Re: mill servo conversion ?'s

Something doesn't make sense to me. How can you run 36V motors though
"fine up to 50V" at 84V? Also, how do you know these are 4ohm motors?
Just from specs that you have, or from some derivation? When Dave says
the motors are 12amp max does he mean that is the stall current, or
does he mean more amperage will damage the motor? Does this mean, that
if you buy Dave's setup, it is impossible to get 600in-oz of torque
even though the motors are rated that high?

Honestly, we're not terribly interested in extemely speedy machining
as we're not a business or anything. We spend a lot more time
designing than manufacture, its just our size manufacturing
capabilities are the limiting factor. We are interested in getting the
most bang for our buck though.

I don't understand how you know the motors I asked about on ebay are
the exact same motors camtronics sells. What exactly should I be
looking for in or asking about a motor?

These motors you linked to on ebay are the exact same ones that
started this thread, that you already said were worse than the ameteks
camtronics sells. So that is kind of confusing.

Really, what I'd like to do is buy motors from surplus center,
encoders from us digital, and find or build a power supply to match. I
intend to use gecko's so that constrains me with 80V and 20A.
Comparing to camtronics, they sell 3 motors and the power supply for
$635. So I'd like to be able to do the same thing for less, or get
more power for the same amount of money. Also, torque is much more
important than speed to me. A maximum speed of 50ipm would be plenty.
We have 2.5:1 gearing on a 4tpi ball screw, so I don't want to run
over 500rpm at the motor anyway. Our computer can only handle 18khz
pulsing, so with 320's anything over a 500cpr encoder makes the
computer the limiting factor anyway.

So, my specifications are coming into focus, and really, I'm just
writing them out for my own benefit as much as to seek advice on them.
I'd like to get dc servo motors with as much stall torque, and as
much torque as possible up to 500rpm. I'd like to get 500cpr encoders
to match the motors. And I'd like to find or build a power supply to
work that will work with the geckos.

So, knowing that, how do I choose a motor? What makes a good motor for
this application? A higher or lower max rpm? Higher or lower voltage?
Does a higher RPM motor mean there will be higher torque at lower
RPM's? How do I determine internal resistance? How do I find out stall
torque (surplus center doesn't usually provide that information)?

Then, how do I choose a power supply based on those specs? I've heard
I need about 2/3 the amps of the stall amps of all the motors
combined. Is that peak amps or sustained? And how do I judge a power
supplies peak amperage? Can some sustain the peak amperage for longer
(it seems so) and if so, how long? What about voltage? How do I choose
voltage with respect to the motor's voltage and maximizing torque? Is
higher voltage always better? Also, I've heard that these power
supplies are rather simple (practically just a transformer, rectifier,
and capacitor) so is there some way I can build my own?

Really, this all seems rather complicated and involved, and I
understand why the easier thing to do is to just buy camtronic's kit.
However, as a student, I see it as something of a learning experience
and an opportunity to stretch our budget. On the other hand, our
organization is an FSAE team, not a cnc hobby group, so in general our
time is better spent working on the car and not the tools, but I know
better tools will help us make a better car.

Thanks for all your help so far, though I'm surprised no one else has
seen fit to chime in. It seems to me that these are fairly basic
questions, someone more knowledgeable then me might be able to put
into a FAQ.

> For brand-new motors with encoder already installed, they are not bad.
>
> > If they are, where is there a better deal? And why
> >isn't it more widely publicized? Also, why do you recommend a 80-120V
> >power supply for the Ameteks when camtronics only sells a 36V for them?
> >
> >
> For performance. To get full peak torque, you need to deliver 12 A
> to the motor. With a 36 V power supply, you can't deliver 12 A to a

4 Ohm

> motor, even standing still. At 900 RPM, the back EMF of the motor is
> approximately 36 V, so you can produce zero torque! This severely
> limits performance. I have a client in town who has an 800 Lb Grizzly
> style machine with these motors and a 36 V supply, and the

performance is

> pretty weak. To get full peak torque at rated speed, you need 36 +

12 * 4=

> 84 V minimum, appled to the MOTOR itself. You need some margin for the
> modulation scheme of the servo amp. too. So, a 120 V supply sounds

good,

> but if using Gecko 320 drives, 75 V or so is the limit. With a 5

TPI screw

> driven directly, you get 180 IPM, which should be plenty for most users.
> But, if belt reduction is used, or if the screw is finer than 5 TPI,
> then the max
> rapid rate goes down, even with the higher voltage.
>
> >I am part of a student organization that currently has a stepper
> >powered benchtop mill. We are, for the most part, satisfied with it,
> >but I would like to get a servo motor setup on the benchtop, with a
> >view towards replacing the benchtop mill with a 9x42 knee mill later.
> >As a student organization we don't have a lot of money, and we're also
> >not in a position to "save up;" we have to spend our budget each year,
> >also the budget is firmly set, so any money we spend, is money we
> >can't spend on something else. However, we are guaranteed a certain
> >amount of money each year.
> >
> >In terms of requirements, we don't do that much heavy machining and
> >the reason we want a knee mill is to increase the maximum volume size
> >of parts we can machine. My understanding is that our steppers (230in
> >oz) would be grossly underpowered.
> >
> >
> Yes, grossly!
>
> >So, all that is in way of asking, what would be a better deal than
> >Camtronics complete setup? I'm not asking how Bridgeport's or your
> >setup is better; I'm sure they both are. I'm asking if I have ~$1250
> >to spend on a servo system, what should I get?
> >
> >
> Look on eBay, or Surplus Center, or C&H sales in Pasadena, or maybe
> Herbach&Rademan
> in NJ. $1250 for motors will go a long way!
>
> >For instance, how do the motors offered in this auction:
>
>http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&rd=1&item=3831771088&ssPageName=STRK:MEWA:IT
> >compare to the ones from Camtronics. Are they better, worse, or the
> >same and why? Yes, to us, $30 matters.
> >
> >
> They are the same, exact motors. If you are thinking of upgrading to a
> Bridgeport,
> I'd go for slightly larger motors. The eBay motors do have
> higher-resolution encoders than
> the standard Camtronics units.
>
> Take a look at these :
>

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=71400&item=3830503711&rd=1

>

<http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=71400&item=3830503711&rd=1>

>
>
> Jon