I have to personally say this site has been a tremendous help with various projects we have needed comparative models or other models to use with design. Wonderful community, wonderful model selection, this will probably be your one stop site for most of your CAD needs.
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No need to reinvent the wheel! Someone else has solved that CAD problem or designed that bolt. The GrabCAD Library has the models and knowledge you need in the world’s largest collection of free CAD files.