Clamping plate to mill.

[timmeh]

Toe clamps are not strong enough. Looks like pieces of wood under the "outside" ends of the clamps, that might be biggest contribution to movement.

 

[raferguson]

I took another shot at this. First I had to make two more clamps, which I probably needed anyway. This time I had four clamps, and instead of trying to make the slot in one pass, I used several passes. It worked.

Thanks to all for the tips, I will keep them in mind for the next time I need to do this.

 

[Cadillac]

The other problem I see is the wood blocks. Wood compresses expectially with the edges of the clamps like how you have. When you load the clamps it might feel tight but give it a minute or two my bet is the clamp could get tightened more. Add cutting oil and it’s worse. I use wood but to protect the part and light machining.