Hello everyone I want to make a 3D printing case . I colsed it up with screws . What tap and holes and screws . how big should that have
Hey Hans! I am from nowhere but living for a while in Nijmegen, Netherlands 
I suggest reading this article I published some times ago Essential Raspberry Pi Peripherals #5 The Pi-Scope lab tool With this kind of design (stl included) take in account that the screws for the Raspberry PI top and bottom case part I have used four Parker screws 3x20 mm
As a matter of fact you should always consider that using Parker screws you should keep the pass-through hole 0.5 mm larger than the max diameter of the screw (PLA has very few temperature deformation and is almost stable). The internal hole should be about 1 mm less than the max diameter of the screw, considering that it is not the max diameter as the screw is conical.
In the case you want using a sort of Allen screw and nut consider 0.7 mm more the screw diameter. In the case shown in the article the PLA plastic assembly is fixed with M4 Allen screws as the support should be robust, but probably in smaller cases also M3 maybe the worth.
Enrico
