Re: Bridgeport servo motors

Scott,
The Ametek's look very appealing initially, but I don't like them for
a Bridgeport for several reasons:

1. Many of the Ametek ones don't have a 2" or longer shaft on them.
You need 2" or better if you want to bolt right up to a BOSS factory
belt drive.

2. I don't think that they have enough power to run 1:1, and the
highest RPM I have seen them rated to is about 2000 RPM or so. So if
you did gear them down to 2:1, you might see a max rapid rate of 150
if they can pull the load.

3. They are used...not sure the condition of them, but they were
designed for a reel to reel tape drive...

I don't have any direct experience with these motors and initially
was looking at them. Surplus center sells a few variations of them.
The 36v model has a 5/8" X 2" shaft that will bolt right up to the
BPs drive belts.

I just bought 3 of these puppies:
http://www.surpluscenter.com/item.asp?UID=2005020410354708&item=10-
1802&catname=electric

I chose them because they will definately have enough power for the
retrofit running 1:1. I don't plan on running them full voltage, so
my rapids will be a little slower...but still capable of over 400 IPM
in theory. I'm going to shoot for the 150-200 IPM range and be happy
with that.

The reasons I chose them are they are cheap @ $80 ea, they have 2
5/8" shafts that are 2.5" long, and they are 9.125" long at the body
so they will fit between the Y axis and the knee. I will have to
shorten the shaft on the Y...but it sure beats trying to EXTEND a
shaft, which is the case with the treadmill motors. So, aside from
shortening the rear shaft and making some type of mounting/face
plate, these are a bolt-in deal. I really didn't want to fool around
with extending shafts, drilling shafts for encoders or coming up with
some type of external belt-drive because the motor was too long etc.
Belt drives are not cheap...

I haven't decided on a servo drive yet. I am leaning towards the
Rutex though because it has more features that the Gecko...although
wiring is a bit more complicated.

Hope that helps,
-Brady

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Polaraligned"
<polaraligned@o...> wrote:

>
>
> There has to be a difference between a new servo motor at $800+ and

a

> brand (not surplus) treadmill motor at $100+.
> I have servo motors that I paid, what some would say "dearly" for,
> from a dealer on e-bay. He has sold hundreds of these surplus

motors

> and has excellent feedback. He states that as many as half have

been

> used sucessfully on Bridgeport retrofits. Of course, he has an
> interest in selling motors so....
>
> Here are the motor specs:
>
> Torque constant: 44oz-in/amp
> Voltage constant: 32.5v/1000 rmp
> Peak torque: 600 oz-in, or 37.5 in-lb
> Nominal voltage: 40vdc
> Maximum voltage: 60vdc
> Demagnitization current: >20amps
> Terminal resistance 2.60 ohm
> Max rpm at 50vdc is about 1200 rpm
> No-load current <.51 amp
> armature inertia .09oz-in-sec^2
> 4 bolt holes in face for easy mounting.
>
> They came with 1000 CPR Renco encoders pre-mounted. I may move
> the encoder to the leadscrew though. The seller states that
> these are the largest servo's that a Gecko will drive. The motor
> also has a heavy duty front ball bearing. At $150 per motor with
> encoder installed, I find them to be quite reasonably priced
> considering they are "real" servo motors.
>
> The big question, If I decide to use these motors, is what my
> reduction ration will be. I need to trade off between torque and
> speed, and the BIG unknown is what torque is REALLY needed to
> drive my Bridgeport. I would rather err with a higher reduction
> ratio than find myself without enough torque.
> If I go with a 3:1 ratio and triple my torque I will get 80 inch per
> minute rapids. More than an inch per second SEEMS plenty for me.

Why

> do people need such fast rapids on a mill? What do you think about
> even a 4:1 reduction? Will I regret it later? Do I ask too many
> questions? I mostly mill small parts, say 6" on each side or less.
>
> A fellow CNC'er here used these same motors on a Rong Fu mill in
> direct drive and said they worked, but were at their limit. My 3:1
> reduction will triple the torque, and after all, the only forces
> different between my Bridgeport and the Rong Fu is the dead load
> of the larger table. (The Rong Fu leadscrew is also finer at .1" per
> revolution). The cutting forces should be identical.
> The BIG question is will I produce enough power with these motors
> to drive my Bridgeport. What do you think?
>
> Scott