Diameter Gauge

This Thing is a Diameter Gauge, inspired by SubSite's Radius Gauge. For my purpose it was more appropriate to measure diameters, so that's what I've got; no real difference (though it simplifies the equations a bit). Also, I needed to measure somewhat larger diameters, up to 36" or so, so my gauge needed a larger span, to improve accuracy on diameters that size. This has a 200 mm span, which is the largest my printer could handle, with the part placed diagonally.

Then I noticed that with a larger span, you couldn't really do smaller diameters. I could have made a bunch of differently sized gauges, but instead I added a pair of removable probes that can be removed and replaced at span distances of 150, 100, and 60 mm. If you're using the 200 mm span, the probes can be removed and stored out of the way.

The caliper is clamped into the gauge very securely, and the slot for it accommodates the hardware found at the end of the caliper. My caliper is an older one from Harbor Freight (I think the newer ones are slightly different), but it also fits my good Mitutoyo.

The pictures show the gauge at the 200 mm span, with the probes stowed, and with the probes at the 60 and 150 mm positions. The probes lightly snap into place so they won't fall out and there's very little room for movement during a reading.

In use, you 'calibrate' the gauge by extending the caliper probe beyond the fixed probes, then pressing the unit against a flat surface so that the caliper probe is then in-line with the gauge probes. Zero the caliper. Then pressing the gauge against the diameter to be measured so that the gauge end probes and the caliper probe are in contact, H is the reading off of the caliper. I don't have an app for it, but the calculation is simple:

@ 200 mm, D = H + 10000/H
@ 150 mm, D = H + 5625/H
@ 100 mm, D = H + 2500/H
@ 60 mm, D = H + 900/H

I'm going to put those equations on a label stuck to the back of the gauge.

The actual equation is D = H + (Span^2)/(4 * H)