Greenlee punches can get expensive but they make clean holes. For round
holes the other option might be one of those multi-step hole cutters. I
don't like the flycutter/boring head idea.

Grant Erwin wrote:
I'm a big fan of punching sheet metal as opposed to drilling it. - GWE

Dave wrote:

Non-standard sized large round holes?
Thin metal (steel or aluminum?) boxes?
Using a drillpress or a mill or something else?

Bill Schwab wrote:

I am getting closer to doing the job that involves clamping open boxes,
DB 9 holes, etc. There will be some round holes, and my first thought
was a boring head. That would be overkill in precision capability, but
I'd like to learn how to use one, so it seemed reasonable.
[...]