Re: [CAD_CAM_EDM_DRO] stepper vs servo

cadcamcenter@... wrote:

> Jan Rowland, Troll wrote: <....."feed back" the position information
> to the controller (computer, etc.), so, if something
> should "snag", the machine will "keep on trying" to get where it was
> headed.....Next question? >
>
> Next question:
> B
> /.
> / .
> / .
> / . 7 steps
> / .
> / .
> /.......
> A
> 7 steps
>
> Suppose a servo system is executing G01 X7 Y7.
> On the third step Y stalled.
> Will X also stall in sympathy or proceed on?

Servos don't stall, unless the physical limits of the machine are reached.
They can slow down, however. Some systems (the Allen-bradley 7320
I worked with before, for instance) had a feature called feed-rate
compensation.
Whenever one axis lagged more than some limit behind commanded position,
all axes cut their feedrate in half, hoping that would allow the lagging
axis
to catch up.

>
> If Y doesn't stall at the same time, but proceed merrily for 2 steps
> before X recovered sufficient torque to continue will the path AB
> look like this?
>
>
> B
> |
> |
> /
> __/
> / .
> / .
> /.......
> A
> 7 steps
>
> Please note, at the end of the move, B is still at x=7, y=7 from A,
> no missed step, but it is not the straight line intended!
>
> Hope this is incorrect, otherwise the servo may be not that much more
> better at avoiding spoilt parts than steppers.

The problem with steppers is that once a step is lost, it STAYS lost.
The servos catch up, and resume accurate work. It is most likely that
lagging of an axis will happen on roughing passes, not finishing passes.
And, it is most likely that the lagging axis will leave extra material in
place,
rather than remove extra material, and a later pass can clean it up.

Servos, in general, have a LOT more power than steppers of similar size
and electrical power. For instance, my 1/8 Hp (continuous rating)
servo motors, with a peak power rating of about 300 W, can deliver speeds
of 120 IPM, and linear forces at the table of 1000 LBS! I'm pretty sure
most stepper systems don't come anywhere near this.

I have snapped 3/8" end mills off in the spindle while jogging carelessly,
without even causing a following error!

Jon