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

> -----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. 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.

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

> 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. 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.

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