Hi Matt

Don't want to teach Granny how to suck eggs, but you seem to be measuring a Jacobs taper which is mounted in the milling head taper and thinking to mount a collet chuck on it.

I'm wondering if you are confused about what you are looking at. The Jacobs taper adaptor is a round lump of iron with a taper on either end.

It's the other taper you want to knock out, the one that goes up into the spindle by hammering from above. The Jacobs taper is irrelevant.

Drill chucks don't make good milling heads because they depend somewhat on the downward drilling force to keep them in place. Once you lose that downward force they can vibrate off.

best

Robin