Inch/metric dimensions

G70	Dimensions in inches
G71	Dimensions in millimeters (standard setting)
G700	Dimensions in inches with calculation of the feed
G710	Dimensions in millimeters with calculation of the feed

Dimensions in millimeters (G71) is active by default. Information on whether the coordinates have to be converted is stored in the machine parameters (Interpreter tab). The basic dimension system in millimeters is also set there by default.

The effects of the changeover

If the basic dimension system is not the same as the current dimension system (set with G70 or G71), then certain parameters and co-ordinates must be converted. The conversion factor required here is stored in the machine parameters, like the basic dimension system. The changeover has effects on the following parameters:

Path information for the path axes (X, Y & Z)
Path information for the auxiliary axes (Q1..Q5)
Intermediate point co-ordinates (I, J, K)
Circle radii (B or U)
Programmable zero shift
Rounding radius (circle and spline smoothing)

There are also parameters that always remain in the basic dimension system, and are not converted. These include the

adjustable zero shift
Tool data
feeds (except under G700 or G710)

Sample 1:

Basic dimension system: inch

...
N10 G71      (metric dimensions) 
N20 G01 X100 (conversion is carried out)
N30 G70      (dimensions in inches)
N40 G01 Y100 (conversion is not necessary, because)
....         (the basic dimensions are also inches)

Sample 2:

Basic dimension system: millimeters

...
N10 G71 (metric dimensions)
N20 G01 X100 (conversion is not necessary, because)
    (the basic dimensions are also metric)
N30 G70 (dimensions in inches)
N40 G01 Y100 (conversion is carried out)

Zero shifts (NPV)

Adjustable zero shifts (G54-G57) always remain in the basic dimension system, and are not converted. In the case of the programmable zero shifts (G58 & G59) the effect depends on the current dimension system when the shift is selected.

Sample 3:

Basic dimension system: millimeters

...
N10 G71    (mm - default)
N20 G54    (activates adjustable zero offset shift)
N30 G58 X100 (programmable zero offset shift)
N40 G01 X0 F6000 (the axis travels to 100 in the machine co-ordinate system)
N50 G70    (inch)
N60 G01 X0   (zero offset shift is programmed under G71 => zero offset shift remains unchanged)
       (i.e. the axis does not move)
N70 G58 X100 (new programmable zero offset shift - now in inches)
N80 G01 X0   (axis moves out by zero offset shift - to 2540 in the machine co-ordinate system)