Rob Ives has a new post on his blog in which he shows a method for experimenting with the locus of a crank-slider mechanism.
A bit of background from Rob Ives's blog:
The locus is the technical name given to the line that a point traces out as it moves. In mechanisms the locus is often not the shape that you would predict. Changes in variables, such as the lengths of the pieces or the position of hinges can make a big difference to the locus.
His experimentation method is simple and ingenious. He uses a few pieces of cardboard, some split pins (the kind that hold papers together), a pencil, and paper. The results are interesting and worth trying for yourself.
Here is where you can read the full article on experimenting with the locus of a crank.