This will probably be boringly obvious to most readers, but I'll post
it in the hope that it might be useful to a few because it worked so
well for me.

I wanted to cut some .700 dia holes in .062 aluminum. After trying
some other approaches I tried making a shop- expedient trepanning
tool.

I turned a bit of 3/4" barstock down to .700" OD, drilled it out 5/8"
and milled slots in the result. No rotary table, just V-blocks in
the vise and X-Y movements. Each slot was made so the cutting edge
thus formed was square, OD of 1/4" endmill moving radially from the
center of the workpiece. I then hand-ground a bit of relief, rake
and bevel (so it penetrates OD before ID) with a Dremel running an
abrasive cutoff disc.

Material was 12Lsomething free-machining steel. I love that stuff!

I case-hardened it with Kasenite which seems to work OK on 12Lxx,
A sharp new file skates on the hardened part. This precision
process consists of heating with an O/A torch until orange, plunging
into the can of Kasenite, heating again to a glowing glob of orange
for a couple of minutes with the garage door open so the smoke had
somewhere to go, and then plunging into a margarine tub of water.
After quenching, I poured a wee dram of Menard's finest muriatic acid
($1.98 a gallon on sale) into the margarine tub to pickle off the
oxides for a few minutes.

I don't think I spend an hour making this tool.

This quick-a-minnit tool works far better than any holesaw I've ever
used. It goes thru 1/16" al in a few seconds at 135 RPM with only
mild pull on the quill and a drop of cutting fluid on the workpiece,
cutting a nice curly chip and leaving a clean and nearly burr-free
hole. I made a slot for poking the slugs out of the center, but
wouldn't have had to. They about fall out when picked with a scribe.

Photo sent to the dropbox as "holecutter".