Re: Servo motor selection

Jake,

Those are good motors. Observe the continuous torque rating of 50 in-
oz. Think in terms of power, as in Watts. 50 in-oz at 6,000 RPM is
222W (torque times RPM divided by 1351 equals Watts). That also
equals 3/10 of a HP (222 / 746 = .297HP). 1HP also equals 550 ft-lbs
per second or 396,000 in-lbs per minute. You have 118,800 in-lbs/min
at your disposal. This means if you want 300 IPM, you have 396 lbs
of "push" available (118,800 / 300). That takes care of the power
part.

Now for the reduction ratio. From the previous example, you want 300
IPM. Assuming you have a 5 turns per inch (5 TPI) screw, it would
have to turn at 1,500 RPM (IPM times TPI). The motor turns at 6,000
RPM so the reduction ratio is 4:1 (6,000 / 1,500).

Now a reality check. A 4:1 reduction on your 50 in-oz motor applies
200 in-oz on the leadscrew. Force (lbs) = in-oz times 6.28 times TPI
divided by 16. For you that is 200 times 6.28 times 5 divided by 16
equals 392.5 lbs; close enough to 396. Use non-backlash reduction
(toothed belt).

Finally, the encoder. Say you want 0.0001" resolution (10,000 counts
per inch). You have a 4:1 reduction and a 5 TPI screw, so the motor
has to turn 20 revs to move 1". The encoder has to have 500 counts
per rev (10,000 / 20). Since there are 4 counts per encoder line, use
a 125 line encoder (500 / 4).

The motor has a 600 in-oz stall torque. This is 12 times the
continuous torque. That means for brief periods of time (1/2 sec) you
would have 4750 lbs of "push" available (12 times 396).

Mariss

--- In CAD_CAM_EDM_DRO@y..., "jbordens" <jake@a...> wrote:
> First, an introduction...
>
> I'm Jake, and I'm new to the group. I'm currently waiting for my
new
> Grizzly G3012 mill to arrive. Its backordered, so I've got a few
> weeks till it arrives.
>
> I'd like to do a motion control setup for it. I'm pretty handy
with
> electronics, and I understand most of how this works. However, as
> for chosing motors I'm at a loss.
>
> I'd like to thank Mariss for the wonderful top 10 list. Its helped
> me greatly. To quote:
>
> "7. Bewildering choice of motors, encoders, servodrives"
>
> I'm offcially bewildered. I'm deathly afraid of taking all this
time
> (and money) and having underpowered motors.
>
> The camtronics people sell peak-600oz/in motors 50oz/in per amp for
a
> price I can afford. My question is: is this a good way to go?
Will
> these be strong enough to handle a mill this size? Should I go
with
> a direct drive, or timing belts? Is there a good conversion factor
> from stepper to servo oz-in ratings?
>
> Any help is appreciated, and I do appologize for the post. I
haven't
> seen a "guide to chosing motors", other than a really confusing bit
> about coiled ropes and such that I found in this forum. Maybe I'll
> put something together with all my notes once I've gained some
> experience.
>
> Thanks for your time.