As indicated above, the Atlas 6" threading dial assembly is made up of four parts, body, shaft, dial and pinion. The dial and the pinion are pressed onto opposite ends of the shaft. See the 1950 618 parts list Page 5 for a drawing of an almost disassembled one. Take it apart by machining a receiver pocket into an aluminum block and pressing the gear off of the shaft.
However, re-assembly is a bit of a PITA to get the gear in the proper position relative to the dial. So I would first try soaking it for a few days in a cup of Varsol, turning the gear every time that you walk by. That should free it up. Try to measure the shaft end float first or as soon as you can turn it with your fingers. If endfloat is less than about 0.005", that could be part of your problem.