After a few more tests...

If you save a perfect circle as a .pdf from Illustrator then Vectric will use G1 in the G-Code (a series of short lines which won't cut at the correct speed in Linux CNC)
If you save a perfect circle as an .svg from Illustrator then Vectric will use G2 in the G-Code (a proper arc that does cut at the correct full speed)

Generating circles and arcs from within the Vectric software uses G2's and G3's in the G-Code that cut at the correct speed, but curves and ellipses from within the Vectric software use G1's.

Anything other than a perfect circle from Illustrator (in .svg) results in G1's being written in the code - for me that's a problem as most of my work originates in Illustrator and doesn't necessarily involve perfect circles.

It would be nice if the software could write the more complicated contours/arcs as G2's and G3's but in theory it shouldn't be a problem - the feed is still set at 1800mm/min, right?

In practice, however, is this a case of my machine acceleration not being set up quick enough (in the LinuxCNC .ini file) to accommodate that quick succession of commands where the machine transitions from moving from one set of coordinates to another, effectively only allowing it to get up to a fraction of its intended speed before having to change direction..?

The video below (apologies for poor quality) shows the machine following another two paths. In the first you'll see (if not hear) a spike in speed where the path straightens out. In the second you'll see a gradual slowing down as the arcs of the spiral get tighter.



I guess that the more sophisticated commercial CAD/CAM software is clever enough to write contours in G2's and G3's, in the meantime maybe I ought to revisit my .ini file...

Any thoughts?