Re: Servo questions

Dan Falck wrote:

> From: Dan Falck <dfalck@...>
>
> In the interest of getting a FAQ started, I thought I might ask a few
> questions for general reference.
>
> This one's for Jon, Matt, or anyone else doing the servo retrofits using
> EMC, Allen Bradley, etc...
>
> What size, rating, brand servo motor do you recommend for a Bridgeport or
> mill/drill retrofit?

I have motors rated at 1/8 Hp, 1725 RPM, continuous. That comes to 1.54A
at 75 V. I run up to about 5 A on these motors during acceleration. These
motors are certainly strong enough, with 2:1 gearing from the motor to a
.2" lead ballscrew. I hit the wrong button once, and the vise moved over and
broke a 3/8" end mill clean off in the spindle (while it wasn't rotating)! It
didn't cause an axis lag error, overcurrent trip, or anything. You can figure
out what the torque of a 1/8 Hp motor would be at 1725 RPM. My motors
work out to about 70 oz-in nominal, but I boost it at peak by about 3. So,
that comes out to 210 oz-in. Remember, that is with a 2:1 step-down
between the motor and leadscrew. So, it would deliver 420 oz-in to the
screw. I think this would be enough for any Series-I Bridgeport.

> What else should come with or be coupled to the motor? (tach, encoder, etc...)

A tach on the motor would save you some trouble. Since I had none of
that on my motors, I added the tach outboard, using some tiny W. M. Berg
instrument drive belt. The encoder should be directly on the leadscrew, and
that's how mine is set up. I have a helical-slot compliant coupling between
the screw and the encoder, to protect its' sensitive bearings.

> Where would a person purchase this type of motor? I know they can be had
> from old tape drives, but one can't always find them laying around.

Herbach & Rademan, in NJ, and Surplus Center in Lincoln, NE, are two real
good sources for motors. C & H Sales in Pasadena, CA, is also good, but they
know the good servo motors and price accordingly.

Jon