Yes, if the box is (earth)grounded it will prevent the SS from picking up the RFI from the drivers and such in the same box. If the SS is picking up noise from the inputs you'll need to optically isolate those as well.

A couple of observations based on my SS. The USB cable I originally tried had no continuity between the metal shells of the connectors - which suggests there was no shielding.  Also the onboard USB 1 ports in the PC didn't seem to work reliably at all.

The only way I could get stable results was to use a shielded 5m USB extension cable which has a hub integrated into the end. I now have that coiled up inside a metal enclosure, with a "normal" usb cable coiled up in the same box connected to the SS.

Aside from my x or y home inputs getting stuck on randomly - it seems to be performing as expected i.e I actually cut some parts with it and the machine is definitely running smoother.

Having said all that - the main source of interference I have is from the VFD/spindle, which, without a complete disassemble and rewire,  is going to be tricky to resolve.

we tried a few more things today, but i have to say its not a reliable product suitable for comercial use.
ive had a reply from greg today and in his own words"I think that a ground loop exists from the PC through the USB
cable," no mate you design and sell this product you should know not think, a cnc machine with a spindle turning with a cutting tool in it can injure or kill a person

ive put mine in the bin its a liability i employ people i cant have a situation of "the only way i could get stability apart from!" my insurance company would have me in prison if it injured someone especially with the knowledge i have of around 50 percent of users having problems, and all seem to be with commercial machines which are better built to start with but are expected to return better results.

ive put mine in the bin its a liability

ill buy it from you if the price is right
Hood

rally.sport,
Putting the SS in a solid metal box would be shield and that shield would be considered 100%. If it were shielded
 in say a fine metal mesh screen  ( there are rough equations or quides on how open the mesh should be depending on what you want to do ) then that also may be appropriate ( but not as good ). You could also just put Al tape, overlap the seams, ( like the tape used to seal duct seams ) around the whole thing and that would shield it.
Now if the noise is being carried to it via wiring inside the the shield then your shield won't be worth much in that sense.

Define the source of noise  / rf, contain or minimise it, shield as best you can, move the SS to it's owne or appropriate location ( like in the computer case ), Use the same ground for the 5v as the gound of the supplied 5v.
Sometimes there is logic to it all ,sometimes i just call it black magic ( because it worked and nobody can realy say why!

Just some thought's, no one stop solution for you,
 
RICH