You will find a lot of examples in the extensive 3D printed astronomy thread. Yes you can 3D print screw threads if they are big enough. In addition if you search of "3D printed" in all the topics you will be overwhelmed with examples. You might even find some of my thread of things...
TinkerCADis a free online 3D modeling program which runs in a web browser. It uses a simplified constructive solid geometry method for constructing models, with designs consisting of primitive shapes that are classified as either solids or holes. Combining solids and holes yields new shapes, which...
While 3D printers are still popularly used to create decorative items, there is an increasing library of functional items that you can make, in addition to any custom parts that you might design yourself. I was tempted to answer the title question about what you can do with a 3D printer by...
For any future passers-by: I spent an inordinate amount of hours fiddling with the design in TinkerCAD and printing test-fit sample slices (small little 20-minute prints to verify dimensions of certain areas). End result: one full print worked flawlessly. Active Cooling Fan Shroud for ...