Let's talk about screws. When I made the acrylic back for the #PewPew Standalone, I made the holes the right size to use M1.8 screws I had left over from hobby servos. They look somewhat unusual, as they have something that looks like a built-in washer on their heads:
But then it turned out that they are pretty much impossible to buy without the servos, and when I run out of them, I had to find something equivalent. So I bought some random M1.8 screws that looked small enough. Turns out that there is more to a screw than the diameter — there is also the depth of the thread, and the ones I got fit into the holes slightly too loosely.
So this time I made the holes 1.4mm in diameter, to ensure a tight fit, and I didn't think much about one more parameter — the length of the screw. After the parts arrived and I assembled them, it turned out that the pointy part of the screw is sticking out a little bit. The PCB is 1mm, the inner layer of acrylic is 2.5mm, and the outer is 2mm, which gives 5.5mm together, and the screws are 6mm long. So I did what every hacker would do in that situation — cut them. But that is not a solution if I want to have them produced in bulk, so I need to find the right screws.
After a bit of looking around, I found screws that are M1.8 and have a flat end — not pointy. Now, the new version will still have 2.5mm inner layer, but the top layer will be only 1mm thick. With 1mm thick PCB that would give me 4.5mm for the length. The screws come in 4mm and 5mm lengths. But wait a minute, they are countersunk, so they will probably sink at least 0.5mm into the larger hole in the PCB. But is the length measure including the head, or not? Hmmm... I could also use thicker PCB, and add that 0.5mm there, if needed. But I need to know the exact dimensions of the screws to do that.
For now, I ended up ordering both the 4mm and the 5mm ones, and I will see how they work.