Re: [CAD_CAM_EDM_DRO] mill servo conversion ?'s

aschoepp wrote:

>Hello,
>
>I have an old knee mill that I'm looking at converting. I've
>previously converted my Taig mill to steppers using G201's. For this
>mill I have some servos I'd like to use. I'm looking at either
>Gecko's or Rutex drives. The Rutex drives are capable of more power
>but I'm not sure how limiting the Geckos would be. The info I have on
>the servos;
>
>Reliance Electric DC Servo motor
>NEMA 34 Frame
>Model E660
>Back EMF Constant, ke = 65 V/kRPM
>Torque Constant, kt = 5.5 lb-In/Amp WOW!
>Nominal Speed Rating: 1750 RPM
>Nominal Voltage Rating: 120 Volts
>
>Encoders;
>HEDS-5605 A06 (500 cpr from my searching)
>
>I don't know if that is all the required info, I've tried to find more
>on the net with not much luck.
>
>

Maximum peak current would be useful. Ahh, wait, the E660 is an old
Electro-Craft number. With the 64 V/KRPM, it is the -05 winding option,
so it has (GASP) 11.42 Ohms winding resistance, and an 8.1 A Max peak
current. 5.5 In/Lb / amp x 8.1 A = 44 In-Lb or 717 In-Oz. This, however,
is the PEAK rating. The continuous rating isn't given simply, but it
comes to
about 110 Oz-In, depending a bit on speed. That high armature resistance
is troubling, too. The armature temp rise is given as 2.8 C/Watt. They
don't
give a weight for this, but I'm guessing this motor is less than 5 Lbs
weight.
To get full continuous torque at 1750 RPM, you'd need 65 V/KRPM * 1.750
KRPM +
1.25 A * 11.42 = 128 V. But, that is only 160 Watts. I'm really
getting the feeling
this is too small a motor for this large a machine.

>I like the idea of the Gecko's since the accept TTL where I should
>convert to differential for the Rutex according to the email I got
>from Vladimir.
>
>Any advice on the Gecko/Rutex question (or info on the servos) would
>be appreciated.
>
>Also on the Taig the steppers were direct. Can you do that or do you
>have to use some sort of timing belt drive? What's the deciding
>factors?
>
>
>

You'd have to use substantial timing belt reduction with these motors, and
I think you'd have to overvolt them to get any reasonable speed, too.
I can't imagine operating, even with ballscrews, with less than about 20
In-Lb
continuous available at the leadscrew (with a 5 TPI screw). I'm using
1/8 Hp rated motors, but they have 1.5 Ohm armature resistance, rather
than 11 Ohms! That really limits how much you can push them.

See my calculations at http://www.timgoldstein.com/cad_cam/motor.htm where
I go through it all for the motors I'm using. The specs on my motors SEEM
to show them as less powerful than yours, but I think the difference is that
I can push mine harder, as they are MUCH larger and heavier motors.
What this really boils down to is that my motors can probably deliver peak
torque for a minute at a time, where yours can do it for 5 seconds, max.
I think you are going to need at least a 4:1 belt reduction, maybe even
more.
with 4:1 reduction from 1750 RPM, you can get 438 RPM to the screw, which
comes out to 88 IPM. I never run my machine that fast because I tend to
break stuff, like running the vise into the end mill!

Jon