Siemens SINUMERIK 840D sl Function Manual page 417

Rounding machine data
All length-related machine data are rounded to the nearest 1 pm when writing in the inch
measuring system (MD10240 $MN_SCALING_SYSTEM_IS_METRIC=0 and MD10260
$MN_CONVERT_SCALING_SYSTEM=1), in order to avoid rounding problems.
The disturbing loss of accuracy, which occurs as a result of conversion to ASCII when
reading out a data backup in the inch system of measurement, is corrected by this procedure
when the data is read back into the system.
2.2.4 FGROUP and FGREF
Programming
It should be possible to program the effective machining feedrate in the usual way as a path
feedrate via the F value in processing procedures where the tool, the workpiece or both are
moved by a rotary axis (e.g., laser machining of rotating tubes).
In order to achieve this, it is necessary to specify an effective radius (reference radius) for
each of the rotary axes involved. You can do this by programming the modal NC address:
FGREF[
The unit of the reference radius depends on the G70/G71/G700/G710 setting.
In order to include the axes in the calculation of the path feedrate, they must all be specified
in the FGROUP command.
In order to ensure compatibility with the behavior with no FGREF programming, the factor 1
degree = 1 mm is activated on system powerup and RESET.
This corresponds to a reference radius of:
FGREF = 360 mm/(2π) = 57.296 mm
This default setting is independent of the active basic system:
MD10240 $MN_SCALING_SYSTEM_IS_METRIC
and of the currently active inch/metric G code.
Table 2-2
N100 FGROUP(X,Y,Z,A)
N110 G1 G91 A10 F100
N120 G1 G91 A10 X0.001 F100
The programmed F value in block N110 is evaluated as a rotary axis feedrate in
degrees/min, while the feedrate weighting in block N120 is either 100 inch/min or 100
mm/min, depending on the current inch/metric setting.
The time required to execute the two blocks can be differ greatly.
Basic logic functions: Velocities, Setpoint/Actual Value Systems, Closed-Loop Control (G2)
Function Manual, 11/2006, 6FC5397-0BP10-2BA0
<axis name>
] = reference radius
Special features of the feedrate weighting for rotary axes in FGROUP:
Detailed description
2.2 Metric /inch measuring system
29