[jdiwnab]

Cuts are usually done on a part by part basis. If you wanted to remove some shape from your part, you would create a sketch of what you want to remove and when extruding/revolving/whatever you use, choose subtract from the menu (might also be called remove, cut, but is one of the three buttons that control how it extrudes). If you want to cut off a portion, I would recomend the slice tool. You create a work plane, or some other plane, that you will cut along. The slice tool looks like a part that suddenly ends at a yellow work plane. You select the plane and the part. you might have to experiment on which way you want to keep, but is mostly common sence. You might be able to pull of a similer action on a complete assembly, but I doubt it. This stims from the fact that it is referencing the parts and would find it hard to cut though one version of the part and not another.