Re: E-Stop switch

Hi,
In reply:

E-STOP in tightly coupled hardware/software system will (and
DO) work (and is NOT nonsense, and IS used in industrial CNC
machine tools) as long as there is also a fail-safe mechanism built-
in in case software fails. This is typically know as a watch-dog.
If software fails to trigger the watch-dog, all systems will shut
down. The watch-dog is external to any software.

I did not say E-STOP should be used for normal machine tool
operation, but in practice, FEED-HOLD and E-STOP ARE the most often
used methods of stopping a machine, and E-STOP, because of its' easy
access and stopping ability is used VERY often. Many E-STOP
initiations are not true EMERGENCY conditions, they are rather
PRECAUTIONARY system halts where the machine operator wants to halt
motion

If one is concerned about cutting power immediately to the
machine , then one should use the POWER-OFF button which should also
be very easy to reach (typically right next to the POWER-ON button).
E-STOP is not, and should NOT be used an alternative to a POWER-OFF
circuitry (although for small desktop type machines that don't have
tightly coupled hardware/software E-STOP systems, this may be a
typical solution).

I would assume that residual movement after shutoff [E-STOP] would
be a concern for almost all CNC machines. Placing a 'simple brake
clamp' on each axis may be a solution (albeit expensive, and not
necessarily so simple); Imagine a 20 HP (or more) spindle spinning at
10,000 rpm (or more). Now hit E-STOP. You certainly don't want the
spindle to free-wheel. Yes, you could put a brake on it too, but in
a reliable, tightly coupled hardware/software system where software
is NOT the problem for greater than 99.999% (my own estimation) of E-
STOP initiations (remeber E-STOP is often used in NON EMERGENCY
situations), software will work fine to bring that spindle to a quick
and safe halt. The other less than .001% of the time that
software 'might' be the problem, the system will be shut down by the
watch-dog circuit.

As mentioned, this is not how I think things should work, this is
in-practice how many large CNC machine DO work (regardless of what
my, or others opinions are).

Jeff
http://www.xylotex.com

> Seams opinons on this vary somwhat.
> I feel that a E-Stop button should not be
> used for the normal opperation of a machinetool.
> It should be located within easy reach of the Operator
> and clearly Marked as such.
> When activated it should cut the Power to all devices
> attached as close to the Powersource as posible.
> Incorporation of a E-Stop in Software is nonsence because
> this very software may have caused the reason for the shutoff.
> Sence the reason is not known all power should be cut.
> On Machines where residual movement after a shutoff may cause
> reason for concern a simple bracke clamp should be installed
> which clamps onto the axes in case of power shutoff.
> Just my 2 cents worth :-)