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I don't know anything about the electronic ones but I live by the cheap mechanical ones.
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However they will not do everything shown in the electronic one's pictures. There are two distinct operations shown, edge finding and surface finding. The latter is important if you need to set the mill's distance above the top. I rarely need that operation. To set a mill so that I can cut a specified depth groove I just bring the mill down until it just touches the surface then I zero my DRO at that point. If I need to set it better I make a shallow cut then use a depth measuring micrometer to see where I am. Put that number back in the DRO and go at it.
Actual edge finding is for me way more critical. I often need to center the mill over an existing hole to establish the part dimensional origin or to set up a boring tool for hole enlargement. In the latter case the error in centering is very obvious. Center then bring the boring tool down to enlarge the hole by 10 or 20 thou. Let the cutter just touch down and pull it back up. Visually you can see how concentric the new cut is to the old and probably anyone can see misalignments of less than .001 at this point. I find that I can consistently center to where the error is too small to see with my technique and a relatively cheap mechanical edge finder.
I center in four steps. (1) Bring the center finders tip into the hole and touch off on any edge. Let that point define the east side for the purpose of explanation. Set the DRO to 0 and move the edge finder to the west side. That measures the length of a chord in the hole. (It may be close to the diameter chord but since you don’t know the center you don’t know how close.) (2) Divide that number in half via the DRO and move back to where the DRO reads 0. That puts the mill axis over the NS diameter chord. (3) Move the edge finder to the north side and zero the other axis of the DRO. Move to the south side to measure the diameter. (4) Use the DRO to divide that number in half then move the table to zero the second DRO. At that point the DRO 0/0 will be on the holes center point. People have told me that there is a slight bias in mechanical edge finders but I think the fact that this method uses two edges i.e. east/west once then north/south second means that the bias cancels.
Originally Posted by TomB 
