Thanks for the ideas everybody. A friend suggested that I try the following and it worked. Project all geometry to zero. At this point what had previously been the problem arc became a Nurbs Spline. Edit, Simplify all geometry to 0.00002. The Nurbs Spline was converted back into an arc and the cutter path now follows it nicely.

It is an arc.

When I chain another arc on the part (of the same radius (0.415mm)), it works fine. This particular radius is the only one out of 4 identical radii that has this issue. G20G0 G17 G40 G49 G80 G90T1 M6G0 G90 G54 X.6581 Y-.196 A0. S8556 M3G43 H1 Z.3602M8Z.3102G1 Z.0402 F6.2X.6195 Y-.1416X.5952 Y-.1279X.5642 Y-.1256Z.2402G0 Z.3602M5G91 G28 Z0. M9G28 X0. Y0. A0.M30

The cutter path is created by 4 straight lines as it makes the radius. When I backplot and step through the path, each straight line is clearly defined as it makes its way around the radius (but not following the radius with an arc). I have redrawn the radius and still have the same problem. Sometimes it even makes one straight line out of it, ignoring the radius completely by going from the end of the line prior to the radius to the start of the next line.

Why doesn't the cutter path follow the geometry as drawn? All other radii in the same file work OK. See attached pic.