@Randy mentioned machining some steels in another post:
I have also machined some unknown steel with decent results. There didn’t seem to be much issue with rigidity of the machine but I think the general issues still apply.
Namely, the high speed spindle isn’t great for steels in general. The surface speeds with any reasonable sized tool is pretty high, even at the slowest spindle speed. Adding to the issues would be the torque curve of the spindle…you want to go as slow as possible but doing so reduces your max torque available.
For me this hasn’t been too terrible because most of what I do leverages tooling 1/8” or smaller. Where the surface speeds are somewhat more reasonable and the tool itself wouldn’t have the strength for higher torque operations anyway, so the lack at the lower end isn’t too detrimental.
All that said, tooling matters quite a bit. Getting some steel specific mills made quite a difference in the cut. Well worth keeping a few material specific tools on hand for the rare occasion I need them.
Disclaimer: I only dabble in machining. I’m curious to see what real machinists have to say about it 