The Hardinge gear train above looks complicated but is essentially very simple:
The spindle drives two gears at a 1:1 ratio one drives a dog-gear forwards, the other drives a shaft to a reversing pair to drive a dog-gear backwards.
Between the two dog-gears is a sliding dog-clutch which drives additional shafts and gears that in turn drive the lead screw. A lever on the front of the lathe moves the dog-clutch into engagement either forwards or backwards, thus the lead screw is driven forwards or backwards.
Critically, because the clutch is in a 1:1 ratio with the spindle and can only engage in one rotational position, the lead screw is always 'in sync' with the spindle .
So, when threading on a Hardinge, the half nuts are left engaged at all times, after a forward cut the tool is retracted with a quick action mechanism on the top-slide and the carriage is reversed using the threading lever. Another forward cut only requires re-positioning of the quick action top-slide lever, plus a little in feed, then the threading lever is pushed forward again to move the carriage in perfect sync. It doesn't matter what thread is being cut imperial metric or DP, because the half-nuts are engaged there's no need for a threading dial.
On the HLV-H there's a threading stop that runs along the apron that can be set to disengage the clutch in a precise manner (<0.001" typically so threading to a shoulder at up to 1000 rpm is perfectly possible).
The whole thing works so well I'm surprised more lathes aren't fitted with similar mechanism. I'm sure if you can retro-fit a tumbler reverse that you could engineer a dog-clutch. (Like this one fitted to a mini lathe : http://www.gadgetbuilder.com/Dog_Clutch.html