CAD CAM EDM DRO - Yahoo Group Archive

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

Posted by Jon Elson
on 2002-05-16 10:41:58 UTC
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

Discussion Thread

Mark Taft 2002-05-16 00:58:33 UTC Smaller machine stepper motor speeds and resolution stevenson_engineers 2002-05-16 01:05:51 UTC Re: Smaller machine stepper motor speeds and resolution turbulatordude 2002-05-16 04:52:04 UTC Re: Smaller machine stepper motor speeds and resolution mart_wid 2002-05-16 05:58:24 UTC Re: Smaller machine stepper motor speeds and resolution bjammin@i... 2002-05-16 06:14:57 UTC Re: [CAD_CAM_EDM_DRO] Re: Smaller machine stepper motor speeds and resolution turbulatordude 2002-05-16 09:12:31 UTC Re: Smaller machine stepper motor speeds and resolution mart_wid 2002-05-16 09:43:00 UTC Re: Smaller machine stepper motor speeds and resolution Jon Elson 2002-05-16 10:41:58 UTC Re: [CAD_CAM_EDM_DRO] Smaller machine stepper motor speeds and resolution Tony Jeffree 2002-05-16 11:19:00 UTC Re: Smaller machine stepper motor speeds and resolution JanRwl@A... 2002-05-16 14:41:23 UTC Re: [CAD_CAM_EDM_DRO] Re: Smaller machine stepper motor speeds and resolution bwrfromuk 2002-05-16 17:17:51 UTC Re: Smaller machine stepper motor speeds and resolution bjammin@i... 2002-05-16 18:14:30 UTC Smaller machine stepper motor speeds and resolution tonyjeffree 2002-05-16 23:13:07 UTC Re: Smaller machine stepper motor speeds and resolution