This might be sound like heresy but I do that kind of work now with a reinforced cutoff wheel. I mount it in the slitting saw arbor, crank the mill up as fast as it will go (which is only 2800 or so) and bear in. It doesn't hold slot width quite as well as a slitting saw will, but most of my cuts are things like slotted arbors which aren't critical. I have used this for slots in yokes, and one can raise or lower a few thousandths and shave a little more off a sidewall for fit if need be. I've got a couple .040" thick wheels which is as fine as I can go, and some thicker. Haven't broken one yet (knock on wood) but since it's mounted in the mill there's no twisting moment on the disk. I think the greatest danger would be feeding too fast causing an unstable wobble, but it will feed in amazingly fast and sure beats that old grunch, grunch, grunch of the low speed slitting saws.
On the slitting saw feed/speed question though, 100 fpm is a good number for mild steel, which would be about 67 rpm for the 6" saw. With 232 teeth, taking a .0005" feed per tooth, would be .116" travel per revolution of the cutter. Times 67 rpm would be about 7 3/4 inches per minute feedrate. In reality, given a little eccentricity of the arbor, you won't be cutting evenly on all teeth so I'd take no more than about 1/4 of that or roughly 2 ipm.
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