Test Nut Hole size device

I was having some problems with M3 nuts not fitting into printed parts. The outside dimensions were fine, but the nut holes were too small due to thermal expansion or bad slicing path or whatever. So I printed this; it makes 5 six-sided holes. The first one is 3mm radius, the second adds 5%, then 10%, then 15%, and lastly 20%. You print it and try placing the nut to see what settings you should use for the desired fit; then you know how much to oversize nut holes for mechanical prints.

For my printer with Slic3r, adding 20% to hole diameter (10% to radius), yields a nut sized hole that is very difficult to push the nut in by hand. Tapping with a hammer or using plyers yields a nice snug fit. Adding another 10% to diameter (5% to radius) yields a nut sized hole that I can push the nut into by hand, but still snug enough for typical mechanical prints.

You could adjust the scad file to make smaller increments if you want to get more exact or print more nut holes.