Still in the calibration stage, but I made a thing that has helped with Z calibration, and I think is extremely important.
Basically, I have printed a gauge, 45mm long, with an offset scale. It starts at +10mm, then 0mm, then -10mm, -20mm, -30mm, -40mm. There are five mm marks beyond -40mm.
[Edit: the gauge is now 50mm long. +10 to -40mm, including a 5mm keep-out zone at each end, so the nominal working range is +5 to -35mm]
The gauge is stuck on to an extension of the ring bracket and sits alongside the body of the router.
Attached to the router body is a pointer, aligned with the gauge. The pointer shows the position of the router body within its range of movement. I’d like to upload a photo, but I can’t from my phone. It’s pretty simple anyway.
This would have been helpful last week when I commanded the Z-axis to plunge 16mm into the workpiece despite only having 2mm of available travel due to having a short bit. That escalated quickly.
Now, when I set zero for Z on Ground Control I can see how much travel is available above and below that point (even if it doesn’t line up with the 0mm point on the scale). Then I know if the router is going to hit the end stops during the job or not.