Re: [CAD_CAM_EDM_DRO] Smaller machine stepper motor speeds and resolution

Mark Taft 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..

No, steppers have their best torque at ZERO speed, and the torque falls
off rapidly from there. Motors specifically optimized for high speed operation
and run with a bipolar driver with sufficient DC voltage may still deliver one
half of holding torque at 1000 full steps/sec. (5 RPS=300 RPM), but most
average motors and drivers don't even do that well. That is why people are
always talking about using 800 - 1000 In-Oz motors, to get a usable
250 In-Oz at rapid feed rates.

>
> 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.

60 TPI? These screws have very shallow threads, therefore very little area
to deliver axial thrust. I have never heard of using a 60 TPI screw in a
machine tool table motion application. A coarser thread and belt drive
sounds like a much better approach. I think you will also have a lot of
friction with such a screw. I think the equation for friction of a screw has
a term for the screw pitch, and the friction gets worse as the pitch gets
finer.

> 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 !

Accuracy, or resolution? Don't confuse the two. Where will you get this
60 TPI screw, or will you have to make it yourself? Anyway, a 200 step/rev
motor directly driving a 60 TPI screw will give 12000 steps/inch, or a step
size of .00008333" Are you specifying a 60 TPI screw so you can direct
drive it with a stepper motor? Note that the bearings in most stepper motors
are not designed to constrain the shaft axially, and they will generally
have significant compliance in the axial direction.

Jon