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Sanding Cupped Wood
http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=27153		 Page 1 of 1
Author:  Dan Pennington [ Mon Apr 26, 2010 10:06 am ]
Post subject:  Sanding Cupped Wood
Shortly after I bought a Big Leaf Maple sides & back set, the wood became U shaped? I've got it with my wood stash under plastic being humidified to 44%. Everything else is staying flat. After two months, the maple remains cupped.
Image
What do you think about me running the pieces thru my drum sander the way they are. First, sand the high spot in the center down flat and then sand the two outside high spots down. There is enough thickness to leave to do that. Woodworkers do that to cupped lumber all the time. Do you think there will be problems later?
Author:  wbergman [ Mon Apr 26, 2010 10:10 am ]
Post subject:  Re: Sanding Cupped Wood
There is not enough thickness to remove the high spots, but most sanders will hold thin wood flat to sand it. It will come out the other side still cupped, but thinner. The cup might even be removed from the heat generated by the sanding.

Have you had these stickered and weighted in storage?
Author:  Link Van Cleave [ Mon Apr 26, 2010 11:17 am ]
Post subject:  Re: Sanding Cupped Wood
What you have isn't a big deal. I would spritz the concave sides with a little water and place them down flat on something with the concave sides down or clamp them the way you have them in the picture and let air get to the outsides, after adding a little moisture to the inside. It looks like you live in MN. Should be fairly dry there. I bet in a couple of hours you can reverse that cup. If it is close to flat at all you can then sand to your working thickness. The thinner the would is the easier it will be to tame or the less power it will have to move things.
A lot of times you can chase the cup back and forth. If you can get it pretty flat by adding a bit of water you could sticker it well and weight it down. Let it equalize for a couple of weeks and see what you have got. All you have to do is get it flat enough to work. Once joined and thinned to your final thickness, braced and glued to a rim it isn't going anywhere.
Link
Author:  Blanchard [ Tue Apr 27, 2010 8:53 am ]
Post subject:  Re: Sanding Cupped Wood
I agree with Link... wet one side until it's flat and then run it through the sander. Once it's down to near final thickness, sticker it and let it dry out again. If is still has too much cupping, You can flatten it out by ironing it. That's right, just lay it on a flat workbench and take a clothes iron to the convex side. The idea is to use the heat to rearrange the cells in the wood. It's just like what we do when we bend sides. I have done this many times with great success.

Mark
Author:  Jason [ Tue Apr 27, 2010 6:54 pm ]
Post subject:  Re: Sanding Cupped Wood
Like the guys said above, moisten it, sand it down and all will be good. :D
We dry our wood a little differently than others in order to set the color and prevent staining. It's almost like a really mild case hardening. Once you take some of that excess thickness off the cupping shouldn't be an issue.
Author:  Dan Pennington [ Wed Apr 28, 2010 4:03 pm ]
Post subject:  Re: Sanding Cupped Wood
I did have them stickered and weighted down in my stash shelter, but they just didn't flatten out without some persuasion.

So, I dampened the cupped side of both side pieces, clamped them down flat with some stickers, and left them to dry for a couple of days. They were still slightly cupped. Today I ran them thru my drum sander with 60 grit for a bunch of passes. At first it just took material off the high areas. Then the rollers that hold the wood down to the conveyor belt flattened the sides and material was taken off all across the faces of the sides. I took them down to .140 in. and they are pretty flat - just a very very small cupping which flattens out when I bend the side as it will be bent in the guitar.

Image

I'll leave them like this till I'm ready to use them. Then, I'll thin them a little bit more to .90/.100 and then smooth them out with 400 grit on a hand sander.

I fell into a good deal on a Delta 18 in. wide drum sander and was itching to try it out for real. I'm happy with the performance. I may have been able to do this with a plane and cabinet scraper, but this way is so much easier.

Thanks for the advice.
Dan
Author:  Beth Mayer [ Wed Apr 28, 2010 6:39 pm ]
Post subject:  Re: Sanding Cupped Wood
Newbie question - what is stickering? I searched for this thread because I have a piece of African Mahogany with one 1/2 of the back cupping. I got it that way from the seller. Now I know how to try to correct it, but want to prevent this in other wood stock I am starting to stash. Currently have my few sets and a couple tops (all but one set unjoined and not yet thicknessed) in my music room (stable 50% humidity at all times). I just have the plates leaning against the wall of the shelving unit. I would welcome detailed storage suggestions and the definition of stickering. Thanks all, Beth
Author:  Link Van Cleave [ Wed Apr 28, 2010 7:07 pm ]
Post subject:  Re: Sanding Cupped Wood
Stickering refers to using stickers. Stickers are pieces of wood or some other inert material used as spacers between pieces of wood to allow more even air circulation. A typical sticker size for say guitar backs and sides would be 3/4"x3/4" by what ever length needed. I am sure someone will have a handy photo to show a stack of wood "stickered". I don't sticker wood that is dry and equalized. By equalized I mean that the wood has had enough time to reach a moisture equilibrium with the environment it is in. If your wood is nice and flat and it has been in that room of yours for 3 weeks or so I think you would be fine to stack it on top of itself on something flat and weight it down a bit. If the wood is starting to cup, twist, move around a bit then you will need to sticker it. Plywood will be fine for stickers for your wood. There is a bit more to this seemingly simple thing but this will hopefully give you a basic idea. There is a lot of info if you do a search. You can see from the above discussion that if you allow one side to dry or get wet more than the other side the wood will move. You can lay a piece of wood down flat on a bench and come back a few hours later and find it quite cupped. Stickers will help to prevent this by as I said above allowing the wood to be exposed to the air on all surfaces.
If you need more info start another thread with this as the topic.
Link
Author:  Link Van Cleave [ Wed Apr 28, 2010 7:21 pm ]
Post subject:  Re: Sanding Cupped Wood
Beth,
Here are a couple of arrangements for stickering wood. One is just some stickers and a heavy piece of wood weighting it down.
The other is some wing nuts to apply some pressure to keep things flat.
Link
Author:  Beth Mayer [ Wed Apr 28, 2010 7:45 pm ]
Post subject:  Re: Sanding Cupped Wood
Great, Link! That's exactly what I needed to know. I think I'm gonna need a bigger room! Thanks so much, Beth
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