Re: [CAD_CAM_EDM_DRO] I'm confused, which are better uni or bi polar ?

Dave!

All else being equal, a bipolar beats a unipolar hands-down! The only
"disadvantages" are you require a DUAL-polarity supply (i.e., instead of just
"plus and minus", you need "plus", "Common", and "minus". Think of two
separate supplies with the+ of one connected to the (-) of the other, and
that tie-point is "common". THREE terminals. Clearly nearly twice as
expensive as a single-polarity supply of same current. Second, the DRIVE is
much more complex, as you need TWO "H-drive" circuits, one for each of two
windings in the motor. The "H-drive" circuit makes it possible to feed DC
into a winding it EITHER direction (left to right, and right to left). While
the drive for a unipolar requires only two "current-sinking" outputs, that
is, simple NPN transistors "to ground". And there are two CENTER-TAPPED
windings on the motor, the CT is connected to "positive-common", usually
through a "ballast resistor" to +24 VDC or higher, and only ONE side of each
winding is energized at a time.

The stepper-motor is VERY inductive. Thus, when a "five volt" motor is fed
with "chopped" 48 VDC so that the AVERAGE current through a winding makes the
average POWER seem like only 5 VDC. Complicated. Requires paper, pencil,
and a bearded old EE-prof with thick glasses to explain. Or Mariss!

You say you have "some unipolar and some bipolar Size 23 motors", and then
ask "which is better/stronger/faster?" Depends on everything else! What
DC-resistance? "Stack-count"? (how many magnetic rotors are in the motors?
A "single stack" (only two "gear-lookin' magnets") is the weakest. Then
there are "two-stack" and "three-stack". You get more torque, the more
"magnetic gears" in there, but more inductance, so, more complex drives
required, and usually, a higher "raw" DC voltage (up to 80 VDC, in some
cases!).

For the drill-press you describe, I'd hope for at least "two-stack" bipolar,
if #23 motors are to be used. Depends on the mechanical loading, and how
fine your bearings and workmanship are. But if everything is very, very
light and you have used perfectly-aligned ABED-5 (are those the right
letters?) and ball-screws, #23 unipolar motors/drives might do. To really
say, I'd hafta SEE your machine.

Links on how steppers work? I am not sure, but I THINK the one by Jones On
Steppers might help, but I'll just give you the link, rather than re-read it
first, and comment on it.

<A HREF="http://www.cs.uiowa.edu/~jones/step/">Jones on Stepping Motors</A>

Lostsa luck! Oh, remember this: Even if you have the "right" motor of the
"right" size for a job, if your bearings are non-sealed and full of sand, and
5° out of axial alignment, and your DC is poorly filtered ("regulation" is
generally not important for stepper-drives), etc., etc., you won't have much
success, and may never realize why! So, work CAREFULLY and "be fussy" so all
is aligned, slop-free, tight, friction-free, and you would not fear flying in
an aircraft made that way! Got me?

Jan Rowland


[Non-text portions of this message have been removed]