For the larger diameter holes (1" / 25mm+): what sort of cutters are y'all using to machine the initial hole? Annular cutter? HSS or Carbide? Recommended brands? Or just start with a 1" drill and then lots of time with a boring head?
Water jet / plasma / laser is cheating.
I've used a bi-metal hole saw on 1" thick thick steel a couple of times...that was painful. And that was with a clearance hole for chip evac.