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Author Topic: When Is A Circle Not A Circle?  (Read 6392 times)

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When Is A Circle Not A Circle?
« on: August 14, 2008, 02:25:59 PM »
Answer - When it's a polygon!  Today I made a couple of donut-shaped pieces of aluminum.  The center holes came out perfectly smooth and round.  The outsides, however, came out as ~32-sided polygons, the segments all appearing to be a very consistent size!  These circular paths were programmed as a single G02 command to make the entire 360 degree cut.  This has never happened to me before, and I haven't changed *anything*.  Is there some way to tell Mach3 what precision to use in calculating the segments of an arc?  Resetting and re-booting the PC appears to have perhaps made it better, but still not good.

Regards,
Ray L.
Regards,
Ray L.

Offline Hood

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Re: When Is A Circle Not A Circle?
« Reply #1 on: August 14, 2008, 02:41:32 PM »
please attach your xml and code, also what does the toolpath show?
Hood
Re: When Is A Circle Not A Circle?
« Reply #2 on: August 14, 2008, 08:51:56 PM »
XML attached.  I didn't look at the toolpath in any detail, but, as I said, we're talking about a single G02 command here.  I could load it up again and see what it looks like, but what should I be looking for?

Regards,
Ray L.
Regards,
Ray L.

Offline ger21

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Re: When Is A Circle Not A Circle?
« Reply #3 on: August 14, 2008, 09:02:15 PM »
What is your CPU usage at when running the part?
Gerry

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Re: When Is A Circle Not A Circle?
« Reply #4 on: August 15, 2008, 12:55:21 AM »
Gerry.

   I don't know.  I've learned not to mess with the computer when Mach is working.  The PC is a pig - only 540MHz.  But, it's been working fine up to this point, and turned out a ton of very nice work with no evidence of this problem.  This seems to be a new behavior just today.  Will Mach3 start getting less accurate like that if the CPU gets bogged down?
   Ya know, it does occur to me, the only thing that's changed in the last few days is I turned on limit switches.  I had been running without them previously.  Could polling those possibly be affecting the quality of the trajectories it's creating?  Seems like a long-shot, but something has changed...

Regards,
Ray L.
Regards,
Ray L.

Offline RICH

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Re: When Is A Circle Not A Circle?
« Reply #5 on: August 15, 2008, 06:19:27 AM »
The only time I say the "polygon" effect was when using DeskcNC, where for a circle you broke the arc's into lines
and there was a setting that had to be set very small,, and if not set to a small value, you would get small flats all around.
Additionaly from a cad program when exporting and it was not truely a circle. But you see the generated code as small lines. Not experienced the polygon effect in MACH using the G02.
I still use a 400 Mhz computer / 512 memory and when cutting a circle it will come to a crawl as it approces the transition point as compared to a faster computer. So I find your post interesting but have no solution. Something has changed though.
Have you tried a few test cuts justs to see if it's now consistantly doing it?
RICH
« Last Edit: August 15, 2008, 06:21:35 AM by RICH »

Offline Hood

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Re: When Is A Circle Not A Circle?
« Reply #6 on: August 15, 2008, 06:31:51 AM »
Running a few jobs on the mill at the moment but I will test out with your xml when I have done. Think possibly it could be the computer that is the problem, maybe even turning off the toolpathg would help.
Hood

Offline Hood

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Re: When Is A Circle Not A Circle?
« Reply #7 on: August 15, 2008, 07:27:58 AM »
OK just loaded your xml to my mill, left everything as is except changed the pins for limits and motors to suit my setup. G2 and G3 run fine and perfectly round so your xml doesnt seem to be the problem. Certainly looks like your computer is struggling. As Ger says have a llok at your CPU and see what load it is under. Also have a look and see if there are any process's running that you can kill.
Hood
« Last Edit: August 15, 2008, 07:29:29 AM by Hood »
Re: When Is A Circle Not A Circle?
« Reply #8 on: August 15, 2008, 01:36:10 PM »
In the words of Lewis Carroll's Alice:  "Curiouser and curiouser....".  I've stripped the XP install to the bone, so there are only about 15-20 processes running, and none consume any significant amount of CPU.  When Mach is running, the idle process never seems to drop below 50% of CPU usage, and Mach never goes above 49%.

Today, it seems to be back to cutting perfect circles.  Go figure!

But, today I also found when doing rapids, I'd get periodic "clunks" from the drives, which usually indicates erratic timing in the step pulses to the servos.  This was really severe on first boot fo the PC, so I rebooted, and it was much better, but still there.  The "clunks" occurred at very regular intervals, roughly once/second.  Disabling all limit switch inputs and E-Stop input reduced the severity of the clunks considerably.  Reducing rapid speed from 75IPM to 60IPM completely eliminated them, even with the E-stop and limits all enabled.  What is that all about?

Regards,
Ray L.

Regards,
Ray L.

Offline Hood

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Re: When Is A Circle Not A Circle?
« Reply #9 on: August 15, 2008, 01:50:31 PM »
Just wondering if the mobo is having issues, have a look especially around the CPU area, and see if any of the capacitors are bulging or leaking. Could be that voltage issues on the mobo are making things a bit erratic.
Hood