Re: [CAD_CAM_EDM_DRO] Re: mill servo conversion ?'s

carlos_is_dead wrote:

>This response has really created a lot of questions for me. Hopefully,
> someone can answer them, or point me to somewhere I can find the
>answers for myself.
>
>Are the the Ametek motors from Camtronics really a bad deal? I don't
>mean in terms of the high resistance. I mean in terms of what you get
>for the money.
>

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