Squares and rectangles are easy with a reciprocating saw, but what about round holes in a machine enclosure? 6" or 8" Ø? I want a nice round hole and not something that looks like a complete hack used an axe. Idears?
The obvious answer is a plasma cutter and a template. An air-nibbler and a template would do the job if the sheeting is thin enough.
Another method is to make a circle cutter, in-place:
Drill a hole at the center-point and put a shoulder bolt in there with some nice big washers or pieces of plate for stability (to limit flex).
Pivoting on the shoulder bolt, attach a piece of flat bar wide enough to bolt it to your wood-router.
Drill a starting hole and buzz the circle out with your router. If it's on a vertical surface, leave a tab at the top about 1/2" wide.
Cut the final 1/2" tab by hand, then de-burr the hole.
If you start getting too much flex or chatter half way through, use some short fasteners with nuts and washers in the cut groove to support the cut area, or just leave about 3 tabs and cut them after the router work.
A 1/4" or 3/8" end-mill will work, but the helix angle causes chatter in the sheet-metal. You'll get less chatter with a straight flute cutter.