FEATURE
Absolute vs. Incremental CNC Programming
To write and edit CNC programs safely, you must understand when and how to use absolute and incremental positioning. Most programs use absolute coordinates, but incremental positioning can make certain operations easier. Confusing the two modes can cause unintended machine movement.
What Is Absolute Programming?
In absolute programming, every coordinate and movement value is measured from the established origin or work-offset zero point. The modal G-code G90 activates absolute positioning.
Most CNC programs are written primarily in absolute mode because the coordinates are easier to understand, review, and edit. Each programmed position shows exactly where the tool center should be relative to the work offset, regardless of its previous position.
What Is Incremental Programming?
In incremental programming, each commanded movement is measured from the tool’s current position rather than from a fixed origin. The modal G-code G91 activates incremental positioning.
For example, suppose two holes are spaced 4.000 inches apart and the first hole is 4.000 inches from the starting position:
- In absolute mode, their X coordinates would be X4.0 and X8.0.
- In incremental mode, each move would be programmed as X4.0.
Why Absolute Mode Is Usually Easier
Absolute programming is generally easier to read and troubleshoot. In G90 mode, a command such as X1.625 Y-0.875 sends the tool to that specific position relative to the active work offset, no matter where the tool was previously located.
In G91 mode, the same command moves the tool 1.625 units in the positive X direction and 0.875 units in the negative Y direction from its current position. Reaching a particular absolute location therefore requires knowing the current tool position and calculating the necessary displacement.
Incremental programs can also compound errors. If one move is incorrect, every later position based on that move may be displaced because each command begins from the preceding location.
Using Both Modes in One Program
A CNC program can alternate between the two modes. When a short sequence is easier to express as relative movements, activate incremental mode with G91. Before returning to fixed work-offset coordinates, place G90 on or before the next absolute move.
Because G90 and G91 are modal, the selected mode remains active until another command changes it. Always confirm the active positioning mode before running or editing a program.