RE: [CAD_CAM_EDM_DRO] What ratio to use for stepper motor drive?

No, Jan does not necessarily have it backwards in regards to steppers.
Stepper motors fall off in torque quite rapidly as you increase the RPM. So,
putting a smaller pulley on a stepper often has the effect of reducing the
top feed rate because while the torque requirement is about half of what it
was at any given feed rate, the required RPM is now double and a stepper
will often not have the torque at that high a feed rate.

I ran into a similar situation on the Z axis of my Shoptask. The reduction
was such that it took 5600 half steps for 1" of quill movement. My Max feed
was about 18 - 20 ipm. I cut the reduction in half and now have 2800 half
steps per inch and my max feed rate is now up to 40 ipm or so.

With a stepper putting on a smaller motor pulley will give you more torque
at the lower RPM range, but getting enough torque at a low speed is not
usually the issue with a stepper. Another oddity is that going to a larger
stepper motor will not always increase your speed even with a controller
that is up to the task. the larger motors have greater inductance and often
fall off their torque curves at a lower RPM than a smaller motor.

Odd, but true.

It would be well worth the trouble to look up a stepper torque vs. RPM graph
on some of the manufacturer sites and you will quickly see what I am talking
about.


Tim
[Denver, CO]

timg@... <mailto:timg@...>
http://www.ktmarketing.com

> -----Original Message-----
> From: JanRwl@... [mailto:JanRwl@...]
> Sent: Tuesday, April 25, 2000 8:57 PM
> To: CAD_CAM_EDM_DRO@egroups.com
> Subject: Re: [CAD_CAM_EDM_DRO] What ratio to use for stepper motor
> drive?
>
>
> In a message dated 25-April-00 21:52:30 Central Daylight Time,
> bryan@... writes:
>
> << I find that the motors lack power at higher feed rates and have begun
> to think that, if I put bigger pulleys on the motors, I will be able
> to travel the same distance with lower motor RPM and greater torque.
> I don't need that much resolution. >>
>
> Bryan: It may still be April, but the first is long-gone! You
> seem to have
> things backwards in your mind! You would put a SMALLER pulley on
> the motor,
> and this would give you more torque. It would ALSO give you MORE
> (unneeded!)
> resolution!
>
> Lotsa luck! Jan Rowland
>