Re: Max stepper motor speed - rpm

Great info. I have (2) 7/8 dia ball screws and my (4) 36" long THK
rails are limited to about28" of travel when I take out the length and
spacing of the 2 bearing blocks. So I am looking at about 28x28" for X
& Y and I am still up in the air as to Z stroke. I was thinkg
somewhere around 6-10". I want to be able to do flat work and also put
a 4th rotary axis on the table to mill round pieces. Kind of a
universaly capable machine. I have a great scrap pile that I have been
able to salvage some pretty sweet motion items from. My table is a
3x3" X .25" wall tube frame. It was made for something else but I am
able to work with it. I weighs in around 300+ lbs. It was to much for
me to lift up onto the trailer so I just rolled it on.

I have a question about the ball screw. Do you, or have you seen one,
that preloads the screw itself in slight tension to get better frame
stiffness? There are many things to consider like thermal growth and
shrink but I have a very easy way(self adjusting) to do this and was
thinking to stiffen up the frame I would do this. It does put a small
load on the screw bearings and more drag for the motor to overcome but
I think it would give much tighter machine dynamics. I have been
looking around the web at many different designs for about the last 8
years and I definately know what to stay away from for the accuracy
and strength of machine I am looking to build. It wont be a plywood
and round rod guide machine. ;-)

For fun I have built a egg plotter that uses 2 steppers kind of
arranged like a lathe. One stepper with a rubber cup on it holds a egg
with a spring loaded rubber cup on the other end. The second motor
sits at 90° to the first and has a arm to hold a pen or pencil. It was
a fun little project and produced some cool Easter eggs. The only
problem I had was the IMS drivers I had were full/half step only so my
resolution with the pencil looked a little rough. I am hoping to use
the 10 micro step Geckos to see how much better it can look.

Thanks a bunch! Great info.

Scott

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "caudlet" <thom@t...> wrote:
> --- In CAD_CAM_EDM_DRO@yahoogroups.com, "Scott Riddle" <sariddle@l...>
> wrote:
> > I am looking at setting up a CNC router and I am starting to size
> > components like the ball screw and its pitch along with the stepper
> > motor speed and torque needed. One question I have is how to
determine
> > the max rpm you can get out of a specific stepper. There are
basically
> > 3 parts, the power supply, driver, and motor. Assume that the power
> > supply is sufficient how do you calculate the max rpm capability from
> > the driver? Say I start with a Gecko drive that has a freq=200kHz and
> > it is a 10 micro step drive and the motor os 200 steps per rev. Would
> > it be 200,000/(200*10)= 100 steps per second. Times it by 60 to get
> > 6,000 RPM. So the drive is capable. Now is the motor. Would you just
> > have to look at the speed torque curves and see for the desired
torque
> > what speed you can run at? Is there some way to determine this if you
> > do not know the Mfg of the motor?
> >
> > My main reason for this question is that I am thinking of buying a
> > stepper that puts out around 600 Oz-in torque and direct couple it to
> > my .200" pitch ball screw. This would give me good speed capabilities
> > when routing on wood. I am trying to avoid to have to add the extra
> > complexity of a belt system.
> >
> > Thanks
> > Scott
>
> Scott: First let me apologize for Rowlands tactless appoach to
> helping out a new person in a fantastic hobby. The Max RPM of a
> stepper is controlled by several factors. Steppers "jump" from one
> pole to the next. Microstepping tends to smooth that jump factor out
> but there is still a finite time to charge a coil and cause the motor
> to rotate then discharge it and have it charge the next coil, etc.
> What you find in practicality is most steppers are limited to 1000 to
> 1500 RPM and only then if you smack them with sufficient voltage to
> shorten the coil charge time. The other factor to consider is that
> the motor torque curve will drop off with higher RPM. That means the
> you can count on the RPM for non-cutting rapids but not to run a 1/2"
> bit through oak (;-). Most routing is not done at 200IPM and you will
> find that a machine with 120 to 150 IPM max speeds will serve you for
> most wood routing unless you want to invest in a large ($$$$)
> commercial spindle.
>
> Given the constraints listed and your desire to use a .2 pitch
> leadscrew (and direct connection) the math works out like this:
>
> target RPM of motor = 600 RPM = 10 RPS
> .2 in /rev yields 2in/sec = 120IPM
>
> To get that with microstepping, you need 2000 pulses per revolution or
> 20,000 per second to hit 10 RPS. Most of the PC based controllers
> will do that easily for three or four axis.
>
> The 200,000 max input pulses on a gecko is a number that is "there" to
> be observed but few pulse systems can output that frequency of pulses.
> (and your motors couldn't spin that fast anyway)
>
> You can probably push the rapids on up to as high as 200IPM but you
> start to approach the physical limits of the motors, the "whip limit"
> of the leadscrews (a function of length and diameter at a given RPM)
> and the ability of the software to get enough pulses to the driver
> without using step multiplication or an external pulse card.
>
> Keep in mind, I speak in generalities. I have seen claims where
> steppers are being driven to 3000 RPM but those are on optimized,
> tuned, matched systems.
>
> Depending on the size table you plan on building you might find that
> 600 oz-in motors are overkill. A lot of good screw driven router
> tables have been built with 250 to 350 oz-in motors. The screw gives
> you about a 6:1 torque multiplication so 600oz-in turns into 3600oz-in.
>
> Let us know what you plan on cutting and the work area you are needing
> and we can comment in more detail about the mechanical and electronic
> needs.
>
> Thanks for your post and your participation in the form.
>
> Tom Caudle