Re: [CAD_CAM_EDM_DRO] Servo not stalling, a problem?

r_fl_z@... wrote:

> I'm considering switching my design from steppers to servos. I've
> read that a servo NOT stalling can cause design problems. What
> problems are these?

If some kind of NC program error causes the CNC control to issue
1,000,000
steps/second to the stepper motor, it won't move at all, it will stall.

If a similar error causes the CNC control to command a velocity of 1,000
IPS,
and the servo amp can accept such a command, it could break the
ballscrew
or send the machine table through a wall. This is actually possible on
some
of the highest-end gear now being made.

It is not likely in anything we are building here, however. Most servo
amps
in the under 1KW range are not capable of this sort of disaster. The
CNC
control should not accept invalid commands and cause the machine to
exceed the limits set for it. EMC certainly allows you to set
reasonable
limits on acceleration and max velocity.

At least with my servo amps, when a crash ocurrs, either the servo amp
gets an overcurrent fault and puts the entire machine in E-stop, or
whatever
was interfering breaks off, mostly by the inertia of the table. I HAVE
broken
off 3/8" end mills in the spindle of my machine when carelessly jogging
at
high speed. In that case, the inertia snapped the tool off without even
causing
a fault.

If a servo amp fails and causes a runaway axis, the CNC control should
quickly detect a large positioning error and cause an E-stop. EMC will
definitely do this within a couple of milli-seconds. I've been running
with
my servo amps for about 2 years, and never had anything like that
happen.
I can't comment about experience with other systems.

Jon