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+.

There is a difference between a real servo and a DC motor. Servos
have lower rotor inertia, commutators optimized for smother
operation, and more peak torque reserve.

> 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.
>

Well, many of the Bridgeport retrofit packages come with
approximately 30 lb-in servos. So you are right in there.

> They came with 1000 CPR Renco encoders pre-mounted. I may move
> the encoder to the leadscrew though.

1000 cpr is 4000 counts quadrature, a fairly high count already, with
that much resolution and the encoder already being nicely mounted on
the motor you may want to leave it there.

> 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.
>

Well, this is what my research shows on required torque, speed, and
reduction ratio.

Most people on this forum agree the servo size used for commercial
Bridgeport retrofits is around 30 lb-in or 480 oz-in. Also I seem to
be getting the consensus that the pulley reduction is around 2-1 to
2.5-1.

Now let's look at a practical example. A friend has been using a
Bridgeport for 15 years. He is estimating that on average with a
heavy cut he is pushing 5 lb on the hand wheel. The hand wheel has a
3" lever length. So 3" x 5 lb = 15 in-lb continuous torque needed.

You motor has a peak rating of 37.5 lb-in. Estimating constant
torque we would divide peak by 5 = 7.5 lb-in. So you need 2x this
number for the estimated 15lb-in force required. This tells me you
need a minimum of 2-1 belt reduction.

As you noted earlier more reduction gives you more power but less
speed. A 3-1 reduction would give you a 22.5 lb-in at 80 IPM. A
nice continuous torque rating but a little slow on rapids.

Lets talk about 80 IPM, I can tell you this. A typical power feed on
a Bridgeport will give you a maximum speed of around 90IPM. If you
have access to one of these you can see for yourself if you like this
speed.

From a personal standpoint, and this is coming from someone who has
no patience….I get irritated using the power feed to go from one side
of the table to the other. One time I actually disengaged it and
turned the crank by hand because I thought I could go faster (I was
wrong). The reason for going from one side of the table to other was
I had 2 setups on the table, one on each end…

With that in mind I am shooting for a rapid of 120 IPM minimum on my
machine.

With all that said, if I were you I would probably go with 2.5 or 3-
1 ratio, a fast rapid is nice. But if you can't take the cut you
want it doesn't do you any good.

I also think you should use a 70 volt supply, with your current limit
set to 20 amps. This way you will get maximum RPM without running
over current.


Wally

> 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