[Tenthumbs]

Very new to this. If I download a stl file, how will I know what tools are required and when to change them?

[CitizenOfDreams]

STL file is nothing more than a 3D-drawing of an object. You as a machinist will have to decide how to reproduce that object - depending on what kind of machine you have, what tools you have, what is the material, what is the size, what tolerance and finish quality you need, what is the acceptable machining time, what is your CAM program capable of, and so on.

For instance, if your part has 5mm features, you (generally) cannot use a 10mm tool to make it. But you cannot (generally) use a 1mm tool either because the machining time will be unacceptably long.