Re: [CAD_CAM_EDM_DRO] New to servos, please help/advise?

>I have 5 TPI ballscrews, and plan on using Gecko 320's and encoders
>from us Digital. Rapid rates are not a concern as it's a Hobby
>machine. My thought is to use the third servo to drive the knee, and
>later on get a smaller one for the quill so I can use either/or
>depending on the job.

I'm using two of these motors on my Comet CNC conversion:

<http://www.smidgie.com/928/machineshop>

>Power supply amperage? I'm confused.
>
>3 geckos @ 20 amps max each = 60 amps. I've never seen anyone here
>recommend a 60 amp supply.
>
>3 servoes at max continuous - 26.7 amps. So I suppose it should be
>somewhere in between but I'm guessing because I don't really know what
>I'm talking about yet.

I'm currently using an 800w 45vac (~65vdc) transformer but I have it
wired so that I can drop in a second one if it is necessary. The
transformer has an output of ~18amps, I expect that I will rarely, if
ever, have two servo's at peak power - the 800w transformer will handle a
single servo at peak, or two at continuous.

>Power suply voltage?
>This will affect RPM's and not torque, correct?

Correct.

>
>Encoder resolution. What are the pros/cons to teh different
>resolutions? I'd like the ability to thread mill if that means
>anything there.

My understanding, from the recent long discussion here, is that in order
to do thread milling, you will need control over the spindle as well as Z.

Deciding on encoder resolution is really dependent on your drive
configuration. At 1:1, you could get away with a 1000cpr encoder, at
2:1 500cpr and so on. Those will provide 0.00005" resolution which is
probably overkill - a 500cpr or 250cpr would probably be more appropriate
for the machine you're converting.

At 2:1 with a 500cpr encoder, you will need 4,000 pulses for one
revolution, or 20,000 pulses to move one inch. If you're using Mach1,
your maximum feedrate would be 1.25"/second (~25khz) or roughly 75ipm,
with Mach2 and a reasonably fast machine you could get 2.25"/second or
135ipm. Going to a 250cpr encoder would double those maximum speeds.

At 60v, those motors will provide roughly 2,000rpm, which gives you a
maximum (motor governed) speed rate of 200ipm using a 2:1 reduction.
Ideally, you'd probably want to use a 3:1 or 4:1 reduction, but your
pulleys get really large.

--C.S.

>Sorry this is so long, I'm not even asking all the questions I have
>yet :)
>
>
>Thank you!
>David