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.

[gatz]

Makes sense. I was going to pose in my post "what happens if one oils the EF small end?"

I think your assumption re the offset movement is correct.
I might see if that's the case tomorrow.

[olderdan]

In circumstances where logistics make it difficult to use an edge finder the old dodge of using a cigarette paper stuck to the edge if the part with a drop of oil works well. You can use the rotating cutter or drill to just flick off the paper and you will be .001 inch or .02mm from the edge. Near enough for most applications. Will not mark the part either.

[gatz]

^ I've done that many times.