Re: Force through ball screw based on stepper motor torque??

Thanks Mariss,

So, based on what I have

3.14 * 5 * 200 / 8

15.708 * 25 = 392.7 pounds of force

Now, compared with what is actually measured, 75 pounds of force

Saying that a ball screw is 80% efficient 392.7 * .8 = 314 Pounds of
Force. 20 percent loss is a lot for a ball screw, but these are
running with dry lube so I am trying to account for that.

314 - 75 = 239 pounds of force Missing In Action

So, factoring all that, I am still at a loss of 239 pounds of force -
somewhere?

This is the deep mystery....

None of the slides bind, nothing is cocking or twisting to cause the
binding, etc. Yet, I end up with only getting 20% of the expected
force I should have.

At this point, I am not sure where to look next. I have had several
other guys (very familiar with CNC machine and mechanics in general)
and they too could not find a mechanical problem. So, I am now
wondering if it is electronics or perhaps the drive.

The power supply is unregulated and has more than enough capability
for the drives and motors. At this point I am believing that the
the lack of force is caused by the stepper driver.

Is there a way I can measure the output from a unipolar stepper
driver to see if it is putting out the amps needed? I have 6 wire
stepper motors - unipolar.

Chris




--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Mariss Freimanis"
<mariss92705@...> wrote:

>
> Neglecting screw efficiency:
>
> Lbs = pi * TPI * in-oz / 8
>
> Mariss
>
>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "lcdpublishing"
> <lcdpublishing@> wrote:
> >
> > Hi guys,
> >
> > This problem (if it is) has been bugging me for some time now.

On

> > my CNC router, I can only get about 70-80 pounds of force on any

of

> > the 3 axes. I am using the cheapo rolled ball screws 5/8

diameter -

> > .2" pitch and they are attached to the ball screws - direct

drive.

> >
> > The static load on any of the axis is less than 5 pounds of

force.

> > In other words, to move any axis and overcome it's friction, it
> > takes less than 5 pounds of force.
> >
> > I have two different brands/types of motors. Only one of them
> > is "modern" and it is a chinese made uniplolar 200 oz-in motor
> rated
> > at 3 amps. It is a six wire motor and does run real "zippy" -

much

> > more responsive than the motor it replaced - an older round

style.

> >
> > Anyway, even though this machine is working good, I am using it

as

> a
> > basis for determining the specs for some other machines I am
> planing
> > on converting to CNC. When I did the original calculations for
> > thrust force, I came up with much higher numbers than what is
> > actually showing up on the machine.
> >
> > I am wondering if someone can run the calculations and tell me

what

> > they come up with as to what force I should get on an axis with

the

> > following:
> >
> > Unipolar 200 oz in motor @ 3 amps, 5/8" x .2" pitch screw,

direct

> > drive.
> >
> > Oh, and the driver is running at 40 volts & 3 amps
> >
> > Thanks folks!
> >
> > Chris
> >
>