Re: Smaller machine stepper motor speeds and resolution

At 17:41 16/05/2002 +0000, you wrote:

>--- In CAD_CAM_EDM_DRO@y..., Mark Taft <mark.taft@m...> wrote:
> > HI everyone,
> >
> > I am converting a small mill to CNC and have been considering
>stepper motor
> > speed and resolution of the z feed.
> >
> > Its obviously a compromise using steppers, so you need to gear the
>feed at a
> > rate which is practical ( fast as possible) while not loosing
>accuracy
> > resolution or losing steps ( being under powered).
> >
> > I have studied the stepper motor specs for the smaller 34 and 23
>sized
> > motors.
> > They seem to get most torque at about 900 - 1000 Hz..
> >
> > With a 200 step motor this works out at about 4-5 Rps (revs per
>second) ,
> >
> > using a lead screw of 60 tpi and a 4 inch travel ( maximum) it
>takes about
> > 60 seconds.
> >
> > I am picking a 60 tpi screw for increased torque.
> >
> > My question is this, from experience , what is the maximum
>practical feed
> > rate for small motors and what sort of resolution should I aim
>for ?
> >
> > 1/2 thou accuracy is ok for me !

As others have pointed out, 60TPI is a bad choice if you plan to do any
serious work, on grounds of speed and fragility.

With 200 steps/rev, a 2.5TPI screw will give 1/2 thou per full step (1/4
thou if you are half-stepping) which is your target accuracy. So anything
finer than 2.5 TPI will apparently meet your spec.

What you don't mention is the size of the mil you plan to convert, apart
from the max travel of 4" (sounds like a very small machine). To put things
in perspective, the small end of the desktop mills (e.g., Taig, Sherline,
MaxNC, all of which have X/Y/Z travel around the 8"/5"/7" ball-park) use
20TPI screws, with 200 steps/rev motors running half-step - giving you
1/8th thou/half-step. Motors of 100-200 oz-in work well with these machines
- my Taig for example started life with 140 oz-in motors - now has 200 oz-in.

If the machine you plan to convert is as small as it sounds, 20TPI would be
an OK choice, and would give you an order of magnitude more resolution than
you seem to need. At 1000 Hz, and full stepping, you would get 1/4" per
second - 1/2 IPS if you go half step.


Regards,
Tony