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,