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BIG digital caliper

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DMIOM:
Joules,

I've done something similar on the manual mill, but totally non-contact (sorry no pics kept though).

I have previously probed items with a basic Renishaw probe, but several years ago I bought a (relatively) cheap USB microscope, which obviously had a very limited field of view - but it did have a cross-hairs option (it also claimed to have a calibrated graticule but I didn't trust/use that).  What I did was secure the USB microscope to the quill of my DRO-equipped mill, and then drove the mill bed around until features were under the cross-hairs, noted down the DRO readings, then moved on to the next feature etc.  - all total non-contact. I didn't take time to try and actually centre the USB device in the quill, what I did though was to secure the camera to the bottom of the quill but locked so it couldn't rotate. The USB microscope had limited depth of field, but on a workpiece of varying height I could move the quill up&down to maintain the focussing without touching the camera at all.

I have even used that rig for something similar to what you're doing: I had a nominally circular centre-less ring item to check, so not easy to spin on an arbor without fitting a web which might have distorted it. Firstly I found the centre by picking-up a point on one side under the cross-hairs, zero the DRO, pick up the other side and the use the DRO 1/2 to set centre on that axis, then repeat on the other axis; and to spot check if it was circular I used the DRO's PCD function to get to a number of radial positions.

Dave

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