Mastering Absolute Dimensions: G90 Modal & AC Non‑Modal Commands on Siemens Sinumerik

What is Absolute Dimensions (Absolute Programming)?

With Absolute dimensions or Absolute programming, the programmed position always refer to the zero point of the currently valid coordinate system (workpiece-zero-point).

G90 Absolute Dimensions or Absolute Programming is Modal G-code. Siemens Sinumerik provides AC (non-modal absolute dimensions) command to access individual axis absolute position.

CNC program example is here Sinumerik Mill Program – Absolute Dimensions Commands G90 AC

SINUMERIK 828D

G90 – Modal Absolute Dimensions

If a CNC machinist want to program a part in absolute programming then simply the best way is to use G90 absolute dimensions command (normally called G90 absolute programming g-code ).
G90 is a modal G-code so will stay active until you program G91 Incremental Programming G-code.

G90 Modal Absolute Dimensions Format

G90

AC – Non-modal Absolute Dimensions

But if you are programming a part in Incremental programming (G91) and just want to access one or some axis Absolute Coordinates then no need to use G90 command you can use Siemens Sinumerik AC (Non-modal Absolute Dimensions) command.
Siemens Sinumerik AC (Absolute Coordinates) command is the simple and short way to access non-modal absolute dimensions for individual axis.
Non-modal absolute dimensions (AC) are also possible for spindle positioning (SPOS, SPOSA) and interpolation parameters (I, J, K).

AC – Non-modal Absolute Dimensions Format

=AC()

AC – Command for the activation of non-modal absolute dimensions
<axis> – Axis identifier of the axis to be traversed
<value> – Position setpoint of the axis to be traversed in absolute dimensions.