Mill Z axis crank wheel

[Toolmaker51]

Actually, the adaptor drawing is not mine, I found it on the net and it was designed to be used with a cordless drill. For me it was just a nice reference for the dimensions of the Bridgeport dogs...
I would highly recommend some form of counter balancing the weight of the knee and table. It is a win/win modification. For a small mill I'd use an actual counter weight with a cable over a pulley, that enables you to exactly balance the weight throughout the movement range making moving the knee assembly effortless whether manual or motorized. It is easy enough to cast a concrete lump for the counterweight, throw in any steel or lead scrap to reduce the needed size. I use counter weights on a couple of drill presses which don't have a rack and pinion. However, that is hardly practical with mills of the Bridgeport's size, hence my use of the pneumatic spring units which give reduced help as they become extended (table raised), but owt's better than nowt.

A counter-weight is feasible. In use by variety of machine tools; especially in sliding heads of jigbores and drills like the Cleereman, operating in the hollow columns. They use a sprocket and 1 or 2 rows of roller chain connected at the upper end of head, weight of course at other end.
None are neutral or weight cancelling, just reduction so the head doesn't float upward.
In the case of a leadscrew raising a machine element, the same principle would apply. Less effort is nice; still want a leadscrew to bear load for backlash compensation. Otherwise the dial or readout will be useless. Lazy google search indicates a 9 x 30 knee & table at 400-460 lbs, 10 x 54's at 500. Seems reasonable target, considering weight of vises, rotabs, indexers etc. Designed load on 9 x 30 is 750lb workpiece.
A pair of flanged bearings outside the column would support a shaft and sprockets through it. They have self aligning feature built in. Place high enough for full upward travel.
A pair of standoffs on the knee would anchor the roller chain. That would ensure upper end of knee contacts column, leverage presses lower end into contact. Place high enough so a pair of weights operate without contacting machine base or floor.