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Curious problem with arc error
« on: August 21, 2010, 02:19:50 PM »
I'm using BOBCAD V19 to created my geometry and gcode.  I'm using MACH3 V3.042.020 to drive a Gecko 540 controller connected to my Taig CNC mill.

I've run into a curious situation that I just can't figure out.  I recently started programming various patterns for my pool cue inlays. On the pocket program for one design (basically a diamond shape with curved sides), I got the error "Radius to end of arc differs from radius to start" when I tried to load the gcode in MACH3.  I searched the forum for this error and found two likely causes.  Wrong mode for arcs (abs. vs. inc) or rounding problems between my CAD program and MACH.  Since I knew that my arc mode was correct, I assumed that it was a rounding problem, so I tried all sorts of modifications to the design in an attempt to counter this presumed problem. 

After driving myself nuts and always getting the same error when the program was loaded into MACH, I realized that I had programmed a similar design previously and did not have a problem.  To my surprise, when I loaded that program (that had worked fine in the past), I got the same error.  Since I had run out of ideas, I tried something that I thought was stupid.  I went into the General Config and changed my arc mode from incremental to absolute (even though I knew that incremental was correct).  Surprised again.  Both programs would now load without the error (although the Tool Path display showed the arcs incorrectly).  Then I changed the arc mode back to incremental (the correct mode).  Surprised again.  Both programs now loaded without the error and the arcs look correct.  I haven't run the programs yet, but I'm assuming that they will work correctly.

I didn't really change anything (I'm back to the same arc mode I started with and the programs were not changed), but now I can load the programs without the error.  Maybe I should just be happy that everything seems to be working correctly now, but it's driving me crazy trying to figure out why I started getting the error and why flip-flopping the arc mode corrected the problem!?!?!?!??!?

Anybody got any ideas?
Bill (the Cat) Shubert

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Re: Curious problem with arc error
« Reply #1 on: August 21, 2010, 03:44:38 PM »
try playing with your G90's on the first line of code.try G90.0 or G91.1

bill
Re: Curious problem with arc error
« Reply #2 on: August 21, 2010, 05:00:17 PM »
Thanks for the tip, but that's not what I'm looking for.  It's working now (re-read the my post).  Besides, I tried all those things when I was having the problem and they had no effect. What I'd like to know is why it stopped working and then started working again when I didn't actually change anything.  
Bill (the Cat) Shubert
Re: Curious problem with arc error
« Reply #3 on: August 22, 2010, 01:18:35 PM »
Eureka!!!!   I finally found the cause of the arc errors ;D

On the first positional move in any of my programs, my CAM software (BOBCAD) was not outputting the X or Y axis position if it was zero.  For example, if the first move was to X1.0 and Y0.0, it only output the X1.0, but there was no Y axis output in the gcode.  When the next positioning command was a G02 or G03, MACH3 would attempt to interpolate the arc motion based on the current position of the axes in the workpiece coordinate system.  Since the initial motion command did not always specify both the X and Y position, the value that MACH3 would use for the unspecified axis was whatever the current position happened to be.  Normally, I position the machine to the location where I wanted to cut my inlay pocket, zero the axes and then load the program.  When I did this, things worked fine.  However, if I tried to load the program before I zeroed the axes, the unspecified axis position was always wrong (not zero).  So, the arc error would be signalled.  I think I just confused the issue by trying to load the program at different times.  Sometimes I had zeroed the axes and sometimes I hadn't.  In doing that, I never saw any consistancy in when the error would be flagged.

So, I've edited all my existing programs to set the missing X or Y axis output in my first move.  In addition, I've changed the CAM configuration to force the positional output of both axes on initial moves.  Under Setup->Coordinate format->Initial position, I changed the X and Y "Initial position" values from 0 to 9999.
Bill (the Cat) Shubert