Inch/Metric Input (G20, G21) - Siemens Sinumerik 840D sl Programming Manual

3.2.2

Inch/metric input (G20, G21)

Workpiece-related axes can be programmed in metric or inch dimensions alternately,
depending on the dimensioning in the production drawing. The input unit is selected with the
following G functions.
Table 3- 4
G command
G20
G21
Format
G20 and G21 are always to be programmed at the start of the block and should not exist
along with other commands in a block. The following values are processed in the selected
unit of measurement while executing the G function for selecting the unit of measurement:
All the following programs, offset values, certain parameters as well as certain manual
operation and readout data.
Figure 3-8
Additions to the commands for defining the unit of measurement
● The closed position is defined via the machine data MD20154
● During changeover, the values of the work offsets are changed completely.
● If the unit of measurement is changed over during program execution, the following must
● The following is to be done after switching the measuring system from G20 to G21:
● G20 and G21 are not used to switch the hand wheel- and incremental weighting. This
ISO Milling
Programming Manual, 06/09, 6FC5398-7BP10-1BA0
G command for selecting the unit of measurement
Programming example
$MC_EXTERN_GCODE_RESET_VALUES[5].
be executed in advance:
While using a workpiece coordinate system (G54 to G59), this is to be traced back to the
basic coordinate system.
All tool offsets are to be deactivated (G41 to G48).
G92 must be executed before specifying the traversing commands for the axes (to setup
the coordinate system).
takes place through the PLC program. The machine data responsible for this is called
$MA_JOG_INCR_WEIGHT.
3.2 Defining the input modes of the coordinate values
Function
Input in "inch"
Input in "mm"
Motion commands
G group
06
06
47