It is often the case that a motion is designed for a Tool, but the Tool is not driven directly. A different Part drives the mechanism. The motion and forces are transmitted through the mechanism from a Driving-Part to the Tool-Part.

Usually, with a Cam mechanism, the Driving-Part is an oscillating Cam-Follower that is moved by a Cam.

There is also the case in which the Driving-Part completely rotates, but with a non-uniform rotating speed. This is a special-case. To make sure this can happen, the Driving-Part must be exactly the correct length.

The Driving-Part becomes a Crank that rotates continuously in one direction as the Tool-Part moves with your motion-design.

This is one application of the Continuous-Crank FB.