Homemade tools made with Harbor Freight tools

[Toolmaker51]

[QUOTE=Iconoscope;79244] There is no shear pin and that is a potential disaster waiting to happen. I would like to devise an adjustable means to disengage the half nut when the carriage approaches the chuck or face plate.[QUOTE]

You'll need to complete a short list of details:
1] Wire will be unsatisfactory unless you can calculate torque [by diameter and grade of material turned x deepest cut acceptable x appropriate feed rate] of the lead screw at full load, in to select section diameter to withstand that shear force.
2] Wire requires close fit to drilled AND reamed hole, or you'll endure backlash issues. A taper dowel would be more conventional. A small OD rollpin would be decent alternative. Rollpins are hardened spring steel; shearing is certain to raise a burr between shaft and coupling. A simple fixture bored to lead screw OD, clamped by a pair of machine screws, and a predrilled guide hole is one solution. Assembly could be removed to mill or drill press.
3] Typically and intentionally that will not be physical centerline, when pin breaks, correct 180 reorientation will be unmistakable. Use a ball endmill for starting pocket. Cutter geometry resists deflection better than web of center drill off-axis from apex of round stock.

SOME lathes use a face clutch. In general, that's radial teeth of gear profile or 45 degrees rigidly fixed to one side. The other side is pushed into engagement by coil spring(s) backed by a simple adjustment, a threaded nut. Increase the pressure until the bare minimum of positive engagement is attained.
Some larger lathes arrange a projection through bottom of carriage that tips off feed of X axis. Can't think of many examples for threading. Adding instantaneous action seems problematic to equip carriage thus.
Then sell it back to HFT engineering at a serious $$ premium.

As usual, I invite email/ posts with questions on anything unclear or neglected.