# Why is GRBL controller taking y-axis into account when using a Chuck rotary?

So I built a chuck style rotary. Basics are:

1.8 degree stepper, 5.18 to 1 gear ratio, 128 Micro steps (honestly not 100% sure but the math is close?)

So using lightburns suggested formula we get:
(200 * 128 * 5.18)/360 = 368.35 steps per 1 degree rotation.

This does not turn far enough. In fact using a 6 inch long “arm” and testing the right answer is 415.69 steps per 1 degree (the number input into the Steps/mm section).

Baffled I was messing with the controller settings and changed the y-axis steps/mm from 40 to 80 and the rotary did 2 full rotations (with 415.69)… what? I was under the assumption that this number was ignored or replaced while rotary was enabled.

So I guess im asking if anyone can shed light on this for me as to why lightburn is using this setting, or how it fits into the equation?

And as a note I tried this in a different machine with a ruida controller, calculated the steps for a full rotation as 16576 and it worked flawlessly.

It’s a machine that moves the rotary just like it was the Y axis, since it is moving the Y axis. The rotary ‘rolls’ back and forth as the table would travel back and forth in the Y direction.

Check this Rotary Setup and see if that helps.

Good luck

P.S. ensure you save your original configuration so you can restore it when you remove the rotary device. Don’t forget to save the rotary devices configuration also.

Bah! Ok easy once I messed around for hours… maybe I just cant understand the way it was written in the “rotary setup” page but here is the solution that makes sense to me:

Motor steps per rotation * gear ratio * Micro steps / steps per mm = mm per rotation
200 * 5.18 * 8 / 40 = 207.2

If only I was better at math, I would have figured this out hours ago!

What controller are you using? Also, how did you come up with your 5.18?

May be this helps: