Finding the center-to-center distance between two holes can be done a number of ways but this tool simplifies the process. The two arms are slid along the rods until the conical points seat in the holes. Then calipers are used to measure across the flats milled at the top of the arms.
Last edited by mklotz; 07-08-2017 at 10:08 AM.
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
A = D - R1 - R2
B = D + R1 + R1
A + B = 2*D
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.
It's very easy to make a version of this for laying out equally spaced holes along a line. Simply make the steel tips a bit longer and sharper. Dial in the correct spacing by using calipers across the half cut-aways. Set one tip on a starting point, align second tip to reference line and tap with a small hammer. Step and repeat.
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