Re: [CAD_CAM_EDM_DRO] RE: Bridgport clone Retrofit

Davis, Marc wrote:

> Group
>
> I'm planning a Bridgeport clone (Alliant) retrofit. I have
>some servo motors from an old Bandit retrofit on a Mori Sieke (sp?). Dana
>7.5A 90V.
> I'm thinking about using the Flashcut signal generator and
>Gecko G320 drives. I also plane to install ball screws.
>
> I think the servo motors I have are big enough to direct
>drive the X-Y. Any comments on this? How about the knee?
>
>

Yes, that is 675 W, almost a full HP. This should be fine for a
Series-I sized machine. I'm running motors
up to about 5-6 A @ 75 V on a Series-I sized machine.

> What methods are people using to couple direct drive ball
>screws. Is a keyed sleeve adequate? What about misalignment?
>
>

Unless your motors are wound for extremely low speeds, a belt reduction
drive give several
benefits. It takes care of all the alignment problems, and allows for a
very stout drive system.
Zero-backlash couplings that can take the torque of a 1 HP servo motor
are VERY expensive.
You can mount your encoders directly on the end of the shafts with a
helical-slit coupler that
DOESN'T have to take much torque.

> What size power supply do I need? 7.5A X 3 = 22.5. Is it
>that simple? I assume that 90V is the max drive voltage for the motors and
>that
> lower voltage will work ok but limit torque and speed.
>True?
>

The Gecko drives are not rated for use above 80 V. You won't lose
torque, but you'll lose speed.
But, you may have plenty of that, anyway. You don't need a 22.5A power
supply. That is a
worst-case peak load. A well-made 10 A supply should suffice, and a 15
A would be just fine.
All unregulated bulk supplies are designed to take a peak load when
motors are starting.
Also, the switching nature of PWM power supplies allows them to draw
much less current
at the input than the output is delivering when the output voltage is
well below the DC supply
voltage. This is the most common condition, where the motors are
accelerating from a stop,
or reversing from low speed. The voltage on the motors is low when they
are turning slowly.
And, you rarely need peak torque AND high speed at the same time.

Jon

>
> So many questions. Well, any light the group can shed will
>be a great help
>
> Thanks
> Marc Davis
>
>
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