This is kind of why I started this thread. I'm not seeing any reference's to NEC codes saying you can't do this, BUT just about everyone shuns the use of multiple outlets on 240V circuits, especially for 50 amps.
As someone else mentioned, it's not NEC, it's IBC (I can't confirm this I'm not a licensed electrician), and probably also state or municipality codes. Just because it's not plastered on a sign at Home Depot doesn't mean it doesn't exist.
I'm not trying to cut corners or save money by doing this, rather build something that is efficient and can be used for multiple purposes, save space, and not have my garage looking like a power plant to plug in 1,000 different things at once. I'm a one man show and at most two thing will be running at once on 1 circuit.
What I don't like is having three 50 amp outlets just for one welder, three 20 amp outlets for one dust collector, and three 30 amp outlets for the saws, mills, etc. etc. etc. Wouldn't we all rather have three 50 amp outlets and plug in our equipment as needed?
Okay, one more thing to consider. Have you looked at 6GA wire? It's not like running 12 or even 10, you start getting into space constraints. Also, take a look at the receptacles. You will not find a single 220V that you can daisy-chain like your typical 110v around the house. So if you wanted to fudge it and daisy-chain you would have to make little pigtails, but like I started out saying, it's not exactly small or flexible and trying to get all that shoved into one box is going to be a major PITA. Alternatively you could go to a junction box and tie them all together, but if you are doing that much effort a sub-panel with appropriate sizes breakers would not add much cost at all...
Rather than put plugs all over the place, put one plug and make you an appropriate long SO cord so you can move your welder around or whatever.
I'm no electrician, which is one reason I ask the questions. However, what stems this whole questions is that my little iPhone or any other charging device for that matter, has I don't know, 20 gauge wire? Lamps, fans, vcr's all have wimpy 18 gauge wire. Yet it is all plugged into a 20 amp circuit...But when you talk about an 18 amp machine on a 50 amp circuit everyone one goes nuts.
iPhone cord is low voltage (5v), the charger plugs directly in the wall. But other devices like you mention all go into a 20A circuit if their wire is thinner than rated it would have internal fusing, either in the plug or the device, and in theory a house *should* have AFCI breakers and GFCI for various areas. The basic 15A/20A circuit going through a house is kind of a unique situation. Most devices on it like you state pull very little current, thus that's why they allow several plugs to be daisy-chained on one circuit. It's a matter of convenience and also that modern devices should have some sort of safety built in (a fuse). But when you are creating circuits larger than 20A, it's because you are having ONE device that is going to be pulling a LOT of juice, so safety starts to become more important. Also while a short on a 20A circuit is bad... Something shorting on a 50A circuit is REALLY bad... Think difference between a little "pop" and an arc-welder sizzling...
Also like someone else said, certain sized "motor" loads are required to be hard-wired and not a plug (5HP and larger). If you read the fine print, certain plugs & switches will tell you exactly how high of a HP motor they will support. Don't ask me why, I'm not a licensed electrician, I've just done a lot of electrical work and know what you should & shouldn't do for the most part. Likewise for those things that are hard-wired, there's code that states there must be a way to disconnect power from the machine within X feet of it (either by a disconnect box or the breaker panel)... Some people don't want to mess with them but to me having that disconnect box for any hardwired device is just good practice because you never know.
I understand what you *want* to do, and yeah many people including myself wish it was *okay* to do that. But the fact is, it's not correct nor appropriate. Usually equipment that requires more power than your 110v/15a receptacle really shouldn't move around, exception being a welder. That's why I mention previously about making a long SO cord for your welder because you never know where you will need it.
A big saw, vacuum, or whatever are kind of stationary. If you don't want to run complete circuits from your main panel, think about running one heavy wire to a little 4-slot surface-mount sub-panel. Then you could come out from there with a pair of plugs, each on their own breaker and appropriately sized. For instance... Start with a 50A breaker in your main panel, run 6GA wire to a small sub-panel. Then you can attach your 50A receptacle (with another 50A breaker), and a 30A receptacle with a 30A breaker. So you aren't having to make multiple runs back to your main panel for each plug, and that is all okay to do.