Trying to dissect his gcode. Can use different way to generate the arc curve with either the I and K offsets, or possibly use the R to add the fretboard radius. Since I’m doing fanfret the radius will always change for each fret, Luckily in Rhino3d there is a “Radius” command to analyze the radius of the curve. Good thing I simplified my fret curves to 2 degrees while maintaining the overall perpendicular fretboard radius of 16″. Still not sure if this will work for fanfret, because the arc generated follows the XZ plane. Each end of fanfret slot is not on the same XZ plane. Maybe adjusting that center point may work, j.

%
(FILENAME: Fret Slotting Toolpath G18 XZ Arcs.nc)
(STOCK/BLOCK, 100.000, 500.000, 6.350, 50.000, 250.000, 6.350)
(TOOL/MILL,0.6000,3.00000,38.000,0.0)
G00 X0.0000 Y0.0000 Z2.0000 (fast travel to z height of first point)
(Zero Fret)
G00 X-26.5000 Y230.9687 Z2.0000 (START POINT fast travel to xyz of first point )
G01 F120.0 Z-2.1667 (linear movement plunge rate down to z point)

G18 F600 (select XZ plane sets feedrate for cuts)
G03 X26.5000 Y230.9687 Z-2.1667 I26.5000 J0.000 K-181.9833 (END POINT Counter clockwise arc from XYZ, with offset)
G01 F120.0 X26.5000 Y230.9687 Z-2.4167 (linear movement plunge rate down to next XYZ)

G18 F600 (select XZ plane sets feedrate for cuts)
G02 X-26.5000 Y230.9687 Z-2.4167 I-26.5000 J0.000 K-181.7333 (Clockwise arc from XYZ, with offset)
G01 F120.0 X-26.5000 Y230.9687 Z-2.6667 (linear movement plunge rate down to next XYZ)

(Rinse and repeat)
G18 F600
G03 X26.5000 Y230.9687 Z-2.6667 I26.5000 J0.000 K-181.4833
G01 F120.0 X26.5000 Y230.9687 Z-2.9167

G18 F600
G02 X-26.5000 Y230.9687 Z-2.9167 I-26.5000 J0.000 K-181.2333
G01 F120.0 X-26.5000 Y230.9687 Z-3.1667

G18 F600
G03 X26.5000 Y230.9687 Z-3.1667 I26.5000 J0.000 K-180.9833
G01 F120.0 X26.5000 Y230.9687 Z-3.4167

G18 F600
G02 X-26.5000 Y230.9687 Z-3.4167 I-26.5000 J0.000 K-180.7333
G01 F120.0 X-26.5000 Y230.9687 Z-3.6667

G18 F600
G03 X26.5000 Y230.9687 Z-3.6667 I26.5000 J0.000 K-180.4833
G00 Z2.0000