Can a cupped board be "un-cupped"?

[Niels J. Larsen]

I've resawn a 2"x6"x8ft plank of shedua (or ovangkol) into two planks.

Now I have two planks which are cupped along their length.
I.e. if I place a 6ft straightedge along one of the resawn planks, I can measure almost ½" cup.
Since the resawn thickness is approx 1" and I need the full length of the plank (8ft), nothing will probably be left after jointing and planing the plank.

Is there a way I can straighten the cupped plank? - e.g. making the cupped side wet or similar?

I know it's probably not doable, but I'm desperate after I just (most likely) ruined an entire plank of shedua (since I need the entire length in one piece).

[David DeCristoforo]

Wet the convex side and put it in the sun for a bit. Some guys will just toss the board concave side up on the lawn for a while. As often as not, the cup will come out. Sometimes the board will stay flat, sometimes not. The idea is to try and equalize the MC on both faces of the board. Nothing to loose by trying.....

[John Keeton]

David, you would know far better than I, but wouldn't putting the concave surface (the driest) toward the ground cause it to pick up moisture and swell, thereby reducing the cup. Just trying to think through this. I usually will wet the concave side.

I realize the sun would further pull moisture from the convex side up, but seems it would work better overall.

[David DeCristoforo]

"David, you would know far better than I..."

Naa. I never put the wood on the lawn. What I said was that "some guys" will. I just wet the convex side and heat it a bit by putting it in the sun for a few minutes. I would guess that the "lawn trick" relies on more moisture getting into the underside (convex) that will be pulled out of the top side (concave). Maybe next time I have a cupped board I will try the lawn thing just to see....

[Edwood Ferrari]

I just watched a New Yankee Workshop where Norm was making a round pine table. He had a wide reclaimed pine board with a bad cup. He put a table saw curf cut on the bottom side leaving some material on the top. He then pressed down on the ends and the board flattened out. He used some type of wood filler to fill the gap. Norm said the filler he used has some give to allow wood movement. I don't think I would try the trick but it seemed to work for Norm. If you're interested I have the episode recorded so I could look up the details.

-Ed

[Stephen Edwards]

What are you going to use the boards for? What kind of project?

If the moisture trick doesn't work and only one side of the board is going to be the "money side", Then the kerfs may very well work for you. I've used that technique for certain projects and it worked out fine. Depends on the project.

[Niels J. Larsen]

I'll be using the planks for the front of a large drawer under a bed, so the kerf trick may or may not do. If I can avoid it I'd like to.

Which side should I wet? (you guys don't seem to agree :-D)

Also - how much water are we talking about?

Should I use the garden hose or is it better with a small flower sprayer?

When the board is wet and hopefully straightened - how long should I leave it in the shop before I mill it?

Thanks so far!

[Dan Forman]

Just for clarity, do you mean bowed, or cupped? It's not quite clear, but the way you describe it sounds more like bow to me. Cupping is measured across the width, bow is measured along the length. I don't know much about getting rid of either, but it would be good to clarify for others who might.

If it's bowing you have, you might consider doing two drawers instead of one, that way you would only have to deal with half of the bow for each piece, and unless you need to store 6 foot items under the bed, a 3 foot drawer would be less likely to bind when pulling it out.

Is this a piece for yourself, or for someone else - in other words, are you locked into a design, or can you improvise?

Dan

[John Keeton]

Which side should I wet? (you guys don't seem to agree :-D)

Also - how much water are we talking about?

Should I use the garden hose or is it better with a small flower sprayer?

Niels, my belief is that the dry side of the board contracts, thus making it the concave side. The wettest side of the board would have expanded forcing the cup, thus it would be on the convex side.

Apparently, David disagrees, and I would have to say he has much, much more experience than do I.

However, this has worked for me before. But, as David says, sometimes it is permanent - sometimes not.

I would use a very wet rag, it will take much more than a misting. I would wait several days - couple weeks - to see what you have before milling.

[Niels J. Larsen]

Just for clarity, do you mean bowed, or cupped? It's not quite clear, but the way you describe it sounds more like bow to me. Cupping is measured across the width, bow is measured along the length. I don't know much about getting rid of either, but it would be good to clarify for others who might.

If it's bowing you have, you might consider doing two drawers instead of one, that way you would only have to deal with half of the bow for each piece, and unless you need to store 6 foot items under the bed, a 3 foot drawer would be less likely to bind when pulling it out.

Is this a piece for yourself, or for someone else - in other words, are you locked into a design, or can you improvise?

Dan

Arrghh, that's what you get from talking to those darn foreigners (me) - nothing but misunderstandings!

Yes, I mean bowed - it's across the length my boards aren't straight (enough).

Sorry for the mistake - I even looked it up in wikipedia before I posted, but apparently I didn't look hard enough.

I've thought about splitting up the design, and I'll probably end up doing so.

However I'd still like to know what I can do to fix my problem :-)

[Myk Rian]

I just watched a New Yankee Workshop where Norm was making a round pine table. He had a wide reclaimed pine board with a bad cup. He put a table saw curf cut on the bottom side leaving some material on the top. He then pressed down on the ends and the board flattened out. He used some type of wood filler to fill the gap. Norm said the filler he used has some give to allow wood movement. I don't think I would try the trick but it seemed to work for Norm. If you're interested I have the episode recorded so I could look up the details.

-Ed

Thanks Ed. There was a lively discussion on this topic a week or so ago.

[Mike Parzych]

I just had to do this with some 1/2" cherry boards I put out on the lawn to redden them. Forgot about them and they were out there all night and bowed.

Wet the concave side and put that side down on the grass in the sun. The time it takes to straighten it is pretty variable. I left those out overnight again and they were pretty flat in the morning. On the other hand, I had some 3/8" walnut panels that only took about 3-4 hours. It does work, you just have to fiddle with the time.

I poured about a 1/2 cup of water on them and spread it around with a paper towel. You don't need much. Then let them sit for a couple days and see if they're still flat.

[Stephen Edwards]

Arrghh, that's what you get from talking to those darn foreigners (me) - nothing but misunderstandings!

Yes, I mean bowed - it's across the length my boards aren't straight (enough).

I've thought about splitting up the design, and I'll probably end up doing so.

However I'd still like to know what I can do to fix my problem :-)

Don't feel bad about confusing the terminology. I guarantee that foreigners don't have a monopoly on that quirk! You should hear some of the woodworking terminology used where I live!

If I'm understanding you correctly, you need the full length of the 8' boards for a drawer front. Does this mean that your drawer is going to be 8' wide? If so, +1 for your idea of splitting the design.

I'm also understanding you to say that both boards now have a bow in them, for example with a straight edge layed on the board from one end to the other there's a significant low spot in the center of the 8' length of the board.

IF these drawer fronts are going to be false fronts applied to the front of a box drawer, you can certainly reduce the bow with kerfs on the back side of the board.

If that's the case, you would make the kerfs perpendicular to the length of the boards. You wouldn't need many kerfs. You would also want to make sure that the kerfs stop short of the edges so that they are completely concealed behind the front of the drawer box. There is no need to fill the kerfs with any kind of filler material. Of course, that's assuming that these are false fronts as mentioned above. If they aren't false fronts, I don't know what to tell you except to consider that as an option in your design.

I've used this technique many times in simaliar situations. It has worked for me. Done properly, with the kerfs concealed, only you and the board will know about the kerfs!

Best of luck with your project.

[David DeCristoforo]

"Apparently, David disagrees, and I would have to say he has much, much more experience than do I."

No, actually I don't. You are completely correct. I must have had my head up my... oh well... never mind that. The "hollow" (or concave) side of the board should be moistened. Of course it is probably moot now that the OP has stated that the "bow" in his boards is along their length, not their width. That's a whole 'nuther "kettle of fish".

[Dan Forman]

Here is another question to consider. Did the boards bow within a few minutes of being resawn? If so, it's probably not a matter of moisture causing the bow, rather inner stresses in the wood were altered when it was cut, and it is now conforming to a new balance. In this case, soaking wouldn't really do much to change it.

If the wood was straight after resawing, but then gradually bowed over the space of a few hours, then it's probably a moisture thing. It would take that long for the wood to dry out on the cut side and produce the bow.

Dan