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Safe to use 1/4" 4-40 metal standoffs?

Posted: Sun Jan 19, 2014 12:32 am
by Elliott B
I'm building a case for my model B (rev 2), and I want to use 4-40 metal standoffs at the two mounting holes. These are usually 1/4" diameter hex on the outside, and I'm wondering if that's too big. Meaning, will the metal make contact with any wiring on the board?
Here's an example: http://www.jameco.com/Jameco/Products/ProdDS/77586.pdf

Re: Safe to use 1/4" 4-40 metal standoffs?

Posted: Sun Jan 19, 2014 12:45 am
by Phill Rymer
Elliott B wrote:I'm building a case for my model B (rev 2), and I want to use 4-40 metal standoffs at the two mounting holes. These are usually 1/4" diameter hex on the outside, and I'm wondering if that's too big. Meaning, will the metal make contact with any wiring on the board?
Here's an example: http://www.jameco.com/Jameco/Products/ProdDS/77586.pdf
maplins in uk sell plastic ones

Re: Safe to use 1/4" 4-40 metal standoffs?

Posted: Sun Jan 19, 2014 12:48 am
by Elliott B
I'm aware of nylon standoffs, but I want to use metal. These same spacers will be inserted in a wood case, connected to countersunk screws on the outside, so nylon won't be strong enough.

Re: Safe to use 1/4" 4-40 metal standoffs?

Posted: Sun Jan 19, 2014 1:01 am
by Paul Moir
I think they'll be too big. There isn't much clearance (sorry, all my rev b boards are in things right now so I can't measure). I used 5mm standoffs and they're fine.
But why so big? I don't think 4-40s will fit the 2.5mm mounting holes anyway.

Re: Safe to use 1/4" 4-40 metal standoffs?

Posted: Sun Jan 19, 2014 1:09 am
by Elliott B
I've tried a 4-40 screw and it fits perfectly. I suppose M2.5 would fit too, but those seem to be harder to find and more expensive.
For example, DigiKey has 25 options for M2.5 standoffs, and 135 options for 4-40. Easier to find unusual/specific sizes.

Looking at the board, it looks like the corner hole is fine with a 1/4" nut. The other hole (near the micro-USB) has clearance on the bottom, but on the top side, it would touch some nearby traces. I don't know whether those traces are insulated, or if they would contact the nut.

I'm thinking about using metal standoffs below, and nylon nuts on top.

Re: Safe to use 1/4" 4-40 metal standoffs?

Posted: Sun Jan 19, 2014 1:49 am
by Paul Moir
Ok, good, I was relying on memory there. I couldn't remember if I had tried 6-32 or 4-40. Or maybe it was 3mm.

But a normal 4-40 nut is 3/16" wrench size so about .216 at the corners, so that might be fine anyway, and if not a fiber washer would be a good idea. Speaking of which, can you get a 3/16" standoff? I have a few so someone must make them?

Re: Safe to use 1/4" 4-40 metal standoffs?

Posted: Sun Jan 19, 2014 10:00 am
by plugwash
Elliott B wrote:I don't know whether those traces are insulated, or if they would contact the nut.
While traces are usually covered with insulating soldermask I don't like to rely on it as you can sometimes chip or scratch your way through it. So if I thought there was any chance of contact between a mounting screw and tracks I would use insulating washers.