I just finished making and testing a neck turning tool.
It's made from 1/8" steel plate, scored and bent in a vice. A 1/4" hole is drilled for a mandrel to spin in. The cutter is ground into a 1/8" drill bit and held by an aluminum block and 4 screws.
I clamp it to my welding table to use it.
The cases are sized and expanded without any flare. Then they're chucked into a hand drill with a Lee universal 3 jaw chuck and stuck onto the mandrel. The mandrel is a tight fit, so the case doesn't spin on it. The entire mandrel spins. It's pretty concentric, but I'll take it out and turn the case 180 degrees for a second pass, just to keep the cutting extra even.
I turned 10x 7mm-08 necked down from 308 cases. Some of these reformed cases got thicker and lumpier than others. Where the cutter is set, a couple of these reformed case necks hardly got touched. Most got cut a lot. I set the cutting depth to leave some low spots on the majority of cases. Seems to work pretty good, but I've never turned necks, before.