They are fixed radius, you cannot to my knowledge use them as a spiral.

In Mach3, you can cheat with G2/G3 to create a spiral, by specifying the end of the arc to be offset from where it should be.

Load this code into Mach3 and you'll see a spiral.

G40 G90

G91.1

S10000 M3

G0 Z0.1250

G0 X0.0000 Y-0.7400 Z0.1250

G1 X0.0000 Y0.1233 Z-0.1000 F75

G2 X0.0000 Y0.1233 Z-0.1000 I0.0000 J-0.1233 F100

G2 X0.0000 Y0.2467 Z-0.1000 I0.00 J-0.1233

G2 X0.0000 Y0.3700 Z-0.1000 I0.00 J-0.2467

G2 X0.0000 Y0.4933 Z-0.1000 I0.00 J-0.3700

G2 X0.0000 Y0.6167 Z-0.1000 I0.00 J-0.4933

G2 X0.0000 Y0.7400 Z-0.1000 I0.00 J-0.6167

G2 X0.0000 Y0.7400 Z-0.1000 I0.00 J-0.7400

G0 Z0.1250

G0 X0.0000 Y0.7500 Z0.1250

G1 X0.0000 Y0.7500 Z0.0000 F75

G2 X0.0000 Y0.7500 Z-0.1000 I0.0000 J-0.7500 F100

G2 X0.0000 Y0.7500 Z-0.1000 I0.0000 J-0.7500

G0 X0.0000 Y0.7500 Z0.1250

M30