Two related questions, Sherlines and Steppers

Please forgive me for this long post .. there actually are a couple of
questions way down at the bottom.


I have a CNC'd Shoptask that's working quite well for me. But a local friend
put his Sherline lathe with milling attachment up for sale and even though
I have no need of such a machine, I bought it anyway !

There wasn't even any static on the home front. It was close to my
birthday, so my wife simply decided that the Sherline would be my birthday
present.

Frankly, I think she hadn't yet bought me anything and this was a simple way
out for her, but who cares WHAT the reason .. a hassle-free new ( to me )
machine is WAY better than the bath salts she was probably going to get me.

I still haven't made much of anything with it but already I'm thinking about
converting it to CNC.

I've ruled out servos, as too expensive and likely unnecessary for the intended
use, which will be 90% of the time as a lathe.

I WAS thinking about simply mounting some of the lovely little gearhead DC
motors I have around and using them as simple powerfeeds on the X and Y.

But then I thought that once a guy has gone to all the trouble of making
mounts and attaching them to the leadscrews, he may as well go all the way
and CNC it.

Now, I have a few choices.

1. I have a couple of 50 oz-in NEMA 23 steppers. They're single shaft and so
a slightly tricky mount involving a pair of pulleys would be required, but no
real problem.

2. My friend DanM has offered to help me out with some double-shaft NEMA23
and appropriate mounts. Quite affordable, pretty simple, and darned
tempting.


Both of these worry me a little bit when considering the Y axis/cross-slide,
as there is a fair bit of mass in the form of the motor levering that aluminum
table up against the plastic and aluminum dovetails.

Also, X axis then requires some drilled and tapped holes into the end of the
ways, making what looks like ( to me ) a bit of a flimsy mount.


3. Mount the Sherline on a metal plate, mount the X NEMA23 stepper on a
bracket attached to that plate. Mount a SMALL stepper on the Y axis, again
connected by a belt and pulleys.


It is this last sentence that I am unclear on .. how small a stepper can I
get away with ? I have a number of small steppers once used in 5.25" hard
drives. The largest of these is

Unipolar
rated ( on the case ) at 24 volts, .25amps
1.5" x 1/5" x 1.5"
.9 degree/step ( I think )
torque unknown

So, here's the questions

1. Is this motor likely to be strong enough, given about a 4:1 reduction ?
( Speed is not an issue )

2. How do I rewire this for bipolar operation ? It's currently 5 wire
unipolar, but the back plate comes off and the wiring is visible and reworkable.

There are eight physical windings, evenly spaced, and 4 electrical windings,
all on one side of the motor.

Obviously, I'm going to parallel two pairs of windings, but I can't quite get
my head around which pair AND the phase of those pairs ( relative to their
current unipolar connection )

Any help/thoughts appreciated.

Alan


--

Alan Rothenbush | The Spartans do not ask the number of the
Academic Computing Services | enemy, only where they are.
Simon Fraser University |
Burnaby, B.C., Canada | Agix of Sparta