Also known as Do-nothing machine, bull-shit grinder or ellipsograph. http://en.wikipedia.org/wiki/Trammel_of_Archimedes This unit works pretty well but you may need to adjust the tightness of the screw to get the best motion...not too tight or loose...just right! Assembled with five #4 3/4" pan head screws. I think this is the last variant I will make...took me quite a while to get the design right and to get the motion to work correctly. Time to move on to other things!