Ever had success eliminating cupping?

[Michael Minton]

I bought 3 back/side sets of Indian Ebony from a guy in India.  Package was sort of dampish when it arrived (never a good sign), but the wood appeared pretty normal.  In the 2 weeks since then (in the 42% controlled humidity of my shop) the wood has been cupping.  I know that means the wood wasn't perfect quarter (but I'll give someone cutting it a little slack since end grain in ebony is super hard to see) but it also suggests to me it may have been milled before it was properly dried.  I'd say about 4 mm cup in the side slats and 5 mm in the back plates.  I have them clamped under pressure now.

Has anyone ever had success with a technique to eliminate cupping in lumber?

Thanks,

Michael

[Paul McEvoy]

I had a mahogany back that was badly warped.  I was in a class with a well known luthier.  He said brace it and clamp it up.  As far as I know that guitar has survived some fairly radical humidity changes and is still healthy.

Your mileage may vary. 

Leaving it clamped up flat for a few months seems like a good idea.  I wonder if it would be good to add some moisture to start, but I'm really wondering about that, not suggesting it.

I think you could potentially use a flat iron to flatten it also prior to storing it, but also don't quote me on that.

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Paul McEvoy
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[Michael Minton]

I have braced one or two backs that had a little curvature to them, and they were fine.  5 mm seems too much to me though, and I really don't know what would happen if I tried to run it through the wide belt like that.  The rollers will push down on plates that aren't that badly deformed, and you can thickness without much problem.  But a wood as hard as ebony cupped that much?  Seems a bridge too far.

The sides present an interesting issue as well.  When you plasticize the wood as you bend it, you are reshaping things.  Will this take the cup out of the sides?  Not sure.   But I still have the problem of trying to thickness sand them down to 2 mm.

My current plan is to leave it clamped and see if I start to get some gradual improvement.  I'm not real optimistic.

Michael.

[Charles Tauber]

The first thing to ensure is that you are storing it properly. Specifically, stickered and weighted. If air can't reach all surfaces, the wood can cup and twist regardless of how it was cut (i.e. quarter sawn) if the wood isn't in equilibrium moisture content with that of the air surrounding it - i.e. in your shop.

As you probably already know, if the grain showing on the face of the wood has "V" or "cathedral" shapes, the wood was not quarter sawn - or it can go from quarter sawn/rift sawn to flat sawn across the width of an 8" board. Used to be that a usual characteristic of instrument wood for backs and sides was that it be quarter sawn or close to it. These days, many "tone woods" are sold that aren't, particularly of the rarer woods. Did the "guy in India" claim that it was quatersawn? If not, I wouldn't assume that it is.

Charles 

[Charles Tauber]

If your sander has a sufficiently strong indeed and outfield roller, you won't have any issues keeping the material flat during thickness sanding or sanding it to a consistent thickness. Reducing its thickness won't make it flatter. 

My experience with ebony backs is to make them a little thinner than, say, rosewood. If the wood is well-dried - and has fully acclimatized to your shop environment - AND you make the guitar in a constant/controlled humidity environment, the 5 mm cupping won't be an issue. The back will take the arch of the braces glued to it. You will need to use a method of clamping the two halves of the back together that keeps them flat - and the gluing seam aligned - while gluing the two halves together. 

As far as the cupping of the sides goes, several factors come into it. The tighter bends will be flatter across their width, the less tight bends probably cupped a bit more. Depending on how much extra width the sides have, you might be able to trim some from each edge to reduce the cupping.  As for the back, thickness sanding the sides won't be an issue with strong indeed and outfield rollers to hold them flat while sanding.

Never assume that the wood you buy/receive is fully acclimated to your shop environment. Building with wood that isn't will generally bring grief.

[Ed Bolles]

I had a plate that looked like a potato chip. I sprayed water on it and put some weight on it. I got busy and quit working on it for a couple of months and then when I got back to it later it stayed pretty flat. It seemed to want to warp again but i kept weight on it and when I braced it , it worked fine. 

Ed Bolles

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From: 'Charles Tauber' via Robert O'Brien Guitar Building Forum <obrien...@googlegroups.com>
Sent: Wednesday, June 26, 2024 4:25 PM
To: Robert O'Brien Guitar Building Forum <obrien...@googlegroups.com>
Subject: Re: Ever had success eliminating cupping?

 

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[Joe Shuter]

Michael, is the cupping on the sides on the width or the length? If it is in the length that can be dealt  with in the body mold. Cupping on the backs can sometimes be braced to eliminate the cupping. Scooter

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[Sean Gilbert]

I've read that cupping can sometimes be addressed by dampening one side and placing it face down. I believe that it was the cupped side. I've tried it in the past and occasionally had success, but often have not. It's worth a try if you can find some of the descriptions online. 

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[Doug Shaker]

I wonder if a clothes iron would provide enough heat and moisture to reduce the cupping.

Doug Shaker

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[jb nb]

If a piece has a cup to it,I simply remember to "fill the cup,and turn the cup upside down"..... (of course this means just to wet the cupped side,but it helps me remember....)....and flatten it down ...

[Michael Minton]

Thanks to everyone who has responded.

Charles: I can't tell if it is QS.  But "the guy in India" didn't say it was, so it probably is not.

The outfield roller is a big boy, so I guess I am OK.

I think I will sticker and clamp.  Maybe moisten the inside side of the "cup."

Scooter: it is across the board, not down its length.

Doug: I may try that on a piece that I'm rating as a total loss.  See what kind of improvement I get.  Spritz a little on and iron.  

Michael