Here is a method for locating a workpiece relative to the spindle that will work equally well on a CNC mill, a manual mill, or even a lathe with a milling attachment. With a little practice, the location method listed below will help to locate relative to a hole with better than .0005″ accuracy, and best of all it is low cost and easy.
To locate relative to a small hole, wrap one layer of tape around the shank of a tapered scribe. Next, chuck the scribe in a collet or even a drill chuck so the tape provides electrical insulation between the scribe and the machine.
Attach an ohmmeter between the scribe shaft and the machine. Since the scribe is insulated from the machine by the tape, the meter reads no connection. However, if the scribe touches metal anywhere, even slightly, the ohmmeter will indicate the touch. Now, using the X- and Y-adjustments of the mill, move the table so the point of the scribe fills the hole. Move the scribe’s tapered section in the Z-direction, into the hole. As the point of the tapered scribe is moved into the hole, the centering must become better to keep the scribe from touching the sides of the hole and showing contact on the ohmmeter.
The accuracy of the hole location depends on how much finesse you have in working with the backlash of your system. To compensate for any runout that might be present in the scribe, especially if using a drill chuck to hold it, just rotate the spindle and see if the scribe touches the sides of the hole. Adjust the table as necessary to get the scribe as deep as possible into the hole without contact.
To center larger holes, use something larger than a scribe. All that is needed is a piece of conductive material that is tapered on one end and has a diameter that is larger than the hole that you are trying to locate.
To locate edges, use a parallel-sided shaft with a known diameter. The shaft can be used much like an edge finder. Simply move the table until you have made contact with the workpiece (go slow here), raise the spindle, and move over an amount equal to the radius of the shaft to position the spindle directly over the edge of the work.
I think you will find this method is often more convenient than using an indicator. It is very handy when limited space makes the setup or viewing of an indicator difficult.
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