On Wed, 8 Oct 2003, Stefan Trethan wrote:
> I see..
> all say: most important: KEEP IT SIMPLE.
Absolutely.
> It just hapens i have some of this Molybdenum grease (in a spray can)
> here. So i will use it.
Give it a try. For what it's worth, moly grease is good for lots of
stuff. I didn't have any tapping fluid, and needed to tap about 30 holes
in some 1/4" steel plate (5-6mm). Turns out moly grease makes a great
tapping compound. It's just messy messy messy.
> The motors, again...
>
> I wonder wich power the steppers should have.
> 200 steps is ok?
200 steps per rev should be fine. If you wind up with a microstepping
driver, all the better. Regarding microstepping drivers: Unless the
driver has been tuned for the particular motor you're using, don't expect
the microstepping to give you more positional accuracy. It will, however,
make for smoother motion.
> Most motors are rated in volt / ampere.... I know it would be better to
> compare them by N/m but the ones i have are not labeled in torqe...
That's a shame they're not rated by torque. You can always measure this,
though. Mount the motor to your workbench (clamp, screws, whatever), and
mount an arm on the shaft of the motor. Hang weights at a fixed distance
out on the shaft while the motor is energized. When you can get it to
slip, you know the holding torque of the motor. Measuring the torque of a
moving motor is a little trickier, but for a first-pass measurement the
holding torque works well.
> I have some, but they are too small i think... So please anybody having
> firsthand experience please tell me what is needed.
You can use the same trick to find out how big a motor to mount on your
machine, but it means you need to build the machine first: Stick an arm
on the leadscrew and start hanging weights. When you can just get the
leadscrew to rotate, you know the minimum torque necessary to move the
thing. If possible, get a motor with at least four times the holding
torque, preferably larger.
> I also wonder if the main axis should be driven by one or two motors. If
> there is only one motor the screw is centered. or two screws are coupled
> by tooth belt... isn't there the problem of carriage skew if the
> spindle is centered and the load applied on one side only?
Yes there is. There is also the problem of skew if the belt slips. A
toothed belt should not slip, but it does happen for a variety of reasons.
Unfortunately this has happened on a machine at work. Luckily nothing was
hurt. If you go this route, you can always rig up motor kill switches
that engage if the bridge skews.
Tom