In Case You Were Wondering How You Get Holes Highly Accurate to the Edge

You use a device called an edge finder.  This is a spring-loaded cylinder .100" diameter.  You move the edge finder to the side of the workpiece.   When the spring-loaded cylinder is exactly parallel to the part above:

you are .100" from the center of the edge.  

If you see this, you are past the edge:

Raise the quill above the edge, move the axis in question by .100", hit zero for that axis.  You typically use an edge finder for X and Y axes.  For Z, you lower the cutting tool to the top in very small steps 1", then .1", then .01".  Usually at very low feed rates.  When the cutting tool makes contact, back up very slightly and zero the Z.  

Fancier mills than mine have tool changers that allow the controller to change tools.  These usually have a table of tool lengths, because every end mill is different. 

Another,  more sloppy way to do this is to move an end mill up to the edge and move it into workpiece in very small steps.  Now raise the quill, and move the axis by half the diameter of the end mill.

This is still pretty accurate, but it is going to mar the edge.  I suppose if I move it in .0001" steps,  it will not be obvious.  This also assumes you know the actual diameter.  The actual diameter and the nominal diameter often disagree.   My .125" end mill is actually. 122" diameter.  My .25" end mill I'd actually. 235" diameter.   When writing code, I sometimes account for that difference.