On Sunday 03 May 2009, Rob Jansen wrote:
>Doug,
>
>I've seen the responses from other stating it's "kind of OK as long as
>you watch out".
>Well, I can tell you it's not OK.
>
>Use the cable for anything but steppers, servos, encoders, limit or home
>switches etc.
>I have seen problems even when the spindle's VFD signals and stepper
>cables were all shielded - that's proper shielded Olflex cables
>especially made for machine controls - and things like unwanted limit
>switch triggers or steppers 'walking away' are not uncommon and very
>hard to fix once you have everything boxed and ready to run.
>
Can you describe this Olflex cable please?
Using the cable I mentioned, with the computer, stepper psu, and xylotex
driver box all on a shelf about 2 feet wide, located at ceiling height in my
little shed. I've had zero problems of that sort. That means the data cable
from the PC's parport to the xylotex is only about 2 feet long, and it comes
into the xylotex box on the opposite side from the output cables to the
motors. I ran a piece of 12 gauge all around the xylotex board, connected to
the data cables ground breakout, the psu's negative terminal, and to all the
ground drain wires in the Star-Quad cables to the motors. I use a PMDX-106
running on a big wall wart to interface with the spindle motors controller,
and all of that is jumpered via soldered joints back to this same piece of 12
gauge. There is no shielding on the spindle motor cabling, in fact I have an
ammeter on 24" extension leads in series with the return wire right at the
motor, and just made a run of 10 cartridge holding blocks (I'm also a
handloader), 144 holes 1/2" each, or 112 5/8" holes, drilled most of the way
through a piece of cleaned up 2x6 a foot or so long. The only gotcha was the
bit got stuck and the z axis couldn't lift it to clear the chips. So the x
axis pulled the top of one hole sideways about 50 thou. Out of well over 1000
holes. :)
That and Porter Cables forstner style bits are dull as can be right out of the
package, I cut spindle current by half just by sharpening it better. I did
play with the peck increments, and probably should have used more cycles, but
I wound up drilling half depth, clearing chips, then to 85%, clear chips, and
a final push to 100% depth, in 3 cycles, in each case I could see the last 2%
driving up the spindle current cuz the bit was stuffed full.
>Just last week I had some problems when I used an unshielded cable
>directly connected to my Mesa 5i20 board to the E-stop. This was a quick
>hack to see if I could use EMC's E-stop input. When the spindle started
>running, at specific speeds, the steppers started rattling randomly.
>A friend of me bought a commercial (hobby type) CNC controller and he
>had lots of problems in this same direction - in the end he replaced all
>his cables and fitted opto couplers on all the I/O lines - now his
>problems are gone.
This I would do, if I ever have the problem. I have also probed for contour
several times and have not had any errors of more than a thou or two in the
data. This using the sliding tube opto-interrupter device one of the guys
here gave me a link to when I asked about that a couple of years ago. Home-
made of course. :)
To repeat, all grounds are done in 'star' fashion with the motor drivers being
at the center of the star, and no cable shielding connected except at the
star. All machine grounds are through the 3rd pin, and all power on the same
circuit to prevent lightning surges from being anything but 'longitudinal',
meaning the whole system bounces in unison so there is not a surge because one
part is plugged into wall circuit A, and the rest of it is plugged in on wall
socket B which may be on the opposite leg of my 240 volt shop feed. When its
all on one circuit, the whole thing can bounce 100KV measured from ground, but
there isn't an inter unit surge since its all bouncing the same way,
microsecond by microsecond.
When dealing with multiple horsepower stuff, this isn't near as easy, and
makes isolation transformers a lot more desirable. Piece of cake with my
teeny stuff. :)
>Regards,
>
> Rob
--
Cheers, Gene
"There are four boxes to be used in defense of liberty:
soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
If all the world's economists were laid end to end, we wouldn't reach a
conclusion.
-- William Baumol
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