Simply put, computer-aided design (CAD) is software that permits you to use a computer to create an object. CAD software typically includes features to realize an object in various 3D views, manipulate the object surface and interior details, as well as change the view of the object (scaling, rotating, etc.). CAD is also referred to as computer-aided drafting, but in this case it refers to the drawing aspect alone. I spent several years learning the art of mechanical drawing.7 Computer-aided drafting revolutionized that aspect of the engineering discipline. In fact, learning computer-aided drafting software is what got me interested in the engineering disciplines.
Advanced CAD software has features that allow you to create a number of objects and fit them together to form a complex mechanical solution (called a model). Advanced CAD software includes additional features that test fit, endurance, and even stress under load. An ultimate example would be the software that automotive manufacturers use to construct engines. I have seen examples of such software that can animate all of the moving parts and even suggest ways to improve the individual components. There are many CAD applications available with a wide range of features. To be used with 3D printing, they must, at a minimum, permit you to design shapes in three dimensions, define interior features like holes for mounting the object, and basic tools to add a surface or facing to the object. The various CAD applications save the models in a specific, sometimes proprietary, format. This limits the possibility of using several different CAD applications to manipulate an object. Fortunately, most permit you to import models and objects from various file formats. More importantly, the software must permit you to create an object that is manifold (has an inside and outside surface with no gaps). This is important because the slicer needs to be able to create paths for the filament to follow, and gaps or holes means there is a break in the path. Attempting to force the slicing and printing of a nonmanifold object will result in an undesirable end result. I tried it once with a whistle and ended up with a solid block in the shape of a whistle—it filled the interior with solid plastic. Tragic. If your slicer program displays an error that your object is not manifold, you can use an online tool from NetFabb to fix the holes. Visit http://cloud.netfabb.com, select your object, enter your e-mail address, agree to the terms and conditions, and click “Upload to cloud”. After a few moments, you will get an e-mail with a link to the fixed object. I have fixed a number of objects like this. While you are there, check out the cool online 3D printing tools they offer. Remember, CAD applications for 3D printing must be capable of creating a standard tessellation language (.stl) file so that the CAM software can read the file, slice it, and create a printer instructions (.gcode) file for forming the object in three dimensions.
