Planer cupping the flat side...

[Steve Wurster]

Hello,

I'm running a bunch of cherry through my Jet 8" J/P combo machine, and am noticing a problem. After I run a board through the planer with the flat jointed side down, that same side will come out cupped after the rough side is planed smooth. The cupping is actually convex, meaning the board is higher in the middle / thinner at the edges.

I check the jointed side before planing and verify that it is flat across both the length and the width. I then use the planer to smooth out the rough side, but only until it's smooth and not yet the final thickness. Checking the freshly planed side, I find it flat across length and width. Flipping the board over and checking the jointed side, I find it flat across the length but now convex across the width. The board actually seems to be thinner just on one side, and flat across most of the width.

What the heck is causing this? Is this just the forces in the wood releasing, or could it be the planer table is not flat across its width and actually bending the wood? At this moment, I'm ruling out the knives not being parallel to the table, only because this is the jointed side that is getting cupped and not the planed side.

Any ideas here?

Thanks,
Steve

[Brendan Plavis]

Oh my... that does sound like a problem.

I wouldnt completly rule out the blades. You said that the sides are thin, and the middle is higher. Couldnt that be caused by the blades doing something like: /\(sorry for the drastic angle, you get what I mean.) Just a shot in the dark..

[Rich Neighbarger]

I have found that the dial indicator is my best friend when it comes to troubleshooting tools. I own a few, but my favorite is the Oneway Multi-Gauge. It has quite a bit of heft to it and is useful for many tools.

[Neil Brooks]

Could this be a case of reaction wood?? (http://woodzone.com/Merchant2/articles/reaction_wood.htm)

[glenn bradley]

I'm going with reaction wood but, a dial indicator slid across the table while referencing the blade would certainly tell you for sure. Grab a piece of known-acclimated material out of the bin, joint the face and run it through the planer to see if the effect is the same on different material.

[Van Huskey]

+1 The freshly jointed side is TRYING to bend but the part you are about to plane off are balancing that tension, once it that is olanned off it bends.

But as mentioned I would also check the tables.