Can I use a router for finishing sand cast aluminium.

Heyo.

I'm looking to machine some larger, hollower, aluminium pieces. Only issue is just slapping a large (say 400mm x 200mm x 50mm ) block of 6061 and then grinding 80% of it away is not the best economical choice.

To combat that, I've looking into casting first, then machining to clean up.

I haven't been able to find any examples of other people attempting this. So I'd like to know if it could work.

The main concern I've got is making sure the CNC only finishes the cast piece, and doesn't cut away any material it shouldn't. This would mean setting start and finish points and I'm not really sure how you'd go about doing that. I'm probably biting off more than I can chew but this is the only way I can see going about this.

If you have any experience in this, please let me know. Thanks.