G91 Incremental Positioning in CNC Programming Explained
G91 in CNC programming sets the machine to incremental positioning mode, where movements are made relative to the current position instead of an absolute coordinate. This means each command moves the tool by a specified distance from where it currently is, not from a fixed point.How It Works
Imagine you are walking in a room and someone tells you to move 3 steps forward, then 2 steps to the right. You don't need to know your exact location in the room; you just move relative to where you stand. This is how G91 incremental positioning works in CNC machines.
Instead of telling the machine to go to a fixed point on the workpiece, you tell it how far to move from its current spot. Each move command adds or subtracts from the current position, making it easier to program repetitive or relative movements.
Example
This example shows how to move a CNC tool incrementally using G91. The tool moves 10 units in X, then 5 units in Y, each relative to its current position.
G91 ; Set incremental positioning mode G01 X10 Y0 F100 ; Move 10 units in X from current position G01 X0 Y5 ; Move 5 units in Y from new position G90 ; Return to absolute positioning mode
When to Use
Use G91 incremental positioning when you want to move the tool by specific distances relative to its current location. This is helpful for repetitive tasks like drilling holes spaced evenly or making small adjustments without recalculating absolute coordinates.
It simplifies programming when the exact starting point may vary or when working with patterns and offsets.
Key Points
- G91 sets CNC to incremental mode, moving relative to current position.
- Commands specify distances, not fixed coordinates.
- Useful for repetitive or offset movements.
- Switch back to G90 for absolute positioning when needed.
Key Takeaways
G91 enables incremental positioning, moving relative to the current tool location.G90 for absolute positioning when required.