Cupping on acoustic guitar back

[Kieran J Binnie]

Hi all

I was hoping some one would have some insight into an issue I've experienced with my current guitar build.

Over the winter I braced up a Yellow Cedar soundboard and Red Gum back for a parlour guitar build. Due to other project deadlines (and life getting in the way), I didn't get to glue up the guitar body when I had hoped. I checked both components yesterday and noticed that despite being well braced, and stored with plenty of air circulation to all surfaces, they have cupped across the width so that the show surface is now slightly concave rather than a convex curve (so it's bending the wrong way). The soundboard isn't too bad, but the back has a cup of 4mm across the widest part of the lower bout.

The wood is well seasoned, and has been stored in a heated (but not humidity controlled) room. The bracing patterns were all very standard, and I'd achieved good gluing surfaces on all the braces and glued up with go bars (so a good joint was achieved). I'm sure this is a result of change in humidity now that we're heading into summer months, but have never seen this happen before asI usually assemble the guitar soon after completing the back and soundboard.

So, my question is:

1. Are the back and soundboard rescuable by keeping them in a drying cupboard with low humidity until I come to assemble the guitar; or
2. Is it a case of planing off the braces and re-bracing, then assembling the guitar in short order; or
3. Are these components now glorified firewood (which I really hope is not the case).

As I say, I've not seen this happen before on any previous guitar builds, so the combined experience of the MIMF forum will be gratefully received.

Thanks in advance!

Kieran

[Todd Stock]

Two things at work here...change in RH of the top and possible distortion of the braces as a result of release of stress during carving. The short answer is that it's time to engage in a carve-off and retrace at a lower RH (I like 38% myself, and never above 40%). Rebrace and get the plates on the body ASAP.

[Simon Magennis]

Pretty much what Todd said. Plane off the braces. I would think twice about the bracing stock you are using.

I would also get some sort of humidity monitoring. After planing the the braces off, I would let the top and back settle for a while to see what happens. Regarding the the back, if there is a little bit of cupping in a board then I use the cupping for the curve. No point in forcing a curve into something that wants to curve the other way.

Some professionals here in Germany once told me they like to get the box shut up inside of 24 hours. So don't brace tops or backs until you have everything else ready to close the box. I aim to get it done over a weekend.

[Michael Lewis]

As pointed out you have a humidity control issue. Your top and back plates need to be well dry when the bracing is glued on, and really try to avoid introducing any more moisture to them than is in the glue. It can help to warm them with a hair drier for a minute or so just before gluing the braces. Do whatever you have to do to control this situation.

Your braces are probably good material, and they do not cause the cupping. The cupping is caused by the plates losing moisture (drying) and shrinking. Since wood expands and shrinks across the grain as it picks up and loses moisture, and one side (inside) is trussed up with the bracing the surface that is not trussed up (outside) does the shrinking. This is why your action goes up in humid weather and down in dry weather.

[Joel Nowland]

Kieren

I solved this by basically bracing my braces temporarily. The moment I remove the top and back from the go-bar bracing forms I spring clamp slightly spring loaded plywood bars across the braces for at least a week.

When they have normalized they stay put.

Joel

[Alan Carruth]

If the humidity changes after you've glued the races on, the width of the back plate will change, but the length of the braces doesn't change nearly as much (virtually not at all). That's what causes the problems, and it's why you need to have really good humidity control, particularly if you're not able to get things assembled right away after bracing them.

A few years ago I put a new guitar into a gallery show. It opened the first week in January, and we left the instruments off a week early so they could put the thing together. When I dropped it off I talked to the curator about humidity; evidently she wasn't listening. When I went in for the opening the top looked like corduroy, and the back had gone from having it's built in 25 foot arch to being about 1/4" concave. I gave them money on the spot to get a humidifier, and they did get one the next day. Sadly, the weekend following was the MLK birthday holiday, and the coldest day of the year, and they didn't go in to fill the thing. I got an e-mail the following Wednesday that the top had cracked. *sigh*

[Kieran J Binnie]

Dear all, many thanks for the comments and advice - it is much appreciated!

All my stock is well seasoned (no more than 6% moisture content) so it looks like I need to face up to sorting out my humidity control - I moved into a new workshop last year and this has languished on the to-do list - and then rebrace the soundboard and back.

Thanks again!

K