G2 causes clockwise circular motion to be generated at a specified feed rate (F). The generated motion can be 2-dimensional, or 3-dimensional (helical). On a common 3-axis mill, one would normally encounter lots of arcs generated for the X,Y plane, with Z axis motion happening independently (2 axis moves in G17 plane). But, the machine is capable of making helical motion, just by mixing Z axis moves in with the circular interpolation.
When coding circular moves, you must specify where the machine must go and where the center of the arc is in either of two ways: By specifying the center of the arc with I and J words, or giving the radius as an R word.
I is the incremental distance from the X starting point to the X coordinate of the center of the arc. J is the incremental distance from the Y starting point to the Y coordinate of the center of the arc.
Examples:
G1 X0.0 Y1.0 F20.0 --go to X1.0, Y0.0 at a feed rate of 20 inches/minute
G2 X1.0 Y0.0 I0.0 J-1.0 --go in an arc from X0.0, Y1.0 to X1.0 Y0.0, with the center of the arc at X0.0, Y0.0
G1 X0.0 Y1.0 F20.0 --go to X1.0, Y0.0 at a feed rate of 20 inches/minute
G2 X1.0 Y0.0 R1.0 --go in an arc from X0.0, Y1.0 to X1.0 Y0.0, with a radius of R=1.0