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

Mariss,

I finally got to run the test, not on the same machine, but the same
drivers etc.

The current readings go from -.67 amps to +.26 amps (minus .67 to
plus .26) for what I am interpretting as a total of .93 amps. This
driver is setup for 1/4 stepping and for 1.4 amps - it is a unipolar
driver.

Now, if I was getting around a .43 total amp reading, I would feel
comfortable saying the "relative performance" between the two test
machines is the same. However, I can't say that for certain. But,
based on the ammeter, the driver is showing that it is providing
less current that it should - about 40% less.

Am I interpreting this correctly?

Thanks

Chris






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

>
> 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
> > > >
> > >
> >
>