RE: [CAD_CAM_EDM_DRO] CNC Servo motor gear ratio: overheating. EMC step rate. Tablespeed

On Sun, 2003-06-08 at 10:58, Tim Goldstein wrote:
> > -----Original Message-----
> > We are using EMC as the controller. I've read it has a
> > maximum step generation rate of 5 KHz.
>
> This is not true. I am not sure where the rumor started, but it comes up
> frequently.

My reference to it came from the EMC users manual, the chapter about
setting up the ini file settings. I don't have the manual available at
the moment, but there is a maximum pulse sort of setting in there and
the manual recommends time constant such that the max is 5 KHz. I am
happy to hear it will go above this, but I wish I would have known this
before ordering the motors.

> I am running an old version of EMC on an AMD K6-450 and very
> successfully getting 25 KHz. The pulse stream is not as evenly paced as
> some other controller programs, but with Gecko servo drives it does not
> matter.

err... OK. What do you mean by "not evenly paced" and doesn't an uneven
pulse stream affect the co ordination of the axes ?


> > 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 600 oz/in figure is a peak measurement and you can not expect to
> drive them at that level for more then a very tiny amount of time.

I'm surprised to hear this.

> You can guestimate the continuous torque rating to be 1/5 - 1/10 of the
> peak.

That would make them tiny in comparison with a stepper motor ! Are you
sure about this ?

> > 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.
>
> Yes your motors will suffer major heating problems set up that way. If
> you want to run at that low an RPM you need to use steppers. If you will
> use a reduction drive on the servo motors and get them spinning you will
> have a very successful servo machine.

What is the upper RPM limit on these motors then ? It seems like I
don't have enough information to design this system.

BTW: how are you figuring that they will heat up ? ie what sort of
equations are describing the system ? Do servo motors have practical
low RPM limits that have to be observed ? Is there a cutoff for low RPM
operation at certain torque values ?

> I would say that you have two
> choices to use the servo motors you all ready have. First off you can
> expect EMC to give you 20 - 25 KHz pulse rate. Then you can either get
> lower resolution encoders to drop the required step rate with the motor
> geared down or you can get some Gecko G340 or if you have the G320 get
> the G901 pulse multiplier. Either way will allow you to get the motors
> running at the RPM they need to generate the speed you want without
> burning up the motors.

What speed do they need to run at ? This is the first I am hearing of
this.


> Tim
> [Denver, CO]
> Sherline products at Deep Discount
> Mach1 & DeskCNC with credit card ordering
> www.KTMarketing.com/Sherline
>
>
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