Rockwell Automation 8520 9/Series CNC Lathe - Manual (Page 406)

Spindles
Chapter 17
17-8
When programming M58, the M59 code is cancelled and the G96 mode
becomes active again. The spindle maintains the same surface speed
that was in effect prior to the execution of M59 unless an S-code was
specified in the M59 block.
CAUTION: Restoring the constant surface speed mode might
cause the spindle speed to change rapidly depending on the
cutting tool position.
Displayed spindle speed during CSS
The CRT display normally shows the current spindle speed in RPM
following the S-word. This is true during CSS also. The display shows
the actual spindle RPM, not the surface speed.
CSS R-word programming
The optional R-word lets you specify a surface footage value to be
applied at a point other than the current axis position. The R-word
defines the incremental distance from the current tool tip to the spindle
centerline. The sign of the R-word defines on which side of the spindle
centerline the tool tip is positioned.
The R-word is recognized only in a G96 block; its value is valid as long
as CSS mode is active and may only be changed when a G96 is
programmed in the block.
If you change the S-word (surface footage) while in G96 mode and if an
R-value was previously programmed in a G96 block, the R-value is not
cancelled.
Important: An R0 and no R-word do not mean the same thing. An
R-word of zero means that the spindle centerline is the tool tip position.
No R-word means to use the current position.