My latest hare-brained project involves a stepper motor. But how do I know which stepper to choose?
I'm adding a stepper to the lead screw of my lathe. It will be used for feeds and threads. For feeds, the behavior of the stepper isn't critical; it just needs to move at a constant speed. For threads, however, missed steps are terrible; when making successive passes, the tool bit won't line up to the previously cut groove as well as it should. This is an open-loop system so once it goes wrong it stays wrong.
The carriage (the part being moved) is about 10 lbs. It rides on well-lubricated 'ways' but mass is mass.
I don't care too much about the speed of the carriage. When threading, the spindle turns slowly, maybe 1 rev per second, and in many cases even slower. Basically, there isn't a case where the carriage needs to move fast. In the most extreme case, a 4 TPI thread, the carriage would travel 1" in 8 seconds. That's .8 RPS for the stepper for my configuration. Feeds would be much slower.
As stated before, the carriage must not get bogged down and lose a step even as the tool bit suddenly transitions from air to steel. Torque, baby, torque.
When threading, a few thousandths of material is removed per pass.
Advice on how to size a stepper would be appreciated.
I just performed a calculation I found on the Gecko site. According to them, I need a stepper about the size of a thimble. It feels like adding a stepper to a lathe lead screw is a corner case since the movement in inches per second is very low. If I recall, this means the motor would be very torquey but would get hot; current and movement are inversely proportional. So I may need to run the stepper faster and gear it down.