I attached page cutting from 840D Function Description Part 1 (FB1) you can download the manual from Siemens web or Google , search key words " FGREF". followed there 's example 

mrezana

 

Pee Ell Sea

Hi,

Mixing rotary and linear axes frequently leads to incorrect feedrates. You can use the rotary axis to define the feedrate by a command called FGROUP.

You can get the system to calculate the correct feedrate by defining the radius you are working at with FGREF

Traverse rotary axes with path velocity F, FGREF
For machining operations, in which the tool or the workpiece or both are moved by a rotary
axis, the effective machining feedrate is to be interpreted as a path feed in the usual way by
reference to the F value. This requires the specification of an effective radius (reference
radius) FGREF for each of the rotary axes involved.
The unit of the reference radius depends on the G70/G71/G700/G710 setting.
All axes involved must be included in the FGROUP command, as before, in order to be
evaluated in the calculation of the path feed.
In order to ensure compatibility with the behavior with no FGREF programming, the factor 1
degree = 1mm is activated on system powerup and RESET. This corresponds to a reference
radius of FGREF=360 mm/(2π)=57.296 mm.

Pee

 Hi Pee

We have a machine with siemens 840D control and A axis.

Can you tell me how to define FGREF?

When i am using it in the program this error appear