Edge finder

[mklotz]

Edge Finders have always fascinated me. Ive been a machinist / tool & die maker since the early 70's and even in retirement use one frequently in my home shop on the ol' DoAll mill. But, i never understood how they work. What is it that makes them go off-center so accurately?
And, who was the lucky guy that first encountered the principle?

Let me take a shot at it. A little gedankenexperiment may help.

Imagine spinning up a tire and then dropping it on the ground while it's still spinning. As soon as it gets purchase (an olde time word for friction) on the ground it will begin to roll away in a direction determined by the direction of its spin vector.

A similar thing is happening with the edge finder (EF). As soon as it touches down on the part, it gets friction and begins to roll away. It's rolling is quickly stopped by the spring by which it is attached to its shaft but it is displaced far enough that we see the offset. Of course, it won't see any friction until its center is one probe radius from the edge of the part.

Try an experiment. Edge find on an edge parallel to the Y axis on the mill. Approach from the right towards the left with an EF spinning CW as seen from above. If what I wrote above makes any sense, then the probe should deflect in the -Y direction, i.e., towards the operator.

I haven't actually tried this so please report your results.