I have a metal case that I plan to use to house some stuff; an Arduino, a Raspberry Pi, a few power cables with adapters and such.
How should I fix the Uno to the back of the plate of the case? I know it has 4 holes for mounting but should I put something between the Arduino and the metal case back?
I have used nylon screws and nuts. Using metall screws and isolating washers could also work.
There are various spacers or [u]standoffs[/u] that are commonly used. Some kind of plastic hardware is "safer" than metal because you won't accidently short something out.
Commercial products usually have threaded metal standoffs swaged (pressed) into the metal box but that takes an arbor press with the correct tooling.
Several years ago when I was going-through a "lazy phase" I was super-gluing 4 "cubes" of cut [u]foam weather stripping[/u] between the case and a perfboard. 
(I glued it where there was no circuitry.) It's easy, fast, and cheap, and it "worked perfectly and permanently" (I never had a board fall off) but it's kind-of "Mickey Mouse"...
i was trying out some nylon standoffs I have and was able to get 3 of them in, but the 4th one, near the reset button I couldnt get the nut in because it overlaps the last pin and the ICSP pins. But I get the gist of it.
Thanks
I found online that the size of the Arduino mounting holes are A=0.125" which translates to 3.175mm.
https://images.app.goo.gl/trAPx1C18e9MWW8x5
Then I found that the mounting holes on a raspberry pi (because i need to mount one of those as well) are 2.5mm:
https://images.app.goo.gl/8R1w3f8mnw8k4Gs79
So I could get the same size screws for both so long as theyre 3.175mm, right? I mean I know the math is there, but Im wondering if I could expect any noticeable wobble? I dont have any 2.5mm screws, Ill go get some tomorrow, but if I could get the same size for both devices Id go ahead and do that.
You can buy M2 standoffs on eBay.
They work with all the Arduino mounting holes.
Don’t forget to get M2 screws and nuts and washers (insulated).
try 2-56 couplers if 2 mm is not common where you live. I get mine a Ace hardware in a rural community.
I do not do 3D printing and don't know anyone who does. So I shop on Shapeways. I have a similar requirement for a project I am working on, and I found this mount for the Uno that will work for my project:
I bought a pile of these to have on hand. Cheap, and generally sticky enough if you don't keep pulling it off and sticking it somewhere else. Available in different heights. Fits 3mm holes.
https://www.taydaelectronics.com/pcb/pcb-support-hardware-materials.html
