According to Bill Schwab :

[ ... ]

Sorry - large holes, not necessarily non-standard, thin metal, one
steel, one a mix of various metalic substances. If I really get lucky,
I might be able to negotiate the steel out of existence, leaving just
the dusty/grity/disgusting metal that must be penetrated. That stuff
would probably yield to the cheapest of hole saws, assuming I can get
one of a suitable size (likely).

One thing to consider is that hole saws tend to make rather ugly
holes in metal -- at least based on use in a drill press.

What I always like for aluminum or steel panels is a Greenlee
Chassis punch -- but that can be *very* expensive.

And if you're working with the die-cast aluminum zinc boxes, I
think that the stresses of a chassis punch might lead to cracking.

What I've found to do a nice job of making clean holes in sheet
metal on a drill press is something called a "Roto-Bor". I have two
sets, covering a size range from about 1/4" to 1". They have a
spring-loaded pilot which fits into a center punch mark or a center
drill hole to precisely locate the cutter.

The cutter has two flutes for the smaller sizes, or four for the
larger sizes.

The major disadvantage of this is that as far as I can
determine, these are no longer made.

Note that using a boring head on the mill will not eliminate the
need for a hole to start with -- and the bigger the hole the better,
because a boring head is rather a rather slow way of removing material.
So, a hole saw to produce an undersized hole is a good thing to use
*with* the boring head.

Enjoy,
DoN.

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