I purchased a used Sherline 4000 Lathe on craigslist for $140. It was in excellent shape, but had no accessories included. Using the Sherline History page, I was able to identify the lathe as being made in approximately 1990. It has the 1/2 hp AC/DC (Universal) Motor.
There are a few options I've investigated for a new drive motor and controller. The current motor has a nice amount of power, but the speed controller does not hold the speed very well, and pulses at low speeds. The current speed controller is just a basic AC motor controller, but the motor is AC/DC brush type motor.
Since the motor is an AC/DC 115V motor, a DC speed controller can be used to run the motor. Any 1/2 horsepower, 115V AC input to ~90V-120V DC output controller should work fine. KB electronics is a commonly used brand for newer mini lathes.
SCR Speed Controller - control speed using a potentiometer, less expensive KBIC-120
A large stepper motor could provide fine speed control, and be easily controllable from a CNC controller. Maybe this could be used for threading?
Stepper should be about 5-6 Amps. Gecko G723-400- $59.00
Controller: Gecko G210X $140.00
Expensive, and off the shelf setups don't have the ability to be CNC controlled
CNC control can be useful in more ways than just automation, or making difficult shapes. Threading can be done using CNC instead of the complex threading setup using change gears. By adding an encoder to the spindle pulley, the CNC system can keep track of the spindle position, and drive the carriage to cut the correct pitch thread. The spindle will be turned by hand for most threading operations, and the carriage will follow, being driven by a stepper motor.
Sherline sells a stepper motor adapter for the axis of the lathe and mill. It's part number "Sherline 67102", but I can't find it for sale anywhere, and I suspect that it's pricey, like all other sherline accessories. A2Z CNC is the only vendor I found that sells an adapter. Item Code: CNCMMS
To convert a single axis to CNC, you need the following:
The controller will primarly be an Arduino microcontroller. This standalone controller will be perfect for setting feedrates and thread pitches. Optionally the motor drivers can be connected to the computer for full CNC control.