According to gtslabs :
I set my bridgeport up to cut a thin slot in a piece of steel. So far I
have only broken bits.
I dont know much about hardness but the material I am using has a black
coating. Not sure what that is. But I started off with a 1/8" endmill
taking very light cut at a very slow speed. I am having no luck.
Additionally, I need my slot to be deeper than the endmill itself.

That is a serious problem, and takes *lots* of patience, and
lots of coolant or air blast to keep the chips cleared out -- *if* you
insist in doing it with an end mill.

What I would use for the task is my old horizontal spindle
milling machine, with a conventional milling cutter (wheel sort of like
a saw blade) to do the cutting. The smallest ones which fit my machine
are 3" diameter, with the spacers on the arbor being about 1-1/2"
diameter, so those would not reach to the bottom of your cut (barely
3/4" -- so the spacers would drag on the workpiece before you reached
the full depth), *but* there are larger cutters. I have a couple which
are 6" diameter. The ones which you really want for this are the
staggered-tooth ones, one tooth cuts the center and one side, the other
cuts the center and the other side. And, they are available in various
thicknesses.

What they do not do well is produce neat ends of the cuts -- it
will curve up to the surface at either end. However, if your slot needs
to go from one end to the other of the workpiece, it will work quite
well.

With your Bridgeport, you have a choice of a horizontal adaptor
(a right-angle head, an arbor, and a clamp on the dovetails of the ram),
or a stub arbor. The first choice would be the better in terms of
rigidity -- *if* it can handle a cutter over 4" in diameter.

What I am doing is setting up a 2 brackets to hold a 1/8" x 1.25" x 12"
long cutting bar. I am sure there has to be an easier way but it might
take more pieces than cutting from one.

It sounds as though the slots go to the end of the brackets, in
which case the conventional milling cutter should work quite well.

Enjoy,
DoN.

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