Hello everybody!
I have a general question, please forgive me if the question is so trivial...
My question is: I have noted that all the Arduino modules have a couple of holes having apporximately a diameter of 2.5 mm (see https://it.aliexpress.com/item/4000743799331.html just to have an example...)-
My question is: what's the meaning of these holes? How can be used while designing a custom shield?
Thank you very much and happy new year to everybody!
On the DUE, the holes are 3.2mm diameter which is the clearance size for a M3 screw.
The usual reason for holes in circuit boards is for screws or other fixings.
Thank you very much!
I am approaching now to circuit design... I didn't think about fixing and screwing!
Thank you again!
You are forgiven, as it's hard to imagine how you could fit a normal screw into those without hitting the components adjacent to them... most of the other processor boards I get have clearance for an easy fit of standoffs with screws.
True ...... for the mega and/or uno etc -------- there are four screw holes on mega/uno. And at least 1 screw hole requires us to use 'toe-nail clipper' (or some other strong enough cutter/trimmer) to clip off portions of the head of the M3 nylon screws ----- in order to fit the nylon screw in between the gap between the plastic headers. And then, on the under-side of the mega and/or uno, the hex nylon stand-off can then be connected (by screwing it on) to the thread of the nylon screw.
srnet:
On the DUE, the holes are 3.2mm diameter which is the clearance size for a M3 screw.
Seems to be pretty much spot on 3.0mm on my Due, which is the correct size for an M3 screw
in fact, as M3 screw major diameter is supposed to be in the range 2.874 -- 2.980mm
M2.5 and M3 are the commonest PCB metric mounting screw sizes, I've seen M2.5 on some small
sensor boards and M3 seems standard for Arduino boards.
Nylon push-on (no screw) spacers are commonly used in the industry for small circuit boards.
Leo..
aarg:
...it's hard to imagine how you could fit a normal screw into those without hitting the components adjacent to them...
On all my Raspberry Pi i have drilled the holes to 3mm without any problem.
2,5mm screws ar not that common.
MarkT:
Seems to be pretty much spot on 3.0mm on my Due, which is the correct size for an M3 screw
in fact, as M3 screw major diameter is supposed to be in the range 2.874 -- 2.980mm
On the supplied DUE Eagle PCB layout, the holes are specified as 3.2mm
With the hex stand-offs and little nylon nuts ------ I can very easily remove the stand-offs (if needed). And can also easily put the stand-offs back on again if needed. Once again, noting - the particular nylon screw head in this pic was trimmed on either side using a toe-nail clipper. Any other screw head that has a plastic header getting in the way can be trimmed too. For the MEGA, I usually need to trim the heads of two screws. The other two screws don't need to be trimmed.
The uno requires just 1 of the four screws to have a portion of the screw-head clipped with a toe-nail clipper. I use a big toe-nail clipper for safety. Don't want to have injuries with say scissors ----- too easy to slip and nip a finger etc.
Time to try push-on spacers (keep the clippers for your toe-nails).
They are made for 1.6mm circuit boards, are not much larger than the hole when fitted, and the board can easily be removed by squeezing the locking tab with long-nose pliers while pulling up the board.
Leo..
In most places, it's much easier to find generic threaded stand offs and screws/nuts, than the push on ones. That is a good reason to avoid incorporating the necessity of using them into experimenter boards.
Boards often have "index holes" to hold the board in a known-fixed position for automated assembly.
Wawa:
Time to try push-on spacers (keep the clippers for your toe-nails).
They are made for 1.6mm circuit boards, are not much larger than the hole when fitted, and the board can easily be removed by squeezing the locking tab with long-nose pliers while pulling up the board.
Leo..
I knew about the push-in spacers long ago too (eg. from computer motherboards etc) ------ but just prefer the screw-on ----- as I want to have the flat-bottom of the hex stand-off, rather than the tapered ice-cream cone 'peg-leg' stand-off. The hex threaded stand-offs come in various lengths ----- so I can choose what length I want to use. I bought a couple of big toe-nail clippers especially for clipping the nylon screw-heads.
In most places, it's much easier to find generic threaded stand offs and screws/nuts, than the push on ones. That is a good reason to avoid incorporating the necessity of using them into experimenter boards.
True aarg! True.
Actually, the toenail clippers are generally better for clipping male fingernails. Toenails should be cut straight to avoid causing ingrown toenails - (large) side cutters are the preferred instrument. 
Paul__B:
Toenails should be cut straight to avoid causing ingrown toenails - (large) side cutters are the preferred instrument.
I though I was the only one doing that
I found that toe-nail clippers do a much better job than side-cutters for my purpose of trimming the nylon heads. I know this from experience.
It doesn't take 'much' extra experience with getting the big toe-nail clippers to come in from the side of the nylon head - and then 'clip'. Nice and clean - allowing me to hold the screw and do a precise hassle-free cut. And the toe-nail clipper conveniently stops the cut piece from flying off somewhere into the room and onto the floor somewhere.
Some tools used for some cutting purposes will be very good for other sorts of cutting purposes too. In this case, the big toe-nail clippers has some significant advantages over side-cutters for my particular purpose of trimming the nylon screw heads.
Southpark:
I found that toe-nail clippers do a much better job than side-cutters for my purpose of trimming the nylon heads. I know this from experience.
I do not doubt that at all!
The corresponding hand tool is not side cutters, but one form of end cutters, though even these do not often have the cutting face completely flat and the bevel on the inside only.
Actually, these small nippers are generally designed for a flush cut:
