Calculating mechanical forces

I'm at the buy/build point again for my CNC mill.

I need something between a Sherline and a mill/drill. Prazi looks good,
but is expensive.
Proxxon is more reasonable, but I haven't heard anything about them,
etc.

I'm looking for formulae for calculating the forces while machining
various materials.
For example, if I want to use roundways and linear bearings for my Y
axis,
and I want to cut mild steel with a 2" fly cutter at some SFPM, and
some feed per revolution,
I want to find the force on the Y axis. That way I can find the size
parts I need for
a certain capacity and a certain life span.

Thanks in advance!

Regards,
JJ

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Help everyone. If you can't do that, then at least be nice.