RPCs need to be sized larger than the largest motor you expect to start with it typically. In your case, I'd recommend a 5hp rotary minimum. I will also give the advice I give everyone asking about this, buy something twice as big as you think you'll need, it's a one-time cost, and it'll save you a lot of heartache when you find something larger you want to power later.
I use a 10hp CNC ready RPC from American Rotary, and can't recommend them enough, however I wish I had gotten a 20 or 30. The idle power consumption is minimally higher, but I've turned down machines that would have been incredibly useful and insane bargains because I couldn't power them, and didn't have the time or money to replace my RPC.
On the other hand, if you think you're never gonna use anything else, I'd recommend getting cheap VFDs for each machine. I wouldn't personally ever put a non-NEMA 4x rated vfd on a grinder, but I also wouldn't ever recommend paying for a KB Electronics vfd for machine tools like you're looking to power. It's just not worth the added cost IMO. However, you may have to have 220 input for them, you're going to need that for some things eventually anyway, and using the cheaper VFDs can help mitigate the cost of putting in a couple of 220 circuits for your welder and HT oven as example, if you go ahead and do it all at once.
You definitely dont need a sealed vfd for a mill or even the SG as long as you keep it away from the path of grinding dust, but you should get a sealed vfd for your 2x72.
FWIW, the KBAC 27D will *run* a 2hp motor, but for the record, it will not give it full power with 110V input, derating it to basically 1.5hp. The difference between a 2hp motor powered by 110v vs 220v is obvious if you've ever tried them side-by-side. It's also somewhat contentious whether it affects the longevity of the motors.
I'm also not a big fan of switching a VFD to run multiple things, that's fine for frugality, and assuming everything is within arms reach, but it's tedious and time consuming if not. Walk over the to SG try to turn it on, realize the VFD is switched to another tool, walk back, switch it. Yeah, it may seem marginal, and if you're the super patient type, cool, but for me, time is money, and those sort of aggravations keep me from wanting to use a machine. You also add a lot of infrastructure if anything is separated. Either conduit to run all the stuff and wire, or SO cable to be reasonably safe having cords laying all over the place, and expensive turn lock plugs if you want things dis-connectable.
Personally, everything I have that runs off a VFD, has it's own. (The ebay VFDs, which work absolutely fine, are like $100, you can buy 3 for the price of one KB, and still have money left over), and I still have and highly recommend a rotary phase converter, for anything bigger than 2-3hp.
Another question is; if you go with a single VFD, are you gonna have to pay an electrician to do all that wiring also? If you're not comfortable running your own 220v circuit, you're probably not comfortable doing all that in such a way that's actually safe, and I can almost guarantee, it won't be "legal" in the eyes of a fire marshall. Most electricians don't have a clue when it comes to wiring up things like VFDs and RPCs correctly. I've fixed no less than 5 grinders that were "wired by a professional electrician" that could be stalled with 1 hand and casual pressure, because they wired the motors based on 440V input, assuming that the "high voltage" diagram was what it needed.
On the other hand, almost any of them could run you a couple of 220V circuits to each side of your shop, drop 2-3 receptacles of your choice, and it really shouldn't be expensive, if you find someone who's not looking to rip you off, or your infrastructure (breaker box) isn't massively far away. I'd run a larger circuit for a welder in the future. I've got a single 20 amp 220v circuit for my grinders, (4), which'll power any two of them at the same time no problemo.
3 KBAC VFDs will run you what? Almost $1200 after they're shipped?
