I was working on a project at home where I needed to make some 50" wide circles. I used the nail and tape on a ruler method and it worked but decided to make something more Universal and easier to use. Thus the Yardstick Compass was born! The parts slide on to any standard Yardstick to give you the ability to make any size circle from a foot to several feet wide.
I designed this is 123D design and included that .X3d file. Feel free to modify it and remix your improvements! This design is based on the use of a Sharpie but can be modified to fit other utensils like an Xacto knife or Expo Marker.
This latest version includes some oversize bolts to lock down the sliders where you would like them on the ruler. You can also use a standard sheet-rock screw as well in the center whole. You will need to cut the tip pf the screw so it does not hurt the yardstick.
The sharpie holder should be a snug fit but you may want to lock it down as well. I used a 10-32 sized nut and bolt for the latest design.
You will need supports to print this on a Makerbot or equivalent machine. I oriented them for the best print on a Makerbot with Makerbot Support. The .STL is grouped together but you can easy separate them in Meshmixer. Feel free to write me if you have any questions. :)
Use a 60 degree on the supports so it does not fill the inside of the ruler track.