Re: Servo or sterpper motors. on small mill?

Paul,

Good point. (Higher accelerations for servos)

I wonder how the throughput would be affected if you used a higher
feed rate with exact stop mode instead of CV?

For newbies, the corner rounding Paul experienced is caused by a CNC
feature called CV (constant velocity). This is where the machine
tries to "drive" the toolpath as if it were a racecar driver (as
fast as it can go, only slowing enough to get around the corners)
Which means that the faster you go the more "you" must round the
corners.
There is another mode for CNC called exact stop. This makes the
control stop and start at each "line" of the toolpath. So there is
no corner rounding at all. But the top speed will be less for
shorter lines. Think of that racecar driver again. If he or she has
to stop at every turn; the distance between turns will limit how
fast the path can be driven. BUT, for some toolpaths,
the "straightaways" may make up for the curves.

And sometimes ANY corner rounding is unacceptable.

FWIW, often G64 is used for CV and G61 for exact stop.

Hope this helps,

Ballendo


--- In CAD_CAM_EDM_DRO@yahoogroups.com, "paultitchener" <pt@...>
wrote:

>
> I think this point below, from a link listed by a previous poster

is

> an important one that sometimes people miss-
>
> "1) Dynamic response, accelerate faster when starting and

stopping,

> and changing direction."
>
> Its not quite as simple as saying that servos will have faster
> rapids, they also will be able to do cut workpieces that require
> quick changes in cutting direction more quickly.
>
> I have steppers on my BP sized mill, and its been a great machine

for

> me. However, sometimes the stepper limitations slow me down, last
> time was when I was engraving a prototype case. When engraving you
> want to run as much RPM as you can, 5600 in my case, and at that

RPM

> you'd like to be able to feed at 10 to 20 ipm. However with my
> steppers there was way too much rounding of the corners of the

text I

> was engraving, and I had to slow down to 5 ipm or so to get rid of
> the excess rounding.
>
> I didn't start from scratch with this machine, I bought it with

the

> motors in place and a blown controller, but if I was starting from
> scratch I'd go the servo route, not just for faster rapids but

also

> for faster cutting without accuracy loss.
>
> Paul T.
>