I am trying to clear out some stuff.
One of the items I have is a 1/2 HP variable speed, reversible DC motor. This probably has pretty limited application on old lathes, right? Doesn't DC lose torque as it slows down? Would be great on a jeweler's lathe or maybe something like a Levin.
I only kept it around in case I needed it to power an old lathe or little mill.
Should I sell it?
"DC motor" is overly broad. There are several possibilities. The "main" diff is whether shunt wound or series (potentially "universal" EG: AC/DC) wound.
"Straight" Shunt, compound shunt, and compensated shunt wound DC motors have good self-regulation and constant "reserve" torque at base RPM and below, clear down to locked-rotor. At (or near-as-dammit) locked rotor torque "CAN" go to double, potentially four to six - even NINE times rated torque, if only briefly. Around 60 to 90 seconds, per a Reliance "white paper", before thermal damage occurs.
So no, they do not "lose" torque" as they are loaded down. The opposite, rather.
Unless protected by a power source with inherent limits as to max allowed Current and Voltage, a thermal or current protective device, fuse or circuit-breaker, such a motor will attempt to "sustain or move the load, or DIE trying".
Rare event. as fuses exist, etc., but a useful trait, even when limited. Think hoists, elevators, IC engine starter motors, where the initial load is really, really high, but drops-off once the load is set into even partial motion.
Series-wound AC/DC motors are another horse altogether. Think "legacy" hand power tools for some of the most common applications.
Latest tech, the cordless tribe most of all, may combine "smart" batteries, speed/torque sensing controllers and motor "systems" with active management that are sort of "hybrid" AC/DC tech.
Small DC motor as described?
Just fine for a small lathe. Implementing variable speed is cheaper and easier than a VFD. Simple Variac+rectifier, basic SCR PSU, more sophisticated "DC Drive", or even just a great fat rheostat.
With access to a wound Field rather than permanent magnets, one can even cheaply dial-in a limit to torque so it stalls before it harms something fragile, and .. a DC motor CAN, in more than just a few situations, perform the work of an AC motor of significantly greater nameplate power - 3/4 to 1 HP in this case?
This size is also useful for power traverse on a lathe or mill.
3CW.. and "many" DC motors. KB-Penta Eurothem/Parker.
3-P AC, too. Horses for courses. They all have their place...