SkillsUSA CNC Milling Practice Exam 2025 – 400 Free Practice Questions to Pass the Exam

The use of G-code is fundamental in CNC programming, and understanding the specific functions of different G-codes is crucial for effective machining operations. The G-code flag that enables absolute positioning is designated as G90.

When G90 is active, any positional command given in the subsequent code will refer to the absolute coordinates of the workpiece, meaning that the coordinates specify a point in relation to the origin of the workpiece. For example, if you command a movement to X5.0 and Y10.0 while in absolute mode, the machine moves directly to those coordinates from the defined zero point of the workpiece.

In contrast, G91 enables incremental positioning, where movements are specified in relation to the last commanded position, not the absolute coordinates. This is particularly useful for certain types of machining operations where changes in position need to be more fluid and based on previous locations rather than the absolute origin.

Understanding the difference between these flags is essential for programming CNC machines accurately, as it directly affects how the machine interprets the movement commands.