Rockwell Automation 8520-MUM 9/Series CNC Mill Operation and Programming Manual Documentation Set - Manual (Page 638)

Milling Fixed Cycles
Chapter 26
26-6
Figure 26.3 shows the two different modes available for selecting the
return level in the Z axis after the hole has been drilled. These two modes
are selected with G98 (which returns to the same level the cycle started at)
and G99 (which returns to the level defined by the R point).
Figure 26.3
Milling Fixed Cycle Parameters in G98 or G99 Modes
0 0
Z Z
Z Z
R point level R point level
G98 Initial point return G99 R - Point return
R
Cutting feed
Rapid feed
Important: In the following sections, figures and examples are assumed to
be programmed in the incremental mode (G91) and initial point return
mode (G98).