Using a metal cutting burr with a router...?

[noyloj]

I cut a slot in my table about 4mm deep

and the table twisted by about 2mm with the stress being relieved then I had to take it to engineering place to have it trued up and the top skimmed.

So beware as I did not expect that to happen.

Yes 3-4mm was about what I'd imagined, I would have though that would be safe!.. bugger for your table what a pain!

It's a crazy thought probably but what if I was to try and relieve the stress before milling out the groove?