The measure: 12.5 mm, is the base unit of the STEMFIE construction set; this is also referred to as BlockUnit (BU). Therefore, a typical construction beam will measure 1 x 1 BlockUnits (12.5 mm x 12.5 mm), and maybe 5 BlockUnits in length.
Thinner elements; such as washers, spacers and braces, typically measure one-fourth of a BlockUnit (12.5 mm/4 = 3.125 mm). Therefore, logically, if four of these 3D-printed elements are stacked on top of each other, they should measure one full BlockUnit (12.5 mm).
Unfortunately, a typical 3D-printed part's top layer will yield a slight overflow of plastic, thus adding slightly to the overall dimension of the piece. This dimensional variation will increase and can become a problem if too many parts are stacked together.
One solution to this problem would be to adjust the 3D-slicer software values to yield as flat top-layers as possible (ironing). Such a solution is something to be avoided since the aim of the STEMFIE project is to provide 3D files which are as easy-to-print as possible - preferably using only standard 3D printing settings.
During the beta testing period, Gina Häußge - a 3D printing enthusiast and creator of the popular OctoPrint 3D printer web interface - suggested a smart solution for the stacking problem om STEMFIE braces. The idea was to modify the 3D model in such a way so that only the perimeters around the borders would print and no infill would be created. This turned out to work well, and the solution was simple: the top surface is 0.2 mm too low, and on top of that is a small 0.2 mm tall and 0.8 mm wide strip representing the perimeters.
I'm thankful to Gina for coming up with this simple solution, which makes it easier for everyone to 3D print STEMFIE parts at home.
The image above shows a sample file of the experiment showing the difference in stacked size. Please download the STL from this link and give it a try on your 3D printer. Please note that the four braces in this sample part are intentionally designed to have half of the top surface including the solution and the other part is plain and flat, which demonstrates the stacking issue and dimensional increase.