TinkerCAD is an online tool that gives users the opportunity to design custom 3D models, for free! These designs can then be exported for 3D printing. Unlike other CAD programs, TinkerCAD is very easy to learn and uses a simple three-step design process. Tinkercad uses a simplified constructive solid geometry method of constructing models. A design is made up of primitive shapes that are either "solid" or "hole". Combining solids and holes together, new shapes can be created, which in turn can be assigned the property of solid or hole.
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TinkerCAD.com provides great starter walk-throughs, lessons, and projects for beginners.