A test to mill the pivot hole in 0.5 mm PETG sheet worked perfectly:
The cutter is a 3.175 mm = 1/8 inch router bit, one of a ten-pack that came with the CNC 3018 and to which I have no deep emotional attachment, held in a collet in the Sherline. The hole is 5.5 mm to fit an eyelet. The PETG is taped to a thin plywood scrap.
The hole happened by feeding G-Code manually into LinuxCNC, after touching off XYZ=0 at the center of the pivot and jogging up a bit:
g0 y-1.1625 f1000 g0 z0.5 g2 p5 z-1.5 i0 j1.1625
Yes, I engraved the hairline using a diamond drag tool on the CNC 3018, cut the cursor outline with a drag knife on the MPCNC, then milled the pivot hole on the Sherline. This seems way over the top, even to me, but that’s just how the tooling worked out right now.
In actual practice, I’d probably mill a stack of cursors and pivot holes on the Sherline in one setup, then engrave the hairlines in a suitable fixture. I think I know enough to fit a spring-loaded diamond drag bit into the Sherline’s 10 mm ID spindle or, worst case, conjure a block for the Z-axis carrier in place of the entire spindle mount.
At least now I can remember what I did to make the hole.