Re: G02 - G03

If you are only concerned with a single flat X-Y plane, you may be able to live with the R format. However, the only place that it is really productive is on a lathe, which seldom makes an arc greater than 180 degrees. The mathematics for an R is not straightforward if the arc exceeds 180 degrees. Some systems try to use +/- values to signal what to do CW/CCW, but there is often ambiguity between different systems. The definition of an R format G-code is not exact. It is possible to specify the correct end point and an incorrect arc (too large). This will result in geometry that is not what is expected. Only the IJ format will guarantee that your arcs are exactly what you want them to be. Just because the math is not always decipherable in an IJ arc, it will NEVER result in an incorrect arc. If the end points are incorrectly specified, or the center points in an illegal position, the controller should stop cutting immediately. In most commercial systems, there is a simulation mode in which the controller can check the G-code to ascertain if the arcs are properly defined. It's called Dryrun, quickcheck, etc. Most controllers also have an allowable tolerance of error on the arc end point placements. Typically they expect that the location will be to the nearest .0001 inch. The old Fanuc 5T had the math hardwired in logic circuits & they incorrectly rounded some of it, such that about 1/1000 arcs were specified incorrectly, because of rounding errors in the controller. You had to find the bad line & tweek the arc by changing the I,J, or X,z's by .0001. Then it would work. If you took the numbers out to 5 or 6 places, it was apparent that they were making a simple numerical mistake in the controller, but what could you do?

If you want to do 3D work, however, you will not be able to specify the values with R. It is too difficult to get the +/- direction correct in 3D, and without that trick, the arc can swing to at least two positions in each plane specified by G17,G18,G19.

The best advice, Learn and use IJK. There are a lot of Cad-Cam systems that do the arithmetic for you, it's really pretty simple once you get a feel for what the numbers actually mean.


Best Regards,

Fred Smith
IMService

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