Re: Bridgeport servo motors

--- 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

I don't have the specs of my $800 Baldor servos handy, but I know
they're rated 40 in-pound torque. These are on a knee mill a bit
larger than a conventional 42" table B'port. They drive the lead
screw through a cogged belt at 1:1 ratio. I don't know the lead
screw pitch offhand, but it's about 1-1/4" diameter, so maybe 4 o 5
tpi. The machine rapids at 300ipm max.

This talk of reduction ratios to the lead screw got me thinking. I
don't recall seeing anything other than 1:1 on commercial CNC's.