Earlier, wrote:

I need to cut a panel to mount one 3.125" and one 2.25" What size hole
saws do I use? Do I use smaller sizes, say 2.125" for 2.25" and 3" for
3.125"? Thanks.

YMMV, but I've had good luck making 3-1/8" instrument holes with a
3-1/8" hole saw, likewise with the 2-1/4" saw. The hard part is finding
them in those sizes; you might have to order-in from www.mcmaster.com
or some such. The holes did tend to come out .020" or so oversize, but
the instruments I was mounting tended to be on the large size anyways.
If it's for something important, try it on scrap aluminum of the
correct thickness first.

These days I use a single-point suicide cutter as others mention.

Thanks, and best regards to all

Bob K.
http://www.hpaircraft.com