Re: [CAD_CAM_EDM_DRO] CNC Servo motor gear ratio: overheating. EMC step rate. Table speed

> >We've purchased 620 oz in servo motors from Dan Mauch (Camtronics) with
> >500 step per rev encoders.
> >
> >We are planning to direct drive a 4 or 5 TPI ball screw.
> >
> >My calculations show that 600 oz direct driving a 5 TPI ball screw will
> >generate a force of 1059 lbs.
> >
> >The reason for wanting to direct drive the screw is speed. EMC
> >supposedly has a maximum step generation frequency of 5 KHz. 5 KHz/ 500
> >steps per rev = 10 revs per second or 600 RPM. 600 RPM x 0.2 = 120
> >inches per minute, which is slower than we want. We were thinking of
> >using 4 TPI ball screw to get the speed up to 150 IPM.
> >
> >Question: will the motors suffer from heating problems ? At a feed rate
> >of 5 inches per minute, the motors will be turning 25 RPM.
> >
> It is hard to say. Is there a continuous torque or continuous current spec
> on these motors?

I'll ask tomorrow.

> If so, you can make an attempt to see whether friction
> and cutting forces (if any) will exceed this spec. Always throw in a fudge
> factor for unanticipated drag. Anyway, if 25 Lbs linear force is required,
> add a generous 20% fudge for the ballscrew and you get 25 * 1.2 = 30
> 30/1059 = 0.028, or 2.8% of peak torque. (I'm assuming the 620 Oz-In
> is the peak torque, otherwise, this is a HUGE motor).

What is the relationship between peak torque and continuous torque on a
stepper motor ? I thought the torque curve was constant with RPM on a
stepper motor.

> it seems pretty
> safe to assume that running at 2.8% of peak torque, it is not going to
> overheat.

If one calculates the work being done by the motor, it isn't that much.
The issue is the current required to get the torque to turn the screw
under load. They might be lugging in direct drive. I think Dan uses
them on a mill with 2:1 reduction. All the real VMCs use direct drive
and are speedier than what I am proposing.

> With the Universal stepper controller, I've run some 200 Oz-In size 34
> steppers
> at 4800 full steps/second (48,000 steps/sec into a Gecko201A). That ran a
> mini-mill to 90 IPM with 16 TPI screws. 48000 was the limit for these
> motors, the USC can go to 300,000 steps/sec.

I'm driving special drivers from EMC via the serial port.

> Are those 500 cycle/rev, or 125 cycle/rev encoders? (You get 4 counts from
> every black/white cycle on the encoder disc.) If they are 500
> Cycle/rev, you'll
> actually get 2000 counts/rev.

Dan and I went through a resolution calculation and we'll get 2000
pulses per rev from the motors into the driver. This means the drive
resolution is 500 pulses per rev.

> (you can get more info on my USC at
> http://jelinux.pico-systems.com/univstep.html )
>
> Jon
>
>
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--
Kim Lux <lux@...>