The file format that printapart.com currently accepts is an STL file (stereolithography file). Most CAD packages have the ability to output STL files.

Not at this time.

You need a 3-D CAD program. Most professional CAD programs–and many free ones as well–have the capability to output an STL file.

• BRL-CAD: a sophisticated 3-D modeling program which has been developed for many years for the U.S. Army. It has recently been released as an open-source program. Ready to run versions for Windows, Macintosh, and Linux systems can be downloaded from the project website.
• Alibre Xpress: a free 3-D CAD Modeling program that has an easy to understand and use interface. You can register and download a copy from their website.
• There are other free 3-D CAD programs out there, but some do not have the ability to export STL files, and some of them are quite difficult to use.