One of the keys to success in mechanical design comes from understanding that you can draw things that you can not build. Just because you can make it in Sketchup does not mean your fabricator can make it. You must design being mindful of the capabilities of your fabrication infrastructure. This comes down to the concept of design rules. Design rules are a set of conditions. If you design your devices following the conditions in your design rules, your design will fabricate properly. Your design rules will depend on your fabrication infrastructure. In our case, we are building on our own 3D printer, but none the less, we still need to develop some design rules. In this video we show how to design a simple test device that will allow us to know how small of a line we can print, and how closely we can put two holes to each other. This simple set of two design rules is a good start of successful designing for a 3D printer.