Trouble with arcs

Hi all,

I've previously been doing 180 degree arcs by this method: (starting point X0 Y0.5) G03 X0 Y-0.5 R0.5 but the manual says that arcs approaching 180 degrees are notoriously inaccurate.

Now when I attempt to do a full 360 degress by typing G03 X0 Y0.5 R-0.5 I get a runtime error and TurboCNC exits to DOS. To get around this I combined two combined 180 degree arcs separated by a pause of 5ms, which seems to clear up the crashing issue, but I'm left with a slight pause which I'd like to get rid of entirely.

I was also having a problem of the program ramping down, pausing, and speeding back up at each quadrant. This was solved by turning the backlash off.

When I try to use the I/J/K method for declairing a center to arc around, I get an error message saying that I have a zero or negative radius, when I most certainly do not. Even when running the simple example in the manual I still get this error message.

I'm just trying to do some helical thread cutting and I want a smooth 360 degree arc with no pauses - just a clean quick helical interpolation.

Any ideas as to what I'm doing wrong?

Guess I should fix that runtime error - the case where that start and end points are the same using the R notation is indeterminate. It really should fail gracefully, not dump you to DOS...

Killing the backlash comp gets rid of the quadrant pauses, that's the best way around it for now...

Mind posting your IJ notation arc code? Just the G02/03 and the line above it is enough. I'm guessing it's just something simple. Are you in absolute or incremental mode? For absolute, it should be this (for your example):

G00 X0 Y0.5 (12 o'clock spot)
G03 X0 Y0.5 I0.0 J-0.5 (full circle CCW)

alternately you can try

G00 X0 Y0.5 (12 o'clock spot)
G03 I0.0 J-0.5 (full circle CCW)

which should be slightly more reliable, as the software will realize that leaving out XY means you want a full circle. Add a Z0 in the first line, and something like Z-0.125 in the second, and you ought to be going helical...

Hmm.. I'm really starting to like that second method you posted just using the I,J,K coordinates and leaving out the X,Y,Z. I just cut some real nice threads using that method.

I did find out that I kinda have to keep backlash compensation on, otherwise my circles start to become ovals by the time I finish the Z depth. I could really tell when I took the first cut with the thread cutter as it would cut on one quadrant of the hole and not even touch another! But since I thread cutting has to be run so slow anyways, the pausing on each quadrant shouldn't pose too be much of a problem.

From now on out I will forget about using the R function and just use the I,J,K method for making complete circles. Thanks for your help!

bringing this thread back from the dead...I'm also getting booted to DOS AFTER completing a full circle (useing IJK coordinates). Is there a fix for this?