I am thread milling an M24 x 1 thread in cast iron. below is the sub program for the thread milling process. This sub program was writen years ago and the programer has since retired. I am having trouble following the sub program line by line. The machining center is a Deckel Maho Horizontal with a siemens 840D control.

;%_N_L921_SPF
;$PATH=/_N_SPF_DIR
;PROGRAM FOR DMG 1-8
;THREAD MILL CYCLE
;R2=REFERENCE DEPTH
;R3=FINAL DEPTH
;R5=PITCH OF THREAD
;R6=START OF THREAD RADIUS THAT IS SMALLER THEN TAP DRILL RADIUS
;R7=FINISH THREAD RADIUS
;R10=ADDITIONAL RETRACTION
N0010 G0 G60 G90 CFC Z=R2
N0020 R56=R7+R6 R57=R5/2 R62=-R6 R63=R56/2 R64=-R56
N0030 G0 G90 Z=R3
N0040 G1 G41 G91 X0. Y=R62
N0050 G3 G64 G91 X0. Y=R56 Z=R57 I0. J=R63
N0060 G3 G64 G91 X0. Y0. Z=R5 I0. J=-R7
N0070 G3 G64 G91 X0. Y=R64 Z=R57 I0. J=-R63
N0080 G0 G40 G60 G91 X0. Y=R6
N0090 G0 G90 Z=R10
N0100 M17

Parameters used, (all values below are in millimeters):
R56=22
R52=-10
R63=11
R5=1
R7=12
R64=-22
R6=10
R57=0.5

I am getting lost at line 60, how are the I & J being calculated?

Thanks for the help