I never thought I'd be doing this sort of work ever. I am quite content
to high speed micro mill aluminum. When I was first learning I asked a
lot of questions here and elsewhere. Eventually I put together enough
information to where I can produce satisfactory results most of the time.

When I say micro milling I mean optimizing cut rates with modestly high
speed spindles (24K RPM) and pretty small cutters. Almost no milling
job that I do gets by without a finish pass without atleast a .0625 ball
mill. Quite often I have to use .03125 ball mills to finish some
details. I even have to break out the .0156 stuff once in a while.
That's the range I am talking about when I say micro milling.

For all of that work I use uncoated square end and ball end mills with
6-8% SC520 and water with a constant flood. Cutters last a very long
time and the results are good. Jobs often last for hours. Sometimes a
single tiny little cutter has to work for hours.

Well somebody offered me enough money to tempt me to do some similar
work in 4140HT. I've milled some hardened steels and hard tool steels
before. The numbers say 4140HT should be easier. Its a sort of
compromise. My past experience shows me that a low speed and feed with
an AlTin or TiAlN coated cutter works pretty good. That's fine when you
are pushing a 1/2" 5 flute end mill, but I have to do some of that small
detail work to finish this job. I need to push the envelope with ball
mills as small as .0156 for some of the finish work. I can break it out
so I don't have to finish everything with it, but its still going to
more time than just a few hours. HSM Adviser gives some numbers that
show days instead of weeks, but I wonder if there is anything I can do
to actually get those numbers. With aluminum heat and thermal shock is
not really an issue. With Altin in steal I don't run coolant, but I
admit I do throw a little cutting oil on the part. I've just never done
small or micro milling in harder steel. Well, I have engraved some
knife blades, but that's it.

I'm hoping I can cut down the learning curve, but I am prepared to break
some cutters and ruin some work pieces if I have to. Chip clearance is
probably also going to be an issue. Maybe an air blast to blow chips
clear and see what happens. The thing is we are talking about steel.
Steel chips don't blow clear as easily as aluminum chips.

I am open to any suggestions.

Worst comes to worse I am prepared to tie up my machines for a few
weeks. I certainly figure days even in the best case.