As a really bad 3D designer, I use the very handy TinkerCad to draw 3D parts that I want to print. When I had to print parts that had to fit into each other, I was faced with a technical reality: At the beginning, I had simply transformed the part which was to enter the other into a "hole" to create the hole. But in fact, you need a margin of tolerance, otherwise it won't fit. And this margin of tolerance, I suspect it depends on the 3D printer and probably on the filament and its fineness. In any case, I started to spread the holes a bit at random, and then finally I created this gauge to find out by how much precisely you have to spread the holes in a print for the parts to fit into each other. This model allows you to print a part of which the part to be nested is 1cm on the side (a square therefore). And in the gauge, the holes are linked with 0.1mm more each time, from 0.1mm to 1.0mm. On my Markebot Method for example (what you can see in the photos), the part fits quietly with a gap of 0.4mm. At 0.3mm it rubs a bit, and at 0.2mm you really have to force it. In addition, I added small plates whose orientation changes every 5 °, which must be printed without support, to see with what quality your printer can print angles (this will allow you to know the limits without support) . Hope you find this useful.