Rocking between lid and bottom

[dcdear]

We are having the following problem.

The project in question is a box. We have a top and a bottom.

After cutting both pieces individually, the two don't fit together well. There is an uneven aspect to both. When put together they rock back and forth instead of fitting completely flat over each other. If you take the top or the bottom and put them face down on a flat surface, they aren't flat. The spoil board has been leveled.

I'm not sure if the crown of the wood has anything to do with the problem but we've tried both crown up and crown down and can't really tell if its any better or worse in either direction. The uneven portion appears to be an issue in the Z direction as the router moves in along the Y plane. We can't figure out if its a problem with the spoilboard, Y movement, or Z movement. Z movement seems like the most unlikely.

Spoilboard has been leveled so I assume it's not the problem, so all thats left is the Y movement, but that would mean that the bar would have to be bent, or the router is moving up and down slightly in the Z direction as it travels along the Y axis.

Or something else, 
Any help appreciated TIA.

[Derek the Admin]

I want to make sure I understand fully. Do you mean the lid and bottom rock when you out them on top of each other or that they rock on top of the walls if the box?
Are you facing these pieces down?

[dcdear]

When we put the top and bottom together - there is a slight rock back and forth. They don't fit snugly.
Putting the lid on the cut side on a flat surface there is a slight rock, like an uneven table at a coffee shop.
Does that help?

[tgattis]

Wood is under stress, when you remove even a tiny portion...it relieves a slight amount of that stress. Humidity levels in a shop or house can differ greatly and what may be flat in the house, might be warped in the shop. When you clamp a piece down to the table/spoilboard that has been flattened, you straighten out the twist/warp, machine the surface and then release the clamps, voila..the warp that was there before is still there. Maybe an explanation??

[dcdear]

I'll look at that, thanks for the response.

[Derek the Admin]

I'm going with tgattis on this one. Especially if one or no sides are being faced.

[dcdear]

So we planed the wood, this is both red oak and sapele. Planing the wood with the crown up, seems to have helped a lot or a least got us into the ballpark. We think the issue is tied to the wood and clamping as suggested based on cutting mdf and the same cut remaining perfect flat. Thanks for the help and responses, sorry it took a while to get back to this thread.

[Derek the Admin]

Glad all is well.