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Help, please! After installing three or four frets at the end of a tenor uke fingerboard, I noticed that the board was cupping upward. I took the frets out, and it went back to flat. I put the frets in again, and it cupped again. I can't figure out what is going on.

The board is ebony, and I sawed the slots using the LMI jig and hand saw two or three months ago. I left it at .25" thick until this week when I took it down to final thickness of .18". The frets are Stew Mac medium/medium and fit snuggly in the slots. I got them started in the slot with a hammer and then pressed them in the rest of the way with Jaws 2. I expected the board to back bow, which it did, and is not a problem. But the cupping has me puzzled. I checked the base block of the press, and it was flat. Then I checked the brass insert to make sure it was flat, and it was.

I am afraid to go any further with this fret board. Any suggestions or ideas?

Kevin

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Kevin Mason
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That will happen on relatively thin boards that are fretted off the instrument. I don't think I've done a thin board where the cupping has not occured. Actually I've had it happen on much thicker boards too, even Classical fretboards that are around 6.5 or 7 mm thick.
I made a counterform from wood - the length of the board but with an opposite curve. The fretboard was clamped to this, pulling the edges down.
It worked but dealing with the back bow and then with the cupping seemed to be a lot of messing around.
I decided to experiment with gluing in frets, especially for thin fretboards.
The first thing to do was to see how much better or worse the fret was held by glue, as opposed to being held by the barbs alone. I cut a whole series of slots in a board. Some frets were done with the normal slot and hammered in, others were done with a much wider slot and glued with Hide and Fish Glue.
All frets were left to overhang the board by about 1 inch to facilitate pulling the frets out.
The easiest to pull out were those that had been hammered in. Next the Hide. Those glued with Fish glue were the most difficult. This was all confirmed when a friend did the same test - except he was unaware which frets had been glued. His 'results' were exactly the same as mine.
There are obvious advantages to gluing:
1. No, or at least very little Backbow or cupping
2. The fretboard is under less stress, especially the tongue over the soundboard.
3. It's an easy, simple method that requires little 'skill'.
The slot I cut is wide. I haven't measured it yet but the barbs barely grip the wood. You can push the frets in with a piece of wood. The underside of the fretwire is thoroughly cleaned with Wirewool.
I clamp with a thick piece of plywood, 4 or 5 frets at a time. I'm fairly certain you can do the whole lot in one go, especially when using Fish Glue. Cambered boards will obviously need a suitable caul/form.

Do you mean cross grain cupping due to wood shrinkage, or fretboards backbow because of the frets?

If its cross grain cupping it's due to fretboard shrinkage and it shouldn't be happening, however you will get backbows because the presence of the fretwire and its barb will cause the fretboard to bend backwards. The solution according to Stewmac guitar kit instruction is to introduce a bow to the neck while gluing the fingerboard to the neck, by exerting tension with a broom handle in the heel

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He isn't referring to either. It has nothing to do with shrinkage, the fretboards are perfectly flat before fretting. After fretting the board takes on a huge Backbow. That can be dealt with but there can also be cross grain cupping introduced. It is much more noticeable on thin fretboards found on instruments like the Ukulele.
I can only speak of my experience with Flat fretboards, not of the cambered type.

Obvious solution: glue the fingerboard to the instrument before fretting. It's more difficult installing the frets over the body, but the benefits far outweigh the disadvantages for me.

Not so easy on small instruments with small soundholes. These fretboards are thin and have little mass. It's obvious that on thin boards there is a lot of stress caused by the normal method of securing frets. The obvious solution, as far as I'm concerned is to glue them in. I'm still trying to think of a disadvantage.
Seriously, I've been through all this. I fretted on the Neck in the true Spanish style for years. It's only when I started making Romantic Guitar copies (with their thin fretboards) that it became a problem.

Michael, thanks for sharing your experience. I realize now that this is the first uke finger board I have fretted where I cut the slots myself--all the others were pre-slotted boards from Hana Lima and he must cut the slots slightly wider. I agree that when it comes to uke building, fretting before gluing the board to the neck is the way to go. So, it looks like I pull the frets again, file the barbs off the frets, and glue them back in. If the cupping comes back, pull the frets, widen the slots a bit, and re-glue. Ultimately, I will probably end up preparing a new board. Sure am glad I inlaid only simple paua dots instead of some more elaborate fret board inlay.

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Kevin Mason
Mason Guitars and Ukuleles on Facebook
http://masonguitars.net