Router in router sled not working as expected

[ToolbeltDandy]

I tried to use a router sled (with a bowl shaped router bit) to plane/thickness some pieces of timber salvaged from some pallets (I saw a cool project I wanted to emulate but without buying a thicknesser). The problem is that I would "plane" (can I call it that?) one side of the board and then flip it over and do the other side. The result was never a perfect (not even approaching usable) board. I've tried to illustrate the results with an exaggerated sketch:

Any idea why this is happening? It's not like the router sled sags in the middle; in that direction the cross-section is fine. Do I need to use a hot-glue gun to keep the board still/flat? Until now, I've been using a couple of wedge-shaped particle board pieces that I tap against with a hammer.

 

[big-all]

do you not have a table saw with half the depth capacity ??

 

[Dave54]

How much is it "out", and over what length?
I can think of two or three things that could be going wrong though.
- The work piece is moving
- The surface you are running off isn't true
- You are putting more weight on at the start and end of the cut, "springing" the jig down, and causing the cutter to move down as well.

 

[Harbourwoodwork]

Hi toolbelt What are you trying to acheive?

 

[cajar]

You are trying to take too much off in one pass.