All parts were drawn without any tolerance for the material thickness. For example, if one part is 3 mm thick, then the corresponding hole in the second part has a dimension of 3 mm as well, i.e. there is no tolerance. If the parts were cut with such zero tolerance, it would be difficult to assemble the parts together. Therefore, it's recommended to create a small tolerance during cutting. One way of doing so is to generate the tool paths using a tool diameter of for example 1.5 mm while using a diamater of 1.6 mm when physically cutting the parts. This creates the desired tolerance for an easy parts assembly, in this example either 0,05 or 0,1 mm depending on mutual position of both parts. It is recommended to do some tests on the used material before actual cutting.The fact, that all parts are actuallly smaller by this tolerance is negligible.