Hi,
I am doing a project that involves several 12v valves and pump, relays, 12v motor and a 230v pump.
The plan is to use one or two old PSUs for the power supply, and put all the electric stuff inside a metal connection box.
Would it be sufficient to use the cabinet as a common ground for the 12v components?
Thanks in advance!
Sufficient, Maybe. This is a essentially what cars do. However since you are throwing AC into the mix I would avoid it.
On another note, if you are that hard up for black wire let me know and Ill send you some 
Okay, so its is not really recommended then.
Would it be best to use some kind of power distribution block then? I have at least 8 different 12v components that need to be connected to the same ground.
Just trying make it nice and clean inside the cabinet. Thank you, but I think I will manage to obtain some ground wire 
Well, I´m having a little trouble encountering this.
All of the components in this project are going to be mounted on a steel frame, including the 230vac pump. The 230vac pump housing is connected to earth, which means everything else is also going to be connected to earth. Would there be any problems with a common earth and 12v ground? The project does not involve any sensors sensitive to noise. I´ve tried to do a little research about this and I understand there are a lot of opinions.
The 230vac pump will be switched on/off through an opto-isolated relay board, while all the other DC stuff will be switched by another one.
I´m not really sure how I can avoid connection earth/ground, while everything is connected to the metal frame. (Other than using some kind of plastic enclosure for the DC components).
All help very much appreciated 
I´m not really sure how I can avoid connection earth/ground, while everything is connected to the metal frame. (Other than using some kind of plastic enclosure for the DC components).
It's certainly OK to earth-ground the box. In fact, it might be a requirement if there is line voltage inside. The idea is, if a "hot" AC wire comes loose inside and contacts the metal case, the voltage gets grounded-out, the circuit breaker blows, and nobody gets killed.
It's also OK to connect the low-voltage ground to the same case. In a computer, the low-voltage grounds are connected to the case, and the case is grounded to the power cord.
The question is... Is it good practice to use the box as a ground conductor for the internal circuitry? It's sort-of non-standard, but in some applications it might be OK.
Thank you!
I think I´ll try to join all ground cables from the PSU into something like this, and from there stretch wires to the different appliances.