In the world of CNC machining, precision is everything. One of the most common sources of programming errors—and subsequent machine crashes—is the confusion between Absolute (G90) and Incremental (G91) positioning. Understanding when and how to use each can significantly improve your workflow and reduce costly mistakes.
What is Absolute Positioning (G90)?
When you use G90, every coordinate you input is measured from a single fixed point: the Work Home Position (G54, G55, etc.). If you tell the machine to move to X50, it goes to the 50mm mark on the X-axis relative to your zero point, regardless of where the tool is currently located.
Pros of G90:
- Consistency: It is easier to read and verify dimensions against a blueprint.
- Safety: If a tool change occurs, the machine always knows exactly where the "Home" is.
What is Incremental Positioning (G91)?
In G91 mode, the machine treats its current position as the starting point (0,0) for the next move. If you input X10, the tool moves 10mm to the right from where it is standing right now.
Pros of G91:
- Repetition: Ideal for subprograms, such as drilling a series of holes with the same spacing.
- Flexibility: Allows for quick adjustments without recalculating the entire coordinate system.
Error Reduction: Which One Should You Choose?
To achieve error reduction in CNC programming, follow these best practices:
| Feature | Absolute (G90) | Incremental (G91) |
|---|---|---|
| Main Program | Best for safety and clarity. | Risk of "Stacking Errors". |
| Subprograms | Harder to repeat. | Excellent for repetitive patterns. |
The "Stacking Error" Warning
The biggest risk with Incremental G-Code is the "Stacking Error." If one movement is off by 0.01mm, every subsequent movement will carry that error forward. In Absolute G-Code, the error stays localized to that specific coordinate, preventing a total part failure.
Final Verdict
For most CNC machining tasks, sticking to G90 (Absolute) for your main code is the safest bet for error reduction. Use G91 (Incremental) only for specific sub-routines or repetitive features where it simplifies the logic. Always double-check your safety blocks at the start of the program to ensure the machine is in the correct mode!
CNC Programming, G-Code, Machining Tips, Incremental vs Absolute, CNC Error Reduction, Manufacturing
