Draw Whorl
I made the Draw Whorl plug in/extension for SketchUp for a simple way to generate threads for your models. It allows fixed-radius thread section, as well as an optional tapered-thread section. You can specify the starting and ending radius for the tapered section, and how many revolutions of the fixed-radius and tapered-radius sections. You can specify the pitch of the threads, the length of the threads, and the width of the outer edge of the threads. The help page has additional tools to help you generate settings files for standard bolt and screw threads.
Her is a link to the Extension Warehouse page: Draw Whorl
