Hole center spacing gage

[mklotz]

You're both correct.

The conical tips are replaceable. I never made a selection of tips for this version because I built a larger version that has several sizes of cones that can be mounted to rods that can slide up and down on carriers that ride on a single rod. Their up-down sliding degree-of -freedom solves the problem of different sized holes. This version was for a job where the holes were all about the same size.

For holes of different diameter there's a method using only calipers that will give very good, but not precision, results. Measure across the hole edges using the outside jaws. Call this measurement A. Measure across the hole edges using the inside jaws. Call this measurement B. Average the two measurements and that is the distance between the hole centers. Proof...

D = distance between hole centers
R1 = radius of first hole
R2 = radius of second hole

then:

A = D - R1 - R2
B = D + R1 + R1

A + B = 2*D

So

D = (A + B)/2
the average of the two measurements.

For really precision measurements, put gage pins in the holes and use this method on the pins.