I have watched these instructions several times and I am still puzzled at
what appears an error in the method. I am sure everyone knows what I am
talking about, but I will recap the instructions:

1) Find edge of the hole at a point A using an edge finder (wiggler was used
with a 0.250" ball). Set your Y axis to zero.
2) Move to point B which is diametrically opposite to A on the Y axis. Use
wiggler to find edge at point B.
3) Determine the distance from A to B by counting the turns of the handle
and add whatever figure corresponds to the partial turn on the dial.
4) Divide this figure by 2.
5) Move to AB/2 on the Y axis. Lock Y axis.
6) Repeat the procedure on X-axis. The point arrived at is the centre of the
hole.

My problem with this method is that it ignores backlash.
If backlash is B and the AB distance is the true diameter of the hole then
the actual dial reading is going to be B+AB. On reversing to reach the
half-way point on the Y-axis the backlash has to be taken up again (I assume
the same amount, B) but now we are moving the table by a distance of
[(B+AB)/2]-B. Thus the actual distance the table travels is going to be
(AB-B)/2, i.e the centre is going to be off by half the value of the
backlash.

I would be happy if anyone pointed the fault in my reasoning or outlined the
correct procedure.
I know there is indicators but that is cheating :-)

Thanks,

--
Michael Koblic,
Campbell River, BC