Re: Minimum torque calc. for conversion
For a stepper in this setting, I don't think minimum torque calculation is particularly necessary or helpful.
Steppers have heaps of torque at low speeds, poor torque at high speeds.
Cost of steppers is low, and marginal difference between different models etc.
As advised by others: Get low inductance steppers. Get decent digital drivers. Run the steppers at higher voltages - 48V minimum, higher is better for torque at higher speed.
I would go closed loop steppers, not because they perform better, but because they will fault and signal an error once you lose a certain number of steps.
Yes, a properly designed stepper system should not lose steps, but let's face it, our hobby systems are rarely optimally designed.
I have had a couple of projects with serious fails (over years) due to stepper lost steps / stall. A single one of these stuff ups cost enough in time +/- materials to justify the cost difference of closed loop for the prices they are these days.
(noting closed loop won't save you from every stuff up, particularly in tight tolerance parts)
7xCNC.com - CNC info for the minilathe (7x10, 7x12, 7x14, 7x16)