Mix It Up! with 123D Meshmixer
.STL AND OBJ FILES ARE mesh models inside an invisible shell. The Tinkercad and Design apps let you make superficial edits to the shell such as scale and move it. Meshmixer, on the other hand, puts you inside the shell to edit the mesh itself. Specifically, you can perform the following tasks:
Edit a mesh model You can change a mesh model’s density, alter it with brushes and other tools, combine multiple models, and even heal open mesh models into closed, watertight models.
Create simple 3D models ...