Re: formula ?

Dave,

Reading this just makes me want to slug Sir Poncelet. Or get on a
horse and run furlong ways... And as to what that poor girl Ellie did
to deserve the flames, I just don't know<G, for groan>

"Normally", you'd take a few wraps of cord around a rotating drum
driven by whatever you are testing; fasten the free end to a fish
scale (or better quality device suited to wahtever range of force you
expect to encounter), and then gradually apply tension to the "free"
end of the cord, and read the scale until failure. Repeat til
satisfied of the consistency of the measured results.

In this case, such a setup may not work. Besides, this is an
empirical experiment. What we REALLY want/need to know is how much
REAL force our drive systems have at operating speeds...

So I"d try three different tests. First, add some weight, program a
fixed, very slow speed(slow jog, or slow homing rate). Repeat, adding
weight, until the axis drive fails to BEGIN moving the load.

Next, I'd try tests with accelerating moves; again adding weight
until the axis fails. Again the axis has to "start" the full weight
moving. This would tend to let me know what I REALLY have. And what
sort of "margin" I REALLY have at typical cutting and rapid speeds.

Finally, you could use the results of the 2nd series of tests to help
set up this third series. Here we will use the bucket to "add" weight
AFTER the axis is moving. (This will be similar to the pony brake
method first mentioned above.)

Personally, I'm most interested in the first two series. You know, if
we all did these tests on our "homebuilts", we would have some very
valuable REAL info about motors, drives, screws, etc... and how they
relate to necessary cutter forces...

Another useful (and often asked for) set of data would come from
discovery of just what ARE the NEEDED forces for our typical cutters
and spindles... This would be a bit tougher to determine, but varying
the current in our axis drive (for each "pass" of a test) should give
us better than the "guesses" which seem to guide development now.
Even the motor/screw sizing software provided by most motor mfrs.
requires the input of an amount for "the desired force" of the axis.

I'm sure I'm not the only one who worried about how much force I
NEEDED to cut wood, plastic or metal when cobbling a machine
together. I have to admit, my first machines just used the C.O.P.Y.*
approach, followed by a TLAR** empirical review<G>

Hope this helps,

Ballendo

*C-onsider O-ther P-eoples results Y-our own<G>

** T-hat L-ooks A-bout R-ight (a corollary to TLAR is TSAR; S=Seems)

--- In CAD_CAM_EDM_DRO@y..., "dave_ace_me" <davemucha@j...> wrote:
> Hi Ballendo,
>
> I really did just make an experiment.
>
> the manual set-up that I ran for years would see table travel with
> less than 5 pounds of weight on a string and a pulley. Actually
more
> like 3 pounds.
>
> Now for this test, I can put a bucket on a string and fill it with
> water. and since I need to pull on the leadscrew (too thin to
push)
> I can run the table and lift the bucket.
>
> Assuming the most power if available at low speeds, I should be
able
> to lift quite a large weight.
>
> one thing I have not done. (Seems once you exceed your spec's you
> stop checking)
>
> If I run these VEXTA (5v 1.5a) NEMA23 steppers on a Gecko running
at
> around 65 volts, I should be able to exceed 5,000 Torrecelli
pressure
> on the leadscrew and although it may only be about 0.01 Poncelet,
it
> should lift a 3 slug bucket at around 0.05 knots.
>
> So my question is really basic. Since I can only lift a 3 slug
> bucket 0.003 Furlong, do I just repeat with additonal weight until
> the stepper starts missing steps? or do I need to use the higher
> power, slow steps and not try for a higher speed? Or should I do
> both and see what I can get out of high speed and low speed in
> seperate trials?
>
> At this point I am glad you asked. Since I will be switching to a
1
> inch leadscrew haveing this set of data will allow a valid
> comparrison with that new ACME screw.
>
> questions, questions.
>
> Dave
>
>
>
>
>
>
>
>
>
> --- In CAD_CAM_EDM_DRO@y..., "ballendo" <ballendo@y...> wrote:
> > Hello,
> >
> > And don't forget to de-rate that figure by the friction of your
> 12/13
> > allthread drive...
> >
> > Maybe 30% efficient (if that), so now you're at 153 lbs. Hmmmmm...
> >
> > You know, I'm a bit in favor of the "Mariss" approach myself;
that
> is
> > to use an experiment in addition to the calcs.
> >
> > I wonder what the "real world" result will be if you put a "fish
> > scale" on the driven part of your system... Run it slow, and let
us
> > know, please. (I'm guessing less than 100 lbs actual)
> >
> > Okay, that might be a big fish scale<G> but even some workout
> weights
> > and a rope and pulley will give us some idea of where "reality"
lies
> >
> > Hope this helps,
> >
> > Ballendo
> >
> >
> > --- In CAD_CAM_EDM_DRO@y..., Jon Elson <elson@p...> wrote:
> > > dave_ace_me wrote:
> > >
> > > > Hi Jon,
> > > >
> > > > great explanation. I couldn't get the concept that the lead
of
> > the
> > > > screw was part of the circumference part of the equation.
> > > >
> > > > OF course that makes sense, a 1/2-5TPI 1/2 screw or a 2"-5tpi
> will
> > > > still move the table one inch for 5 revolutions. so the
actual
> > > > diameter or circumference of the screw is not important.
> > > >
> > > > I noticed that we rate steppers in oz/in but power in inch/lb.
> > > > Can I assume that 16oz/inch is the same as 1in/lb?
> > >
> > > I believe this is correct. There are so many torque
measurement
> > units, its
> > > a mess. N-M, N-cm, gm-cm, oz-in, lb-ft, .........
> > >
> > > > That would make a 50oz/in stepper 3.125 inch/lb. Fits close
to
> > your
> > > > 10 inch /lb value. Yours seems to be 150oz/in.
> > > >
> > > > so my 50in/oz at 2:1 is 6.25 in/lb on the leadscrew. The
13tpi
> > > > (0.0769"circumference/0.02448dia/0.012243radius) then 6.25 /
> > 0.012243
> > > > = output force
> > > >
> > > > 6.25 / 0.012243 = 510 pounds of force. That kind of force
> will
> > bend
> > > > the noodle of a leadscrew I am using.
> > >
> > > Yup, sounds good. but, remember, stepper ratings are good at
> zero
> > speed,
> > > only. They drop off very rapidly as speed increases.
> > >
> > > Jon