I published this in my weekly tips newsletter today, but thought I would also share it here:

On Facebook, Sean Ragan shared this Thingiverse file a few weeks ago. It’s for printing adapters that go over the external jaws of a set of calipers to accurately measure the center of holes.

In response to the post, Evil Mad Scientist, Lenore Edman pointed out that you can get the same result using this trick (which I’ve written about before): “You can get center-to-center measurements by measuring your hole’s diameter, zeroing out your calipers, then taking an outside-to-outside measurement. (Zeroing essentially subtracts the diameter of one hole.)”

Sean Ragan responded: “I know that trick, but it’s not terribly accurate because you don’t get good indexing on the curved interior surfaces of the holes in either step, whereas turned, tapered caliper heads can be relied upon to be kinematically on-center in round holes with no wiggle room.”

Both seem like worthwhile options.