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Say one has a cupped board one wishes to make flat.
Which is the correct method, does one repeatably machine the face until flat.
Then edge, then face then edge, face then edge.
Or does one start off, face then edge, face then edge, face then edge then pass through planer?
Thanks.
 

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Cupped wood, unless quite thick, usually burns quite well in a fireplace, much better than it flattens. In general, you joint the U shape with the points facing down. You can't reliably work on the other side until the first is flat. Unless the board is only moderately cupped and pretty thick, you'll wind up with a very thin board that will want to cup again. At least that's been my experience.
 

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Agreed. Points down always. Points up can rock as you push through so you aren't getting consistent results and it will take more passes and you'll lose more wood. I wouldn't worry about an edge until I have the faces flat. In order to get a 90* edge to face you need the board to register flatly against the fence so until you have it flat there is not much point.
 

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It may seem silly but, I've had some limited success straightening cupped timber by wetting the cupped side and placing it cup down on flat concrete. Then putting concrete blocks on it. On a 1X8X24 length of hard maple, it took three days of re wetting every 30 minutes, or so. But, I was able to get it flat enough to work.
 
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I had a bunch of warped ruff sawn elm one time that I cut into 2' lengths. It was 8"-10" wide. I ended up ripping it to 2"-3" wide stripes and gluing them back together and then planing them to thickness. They came out nice and flat and the grain matched so you had to really look at it closely to see that they were glued back together. They came out at about 11/16" thick when finished.

Herb
 

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planing a cupped board flat can work but there is nothing to stop it from cupping again because of the grain..
follow Herb's advice for the best method...
splining the joints is to your advantage also...
 

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If you have a band saw you can try putting the cupped side toward the fence and see if you can cut a flat side on the top. If so then you can run it through the planer. But cutting it into to at least two boards will work better.
 

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I Joint the cupped side on a jointer first, either with a hand plane or a powered jointer first.
Once the first face is flat, I plane the opposite face to be parallel, either with a hand plane or my benchtop planer.
Once both faces are parallel, I joint one edge straight, then rip the other edge parallel on my table saw.

If the wood is dry (6%-8% moisture) it will retain its straightness, unless the moisture content in the wood changes in the wood from one side to the other.
 

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I Joint the cupped side on a jointer first, either with a hand plane or a powered jointer first.
Once the first face is flat, I plane the opposite face to be parallel, either with a hand plane or my benchtop planer.
Once both faces are parallel, I joint one edge straight, then rip the other edge parallel on my table saw.

If the wood is dry (6%-8% moisture) it will retain its straightness, unless the moisture content in the wood changes in the wood from one side to the other.
That is the recommended way to do it, and if you have a 2" thick board,you might get a 3/4" thick board. If you are starting with already planed 3/4" boards it will end up more like 1/2"-3/8" thick, which is Ok for boxes etc.

The warped boards, corner to corner are harder to flatten. Those have to be shimmed corner to corner, run through the planer to flatten one side and then turned over and run through the other side. Over 1/2 the thickness will be lost on flattening those boards.

A lot of time and material can be used and wasted just trying to save cupped and warped lumber, but sometimes it pays off.

Another way I have done it on planed boards is to make one edge straight and rip the boards about 1" wide then turn them up on edge and glue them back together making a 1" thick plank, then run that through the planer. It will come out looking like a cutting board countertop with 3/4" strips 3/4'thick.

Herb
 

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Gaia (Peter), You made no mention in your question about dimensions in you inquiry or what machines you are using. All of which could alter how to do it.


For example if you only had a benchtop planer, you could glue sacrificial (straight) boards to the edges of your cupped board. That would allow you to send the package through your planer to remove only the high areas...then flip to do the other side to make it parallel. once both faces are done you could rip saw the sacrificial boards off.
 

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You can try ripping the board in half (or into more parts) and face/edge jointing each piece individually before gluing them up as a panel. You'll lose a lot less thickness that way. Since they are all the same board, the grain should match almost seamlessly. Of course, there is no single best answer - it all depends on your judgment of the board itself.
 

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This is how I flatten a cupped board, convex side up, using the ski mounted router. Once the first side is flat it can be inserted into the thicknesser or if no thicknesser, use the ski mounted router on the concave side. A member has already mentioned cutting the board into strips and gluing them together as shown with this box lid.
 

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