressure (a single cam clamp) had only a minor effect - I wasn't brave enough to continue applying more force - I was concerned the amount of pressure required to distort the extension enough to close a gap this big might cause something to pop

I would really appreciate some advice on how I can address this. It's intended to be a personal instrument (I only really build for myself) so a purely cosmetic fix to eliminate this gap would be OK with me.

So far the only idea I can think of is to add a thin slice of rosewood to the underside of the extension and then shape it somewhat, leaving it thick in the 'gappy' area and feathering it out towards the areas where the extension naturally meets the soundboard.

I'll be doing mental back-tracking through my build process for this one in order to figure out what the heck I did to cause the issue in the first place! In the meantime, any advice and guidance would be appreciated!

Thanks!
Josh

Put your straight edge on the underside of the extension and see if it is flat. I doubt if it is. That may be half of your problem. Measure the dip in the top with feeler gauges and get back.
Tom

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A person who has never made a mistake has never made anything!!!

2 possibilities
the extension is bowed these should be glued or at least bolted down.

another is as mentioned that the top is sunk, Ibet your extension is bowed . The fret tangs will push against the end grain in the slot causing it to bow.

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John Hall
blues creek guitars
Authorized CF Martin Repair
Co President of ASIA
You Don't know what you don't know until you know it

I'd glue it down and see what happens. Make sure it is properly hydrated.

This area of the guitar really needs to be sanded only with a hard pad. I have a rectangular phenolic sanding pad that I use for ensuring perfectly level sides where the neck joins the body and also for under the fretboard extension. Another concern is inconsistent humidity levels. For your next build, ensure that all those things are solved before proceeding to finish.

As mentioned above, the frets will often cause the fretboard extension to backbow a little bit. This isn't a big problem -- just glue it down. IF the backbow is severe then your fret tang is probably oversized by a couple thousandths or so.

Thanks for the advice everyone!



You're correct, the extension does have a slight bow. I broke out the feeler gauges and it looks like about 0.015 dip in the top and the same for the bow in the extension.


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Thanks, I added more clamps and managed to crank on enough pressure to close the gap without damaging anything. I'll glue it down and hopefully there'll be enough meat in the frets to 'level out' the dip, at least in terms of the fret plane.

It's possible humidity was a problem. I had sanded it flat with a hard block prior to finishing but had switched off the dehumidifier while the finish cured as I figured I wasn't gluing anything for a couple weeks. So maybe that period of greater humidity in the shop caused some movement. I generally try to maintain a constant 43% during actual construction but to be honest I could use some better quality hygrometers... My reading of 43% could be inaccurate.

Thanks again for the advice everyone.


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