Siemens SINUMERIK 840D sl Programming Manual page 603

If the following axis is not yet at the correct position when the master value coupling is
activated, the synchronization run is automatically initiated as soon as the position setpoint
value calculated for the following axis is approximately the real following axis position. During
the synchronization procedure the following axis is traversed in the direction that is defined
by the setpoint speed of the following axis (calculated from master spindle and using the
CTAB curve table).
No synchronism
If the following axis position calculated moves away from the current following axis position
when the master value coupling is activated, it is not possible to establish synchronization.
Actual value and setpoint coupling
Setpoint coupling provides better synchronization of the leading and following axis than
actual value coupling and is therefore set by default.
Job Planning
Programming Manual, 06/2009, 6FC5398-2BP20-0BA0
9.3 Axial leading value coupling (LEADON, LEADOF)
Axis couplings
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