If you have the 'Easy Circle Cut ruler' you will need the 'Easy Circle Cut Rotary', esp. if you want the smaller circles. 45mm works on the largest, I also tried to use 18mm, with no luck.
The reason I know this is because I had purchased the ruler ... GRRR ... Returned the ruler. Then on another shopping expedition, I see the Easy Circle Cut Ruler and right next to it was the Easy Circle Cut Rotary. The other shop did not inform me that you needed both. Why they are not sold together???