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

Very easy. Set your multimeter to 'DC Amps' 10A scale, pick any motor
winding and disconnect it. Put one meter lead to the drive output,
the other to the disconnected winding (ammeter in series).

Single-step the drive until your meter reads a non-zero current.

The meter will read your actual phase current while the motor is
stopped.


Mariss



--- In CAD_CAM_EDM_DRO@yahoogroups.com, "lcdpublishing"
<lcdpublishing@...> wrote:

>
> 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
> > >
> >
>