How to generate an STL file

What is an .stl file? Answer from Wikipedia. Answer from 3D Systems (can take a while to load).

If you'd simply like to try a 3D printer, then select something to 3D print from an internet site such as Thingiverse (maybe good to start with something simple) and you can download an STL file from them - of course there is no guarantee that the STL file is any good unless they report they have test printed it themselves.

More normally, draw the item you want in your preferred drawing package:

Solidworks is available on the Department's DPO and Dyson Centre machines
- Contact Dr Richard Roebuck, RLR20, if you are an Cambridge Engineering undergraduate wishing to install Solidworks on your own machine.
Autodesk Inventor is also available on the Dyson Centre machines.
- Also a free student version of Autodesk Inventor appears to be available from here for student use.
A free version of Sketchup Make seems to be available for personal use.

...and then export the STL file using the following notes to help you:

We'd suggest checking out your .stl file with a viewer for any issues.

For example, the online viewer View 3D STL.

Typical issues include: shapes which do not have completely closed external surfaces - you'll see these as voids or parts of the model vanishing on the above online STL viewer.