Siemens SINUMERIK 828D Function Manual page 179

Boundary conditions
Automatic function switchover
Applying the following functions when the "Free-form surface mode: Extension function" leads to an automatic
switchover to standard LookAhead functionality:
• Thread cutting/tapping (G33, G34, G35, G331, G332, G63)
• Path master-value coupling
• Punching, nibbling
• Cartesian PTP travel
The "Free-form surface mode: Extension function" is then switched on again automatically.
Using the commands of G function group 15 (feed types)
It is not advisable to use the following feed types when the "Free-form surface mode: Extension function" is
active:
• Feedrate per revolution (G95, G96, G97, etc.)
• Inverse-time feedrate (G93)
Time response during feedrate overrides
$ Feedrate overrides (via a machine control panel, $AC_OVR, ...) can extend the traverse time over standard
LookAhead functionality considerably.
Rapid traverse movements (G0)
G0 blocks which are interspersed during free-form surface processing do not switch the LookAhead functionality
over (from the "Free-form surface mode: Extension function" to standard LookAhead functionality or vice versa).
This means that even though the standard dynamic response setting (DYNNORM) is effective with G0, the
standard LookAhead functionality which is preset for DYNNORM ( → MD20443 $MC_LOOKAH_FFORM[0])
does not automatically become effective as well as a result.
By retaining the LookAhead functionality which is currently active, a more homogeneous velocity profile is
achieved, particularly since G0 and polynomial blocks are usually smoothed and connected by rounding.
Number of NC blocks in the IPO buffer
It is generally advisable to significantly increase the configured number of NC blocks in the interpolation buffer
when using the "Free-form surface mode: Extension function":
MD28060 $MC_MM_IPO_BUFFER_SIZE > 100
If the block memory capacity is too low, this may diminish the uniformity of the path-velocity profile.
Basic Functions
Function Manual, 09/2011, 6FC5397-0BP40-2BA0
B1: Continuous­path Mode, Exact Stop, LookAhead
3.3 Continuous-path mode
179