Rockwell Automation 8520-GUM 9/Series CNC Grinder Operation and Programming Manual Documentation Set - Manual (Page 401)

Axis Motion
Chapter 12
12-29
When a G28 is executed in a part program (or through MDI) after the axes
have already been homed, it causes a return to machine home. In this case,
the axes specified in the G28 block simply go to their respective home
position in the machine coordinate system after moving to a programmed
intermediate point. They do not repeat the homing routine of moving to
the limit switches and searching for the encoder marker. For example,
executing the block:
G28 X__ Z__;
in either absolute or incremental mode would return the axes automatically
to the machine home via an intermediate point. The control stores the
intermediate point specified by the axis words (X, Z) in memory to be used
as the point of return for the automatic return from machine home
operation called out by G29.
The return operation generates two axis moves both executed at the rapid
feedrate. The first move is to the intermediate point and the second is to
the axis home position.
Although this command moves the axes at rapid feedrate as if in G00
mode, it is not modal. If G01, G02, or G03 modes are active, they are only
temporarily canceled for the return to home moves.
Only the axes specified in the G28 block are returned to home. For
example:
N1 G28 X4.0; (X-axis is moved to home after moving to 4.0)
N2 G28 X4.0 Z2.0; (X and Z axes are moved to home after moving to
(4.0, 2.0))
12.4.2
Automatic Return to
Machine Home (G28)