Re: [CAD_CAM_EDM_DRO] Crashing CNCs: what gives ?

Kim Lux wrote:

>I keep hearing the same thing: some sort of settable limit should be
>employed to keep the tooling from doing something stupid. The potential
>for disaster bothers me most on our lathe. The more I think about it,
>the more I'd like some sort of movable mechanical limit switch system.
>The range of tools we use on the lathe is pretty big, so this might not
>do much anyway.
>
>

If your CNC control is not reliable, and tends to fly to the limit without
warning, without any command that would ask it to do that, limit
switches are not
the answer. That control needs to be fixed, and if repair is not
possible, it
needs to be replaced. On larger, industrial-size machines, this is a direct
personnel safety issue!

Sophisticated CNC controls have multiple, definable "do not enter" zones.
(I have never even seen mention of such a thing in hobby-class CNC
software.) It is up to the person doing the CAD => CAM process to
define the places where a tool can hit a fixture, part of the machine
itself,
or part of the workpiece, and build a "do not enter" region around this.
The commercial systems I have some experience with (Allen-Bradley
7320, Bridgeport EZ-Trak) don't have this either, as far as I know.
I think some of the newer A-B, Fanuc, etc. controls DO have it, though.

Lathes are tricky, because the tool geometry is tricky, and even a skilled
operator can maneuver a toolholder into interference with a chuck jaw
while concentrating on the tool tip and workpiece only. And, it is common
to have the toolholder AWFULLY close to the chuck jaws during some
operations. Again, limit switches don't really solve it, because the tool
will hit when far forward in X, but if retracted in X, you can run the
tool all the way past the chuck in -Z without danger. A template clamped
to the bed, with a feeler switch located directly under the tool tip
(or any well-known X-Z offset from it) could do this. A plate the
shape of the tool & holder in use would be attached to the switch.
A plate the projected shape of the chuck would be fixed below the
chuck, and moved in the X direction depending on haw far open the
chuck jaws were. If the plate on the switch hits the fixed plate, an
E-stop is commanded. It is messy, but it would work. That is essentially
how the software version of this works, except that it is defined in the
CAM files.

Milling machines are also tricky, because you don't have a 2-D problem,
it expands into a 3-D problem, with situations where a tool will clear
if retracted above a certain height, but it will hit if left too far
extended.
So, you have to have exclusion VOLUMES, rather than simple 2-D
areas.

Jon