Re: Question about steppers

Thank you very much, this is exactly the type of answer I was looking
for. The mill head is quite heavy, think of it as the knee on a bp,
no backlash b/c weight (it also has dual leadscrews and I'm not
positive that they are timed perfectly, ie, some backlash comp.) I
dont plan on slinging it around at 1000ipm like a real mill will, and
the weight of the head will help help with cutting forces.
I will probably have to use a speed reduction I suppose, as its 960
oz-in with out a reduction (it wouldn't click my torqe wrench no
matter how fast I yanked the handle.)15 ipm is kind of slow for a
rapid, so I suppose I will need a larger motor than that one. It
would have 14" of z-travel if I use the millhead as the z-axis, I
think it will be annoying to have to wait 60sec just to home the
millhead. I also figure, its cnc, its not like I have to stand there
and spin a handwheel for a minute and it takes alot longer to run it
up there with the air rachet and my tiny compressor. Unless I'm
drilling a hole, the z just kinda plunges down slow, and then
retracts. It doesn't do everything like the x and y do.
Worst case, I just move whatever motor I buy to the spindle and just
keep running the head up and down with a rachet.
The spindle runs off a rack and pinion and worm wheel arrangement, it
has .075" of lash, and thats after I removed the drill press function
and tig welded all the gears on the shafts. If I cnc the quill I'll
have to come up with a vertical ballscrew arrangement and with 3" of
travel I would have to move the millhead 3 times to spot, drill, and
bore a hole, cam doesn't spit out code like that. Much rather just
deal with slower peck drilling cycles than fool with moving the head
around and making different sets of code for every millhead move. It
would also make the tool library difficult as I would pretty much
have to touch off every tool when I wanted to use it. It's cnc, no
matter how slow it is, its still probably faster than me doing it by
hand. .

William
PS Sorry for the long post


--- In CAD_CAM_EDM_DRO@yahoogroups.com, "cnc_4_me" <cnc4me@g...>
wrote:

>
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "Will" <nmbmxer@y...> wrote:
> >
> > I want to use a stepper to control the quardalift on my shoptask
> for
> > the z-axis. I know that 960 oz-in will comfortably turn the
> > leadscrew, I am planning on reducing it between two and three to
> one.
> > That puts my torque requirement between 320-480 oz-in.
>
>
> I will believe you on the torque you require if you are just

talking

> about raising and lowering the head, and not doing any cutting in

the

> process.
>
>
> > The leadscrew
> > is 10tpi. My question is this, if I use a stepper that is 1.8
> > deg/step, .9 deg/ halfstep, can micro step it to 800 or 1600
> > steps/rev?
>
> NO. Micro stepping is used to eliminate motor resonance. The

micro

> steps them self's cannot be considered true positions. You can

only

> depend on a full or half step for position when using micro steps.
>
> With a 10TPI screw that would be .1" lead. So .1" / 200 full steps
> = .0005" per full step. Half step would be ½ of that or .00025 per
> step.
>
> With that said if you are trying to use the Z screw to move the

head

> and cut material you have other problems to deal with than
> theoretical resolution. For 1 thing the accuracy of the screw is
> probably .003" or so a foot. And you have backlash in your lead
> screw nuts, maybe .03" to .06"
>
> > I am trying to compare the torque needed to lift the millhead

with

> > the Speed/Torque curve to see how fast I can make the millhead
> move,
> > at least in theory. I am having trouble reading the chart. The
> chart
> > for the motor can be found here (it's the N 20 3451):
> > http://www.clickautomation.com/PDF/items/MAE%20Express%20NEMA%

2034%

> > 20Step%20Motors.pdf When the chart lists steps, is that full 1.8
> > deg, .9 deg, or micro steps?
>
> First, good work on looking for a motor with a torque curve.

Without

> this curve you are only blindly guessing if it will really work.
>
> Hmm, looking at that chart I can see your problem. It isn't
> labeled. But most of the time the charts are full step and

sometimes

> show full and half step. Usually there isn't too much difference
> between full and half step. Full step is usually worst case and I
> would think that is what the chart is in.
>
> For your application you would use the 60 volt curve. Gecko drives
> can handle 80VDC max.
>
> The oz-in on the vertical axis is straight forward but look closely
> at the horizontal axis. It is in steps per second (telling me for
> one thing that the chart is full steps, otherwise it would say
> half/steps per sec). To convert steps per second to RPM take 1000
> steps per sec / 200 Steps per rev = 5 rev per second x 60 =

300RPM.

> For half steps the RPM would be ½ of this or 150RPM for 1000 half
> steps. Working in full steps lets just make a quick chart.
>
> 200 steps/Sec = 60 RPM = 850 oz-in
> 400 steps/Sec = 120 RPM = 850 oz-in
> 600 steps /Sec = 180 RPM = 700 oz-in
> 800 steps /Sec = 240 RPM = 566 oz-in
> 1000 steps /Sec = 300 RPM = 480 oz-in
> 2000 steps/Sec = 600 RPM = 270 oz-in
>
> I would say your usable RPM range is up to 300 RPM. Now we can
> calculate your maximum speed the head can move. 300RPM x .1" lead

=

> 30 IPM or 30 inches per min.
>
>
> My father is handling all the
> > electronics for this project; I'm doing the more mechanical side,
> > though that is a bad reason for why I don't know much about micro
> > stepping. Our needs overlap in the area of motors, I know how

much

> > torque and what speeds I want everything to move at, he knows

what

> > drivers we're using (Gecko, I think that limits me to what, 10A

and

> > 80V?)
>
> Gecko stepper drivers can run up to 80VDC at 7Amps.
>
>
> I'm not dead-set on that particular motor, it's just one I
> > found that was cheap and had a torque curve.
> > William
>
> Well now you can make your decision on whether this motor is

suitable

> or not for your application. And you have the tools to use any

chart

> now for motor specs. And calculate RPM and IPM.
>
> Wally