That changes things a bit. Since you have a RPC it's safe to assume that your shop is supplied with single phase power. I would use the RPC to power the lathe and other equipment as needed and run another line from the single phase panel to the mill, then run the mill with the VFD.
The reason you are not getting a definitive answer to your original question is that nobody has run a VFD off of a RPC, so no experience with that arrangement. Will it work? Probably, but the RCP outputs a odd wave form so nobody is willing to say 'Yeah, that'll work''. I'm not going to try to give a detailed technical explanation here, but the wave form of single phase is 2 legs 180 degrees, the wave form of 3 phase from the power company is 3 legs that are 120 degrees, but the wave form from a RCP is somewhere in between and is load dependent. So not sure how the VFD will react.
On a 1/2 hp motor, I might be tempted to get a 120V input VFD (240V, 3 phase out), and just plug the mill into the nearest outlet. Here is an example of that type VFD
http://www.automationdirect.com/adc...ts_(115_-z-_230_-z-_460_-z-_575_VAC)/GS2-10P5
Or just run another 240V line from the single phase panel and run the mill with a single phase input VFD.
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