G02 - G03 - an aside

This brings up a question I have often wondered about. Given that the parameters
to G02 and G03 include the starting position (implicitly), an ending position, a
center point position *and* a radius, what do controllers do if these are
inconsistent values?
For instance if the distance between the initial position and the center point
is not equal to the radius? Of if the ending point isn't on the arc defined by
the start point, center point, and radius?

Even if the G-code writer correctly calculates all these values, all coordinates
and distances are expressed as finite precision values, so it would seem to me
that this would be an unavoidable problem with anything other than arcs that are
multiples of 90 degrees. So the controller can't just call it an error condition
and stop.

If I were writing a G-code interpreter, I could invent several ways of
gracefully handling the error, but I was wondering what real, existing
controllers do...
-Kevin Martin

-----Original Message-----

>From: Art Fenerty [mailto:fenerty@...]
>Subject: [CAD_CAM_EDM_DRO] G02 - G03
>...
> I am trying to update MasterControlCNC to do G02
>and G03 circular interpolation and although I now have working code ( thanks
>to Doug Fortune) I don't quite understand which is the most common form of
>interp.
> My post processor on the software I use the most does
>not generate circ. only linear. Am I better off using R registers or IJK
>interp? or both? Is IJK now pretty much obsolete?