Re: Question about steppers

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Will" <nmbmxer@y...> wrote:

>

Hi Will,

It would seem the quadralift
http://www.ktmarketing.com/quadraliftpictures.htm

can be run from a single motor.

I am not sure how you determined torque for the screw, but a torque
wrench is not going to offer much help.

Also, the knee of a mill, or the head, are not rapid or frequent
movements.

Often used only for set-up.

For a setup that take 15 minutes to load and tram the part, another
minute to zero should not be a big deal.

Also, where is the zero ? the table or the part ?

I can understand the desire to CNC it, but from the pictures, the
rails look like they are not ballslides, but rods and bushings.

I think (personal opinion) that the wear can be excessive in a CNC
environment, unless you pay attention and address it.

Also, with a part zero, the quill would be a good candidate for
CNC. Drilling, especially peck drilling will be hard on the machine
with a head lift as compared to a quill feed.

And, what people do on mini-mills (ie: MUCH smaller machines) is to
add a counterbalance weight. that makes the head lift a matter of
positioning, and not high torque.

Assuming you have a 3/4-6 ACME thread, you need 6 revolutions per
inch of movement.

As you mentioned, there re 14 inches of travel.

so, 6 revolutions per inch and 14 inches is 84 screw revolutions.
(if the screw has some other pitch, you will need to re-calculate)

Assuming you will get 400oz-in at 1,000 steps a second, and 200 steps
per rotation, that is 5 rotations per second. (full, half, micro have
no effect on this at this point)

With 84 totla revolutions and 5 rev per second, that is 16.8 seconds
for end-to-end travel.

Make the pulley 3:1 to triple the available power and you slow that
way down to 50.4 seconds. Allowing for ramping and all that might
drag that out to the leisurely pace of 60 seconds.

Personally (again with the opinion) I would tend to think that is
acceptable for an axis that will not be used as often as the quill.
You can run the motor faster and the screws may be different, so your
actual performance will probably be different.

Now, the comment about microstepping.

The motor has a natural step movement that is based on how the motor
is physically built. and for most of us, that is never going to be
changed. Electronically, we can manipulate the driver to create
half, quarter and microsteps. but the motor did not change.

The motor power may have a slight change, but that will be small and
often not discussed as there are many more power hungry components.

Moving the motor at 1,000 steps a second as noted above would require
10,000 steps a second with a 10x microstepper. Easily attained by
both the driver and controller on almost any platform.

In review, the motor looks acceptable, (let us know the $$ of the
motor) the screws look acceptable. the speeds should be in the range
of serviceable and by adding a counterbalance, the torque should also
be fine. Adding a counterbalance will allow the motors to spin
faster.

Personally, I like steppers for this application, but there is no
clear advantage (in my mind) to use servos over steppers or steppers
over servos in this application. If you want to go a little faster,
considder 2:1 pullys. If you want to go a LOT faster, think servos.
But, this appears to be in the both will work zone, and in my opnion
favors steppers.

My fear, and that surpasses all others is the quadralift rails. they
seem like a major wear point.

I have expressed opnion through out as well as some technical
details. Use your judgement or ask more questions.

Dave