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I'm building something made of aluminium that has mains electronics in it and DC
Also Steppers
It is a CNC machine technically just a very different one
Can I just connect the frame to gnd from the mains? to ground the frame and cladding
And can I connect a metal box to gnd and put my vfd in it for a CNC 3 axis?
Thanks
Yes, of course. You SHOULD connect the aluminium frame to ground. I suppose you mean PE as protective earth, not GND as in DC negative. The same is with the metal box, even that should be connect to the earth of your mains. But you shouldn't daisy chain the earth connection using the aluminium frame as earth bar. Use a common point or a specific earth bar dedicated and designed for the task.
I agree with A_Camera above. I'm not an electrician but if a live wire came loose and touched the outer case it would become live at mains voltage. If the case is connected to earth (PE) then current would flow down the earth line instead of returning via the neutral line and there would be an imbalance between live and neutral which is detected by the breaker, and it should trip. I believe this is set to trip at around 30mA (?), someone might correct me here.
In terms of star earth points then I've always used a fairly large piece of metal as the star point, and then each device is individually connected to it. I'm not sure why I use a large block but I read somewhere years ago that the backplane of a control cabinet should be a certain thickness if used as the PE star point. Perhaps it ensures low resistance so the 30mA is reached? It might also help make sure each device is at the same earth potential, avoiding current flowing along the earth wire but someone might correct me on that one.
The other reason (I think) for connecting the case of an enclosure to earth is for RF shielding, but this is more likely to comply with industrial installations, although still worth having this benefit at home.
I always check continuity using a multimeter between the earth pin of the plug (when not plugged in) and a random series of points around metal cases etc. There is probably a value of resistance it should be below but I've always made sure the value is very low and close to zero.
Those are my thoughts on it anyway. Maybe post some pictures here as first look, but if you are not sure ask a qualified person to check it over.
The only thing you should consider when using a multimeter is that the aluminium frame is not a good conductor because of the surface oxide which builds up immediately after cutting through the material. The oxide layer has very high resistance, which can fool the meter, but if you scratch the surface, or wet it a little, it will show low resistance. This is normal, so if the frame is connected to the earth and you want to measure just to be sure, then you must be aware of this.