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I have a glued-up panel (Red Oak 22" X 22" X 3/4") that has a slight amount of cupping. I recently made Harry-Sin-Style Router Skis and would like to use them to flatten the panel. Which side gets flattened first, the concave or convex side?
 

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Its easier to get the concave side flat to the base than the convex side.
Bear in mind youre going to lose a heck of a lot of thickness by the time you get both sides parallel again. i think it could be as much as half the board thickness.
Might be better to just flatten the side thats going to show.
 

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Without doubt the CONVEX side first because the CONCAVE side will sit flat on the bench, whereas the other way it will rock like a cradle.
 

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The same orientation holds true if you were running it over a table saw, jointer, or planer for the reason Harry gave. In fact, cutting on the TS with the convex side down can be really dangerous as it can cause the blade to bind and toss the work piece.
 

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Have you thought about using your table saw to flatten the board? Put the convex side down on the TS and saw a groove about 2/3 through the board. After that put some glue in the groove and clamp it down on a flat service. It should come out after the glue dries pretty flat.

You might have to shim the board on either side to keep it from rocking while sawing. You could use double stick tape to attach the shims. If you decide to do it let me know how it came out.
 

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My router/planer jig sees a lot of action. But, if the work fits in the planer, I use a sled and shims/wedges. It's faster.
I've used Don's method to straighten 2X4s. But, I use a skil saw and several cuts. It ain't purty but, if it's going to be sheeted, as in shop doors, it don't show.
 

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The same orientation holds true if you were running it over a table saw, jointer, or planer for the reason Harry gave. In fact, cutting on the TS with the convex side down can be really dangerous as it can cause the blade to bind and toss the work piece.
This accident was an early experience with my first table saw. Hit me in the solar plexis and the bruise and pain lasted for months. :crying:

If it isn't flat, I don't put it on the table saw, not anymore.
 

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I do it like Don said . Only I saw it all the way through and joint the edge and re glue it. Then if it needs more in a different spot I do it again til it lays flat, the grain matches so the saw cut don't show. Some times I rip them and turn them end for end. Then after they are glued up I run them through the planer or drum sander to finish them. I find that less thickness is lost by doing this.

Herb
 

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So many different ideas, some more drastic then others, but what ever method you choose, My first knee jerk thought was the Concave down and the Convex up, seems to me that would lessen any rocking or need to shim it. Then flip it. But as Herb said, you would lose a lot of material in thickness that way. I never would have thought of his idea of splitting it and reglueing Please be careful and post your results . I for one would like to see how you do it and what the out come is.

David
 

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I do it like Don said . Only I saw it all the way through and joint the edge and re glue it. Then if it needs more in a different spot I do it again til it lays flat, the grain matches so the saw cut don't show. Some times I rip them and turn them end for end. Then after they are glued up I run them through the planer or drum sander to finish them. I find that less thickness is lost by doing this.

Herb
Herb; if you flip end for end don't you create an issue with grain direction when it comes to the planing? Why not just flip the piece over top to bottom, assuming the quality is the same for both sides (no knots etc.)?
You'd also counter the warp effect by flipping over.
 

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Herb; if you flip end for end don't you create an issue with grain direction when it comes to the planing? Why not just flip the piece over top to bottom, assuming the quality is the same for both sides (no knots etc.)?
You'd also counter the warp effect by flipping over.
Your suggestion sounds good to me. If I flip end to end,I dress it down with the drum sander. That is the last resort because when the finish goes on the grain reflects the finish dark and light.

I can flatten most boards by just ripping and reglueing as many times as it takes.

One time I had a PU load of reject Elm 1" rough sawn boards from the mill given to me. They were straight on the edges but the hump was between the ends ,like skis. I ended up ripping them into 2 1/2" wide strips and then planing them. and nestling them (spooning them) together making a 2 1/2" thick plank ,then resawing them into (2) 1" thick boards.

Herb
 

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So many different ideas, some more drastic then others, but what ever method you choose, My first knee jerk thought was the Concave down and the Convex up, seems to me that would lessen any rocking or need to shim it. Then flip it. But as Herb said, you would lose a lot of material in thickness that way. I never would have thought of his idea of splitting it and reglueing Please be careful and post your results . I for one would like to see how you do it and what the out come is.

David
I am missing Stick´s reply.
 
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