The Arduino Mega has four, count them, four mounting holes in a more-or-less rectangular pattern around the edge of the board. Three of the four have enough room for standard pan-head 4-40 screws. The fourth hole is squeezed between two pin-header sockets, to the extent that no screw in my collection will fit.
Having a lathe, however, makes this situation no big deal… and this time, I put the Sherline drill chuck in the 3-jaw because I wanted to, not because the 3-jaw was stuck. In truth, the lathe chuck simply won’t grab a 4-40 screw at all.
I had that pointy right-cutting tool in the holder, so that’s how I cleared off the burr on the backside of the head.
The chuck did grab the hex standoff just fine, though, and the final result looks like this…
For future reference, the turned-down sections are 0.180 inch in diameter; call it 4.5 mm. The snout on the standoff must clear the pins,so it’s 0.100 inch or 2.5 mm long.
The Smell of Molten Projects in the Morning 
This is one of my pet peeves; mounting holes that don’t leave enough clearance. Or sometimes traces under where a standard screw would short them out. While I love SparkFun, most of their breakout boards have _no_ mounting holes! I can understand why; on some of their small breakout boards a 4-40 mounting hole (with proper clearance) can easily double the area of the board…
I’ve made my own mounting hole libraries with clearance around them for standard nuts, standoffs, washers, etc.
– Steve
clearance around them for standard nuts, standoffs, washers, etc.
Nothing so organized here, as most of the boards I lay out are one-offs for Circuit Cellar projects: mounting holes wouldn’t get used anyway.
But, yeah, why aren’t such layouts in the standard libraries? Doesn’t anyone screw their boards down tight?
Amusingly, Sparkfun has mounting holes with appropriate keepouts in their libraries.
Must be they actually build stuff… [grin]
I’m in a similar situation to sparkfun: a board I’m doing right now would fit entirely inside the outline of a 6-32 screw. Intensely frustrating. But with that said, boards without mounting holes are also really annoying. I built a sensor board that attaches to a bit of machinery the other day and 80% of the surface area of the board is the 4-40 screws. I always make my plated through-hole footprints with a copper area the size of a screw head plus 10 mils just so someone (like me) won’t run a trace through there. But, oh golly does losing all that board area hurt.
80% of the surface area of the board is the 4-40 screws
Argh! That’s about how I felt with the mic amp / GPS dingus: those two screws soak up more area than anything except the audio transformer!
Thank you for the specs used to correct this design oversight. On my project, I’m using Radio Shack’s M3 standoffs (Part# 276-195). Since I don’t have a lathe, I did this with a 1/2″ drill in my left hand and a dremel with diamond grinding bit in my right. The screw wouldn’t stay put in the drill chuck, so I did it entirely freehand. Ended up being 4.5mm diameter and 2.7mm long, not too shabby for a Macgyver job :D
Bash to fit, file to hide, paint to cover… success!
I suppose we could use plastic standoffs and just melt ’em in place, but what to do for the screws? [sigh]
Glad to help!