Rockwell Automation 8520-MUM 9/Series CNC Mill Operation and Programming Manual Documentation Set - Manual (Page 405)

Axis Motion
Chapter 14
14-29
Machine tools have a fixed machine home position that is used to establish
the coordinate systems. The control offers two different methods for
homing a machine after power up.
Manual machine home operation that uses switches or buttons on the
MTB panel provided solely for this purpose. Manual homing is
discussed in detail in section 4.3.
Automatic machine home operation that uses a programmed machine
home code.
Automatic homing is accomplished through the use of a G28 code. When
programmed as the first motion block in a part program (or through MDI)
a G28 will cause an automatic homing operation if the axes have not yet
been homed. Only axes that have their axis words programmed in the G28
block are homed.
Homing will follow the sequence of homing events described in manual
homing (see chapter 4).
The coordinate values which are programmed with the axis words in a G28
block are stored by the control as intermediate point values (described in
the next section).
If all the axes programmed in the G28 block have already been homed
when the G28 code is executed, then the control will consider it an
“Automatic Return to Machine Home” as described in the next section.
Important: When a homing request is made the feedback device for the
axis (typically an encoder) must encounter at least one marker before
tripping the homing limit switch. If the axis is close to the home limit
switch you should jog the axis away from this switch before attempting a
homing operation.
14.3
Automatic Motion To and
From Machine Home
14.3.1
Automatic Machine Homing
(G28)