Monday, July 11, 2016

Interesting Edge Finding Problem

I mentioned a while back that my attempt at boring a hole with my mill had produced an off-center hole because I did not have the workpiece exactly at 0,0.  Normally, to find 0,0 on a workpiece, you use an edge finder to find where x=0 and y=0.  That's easy on a square or rectangular workpiece, but what about a circular piece, like a cylinder?  Finding the left side (x=0) and the front side (y=0) is harder because you don't have perpendicular edges. 

I have thought of drilling a 1" hole in a square piece of acetal.  Then I can use the edge finder on the edges of the acetal.  I just have to find out how far the edge of the acetal is from the edge of the cylinder.  Anyone have a better solution?


  1. You can attach a dial indicator to the mill spindle and move the feeler tip back and forth across the cylinder looking for the point of maximum deflection, then do the same on the other axis.

  2. You can also clamp the piece against stops and find the faces of the vice.

  3. Set a divider at 1/2". Make arcs across the rod from three points. The intersection is the center.

  4. Are you trying to drill a cross hole or are you drilling out the center of the cylinder? For finding the center of any circular feature, a dial indicator mounted in the spindle of the mill works well. There are special clamps made for attaching the dial indicator without having to remove cutting tools. For a cross hole I generally edge find both sides of the cylinder to get the center line, then edge find the end to establish a reference in the longitudinal axis.

    I'm not sure I have all the relevant information about your particular application. If you can put up a sketch or a picture, I might have other ideas.

  5. Use a dial indicator (actually a Test Indicator) in the spindle to center the round piece under the spindle.

    Or, use Osborne's Maneuver: Use edge finder to pick up LH edge. Move Right by exactly the radius. Pick up Top of circle with edge finder, move down by exactly the radius. Move to Left side and repeat, then to Top and repeat. You will be quite close, go ahead and repeat one more time to be sure.

    From Guy Lautard's Machinists Bedside Reader, unfortunately no longer available:

    Here's a lengthy description: