CNC Absolute vs Incremental Programming: G90 & G91 Explained with Real Code

In CNC programming, G90 (absolute) and G91 (incremental) codes define how position commands are interpreted.
A misunderstanding between these two can cause severe crashes or dimensional errors — even in professional shops.

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📌 1. G90 — Absolute Positioning Mode

G90 commands the machine to move to the exact coordinate position defined from the current work zero (G54, G55, etc.).

Example:

G90 G00 X50. Y25.

Moves directly to X = 50.0, Y = 25.0 relative to the work offset zero.

Every coordinate is absolute from the origin.

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📌 2. G91 — Incremental Positioning Mode

G91 tells the control to move relative to the current position.

Example:

G91 G00 X10. Y-5.

Moves +10 mm in X and −5 mm in Y from the current tool position.

Every coordinate is relative to where the tool currently is.

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📌 3. Comparing G90 and G91

Mode	Position Type	Movement Example	Typical Use
G90	Absolute	“Go to X50 Y25”	General machining, setup repeatability
G91	Incremental	“Move +10 from here”	Looping, macros, pattern repeats

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📌 4. Fanuc Example — Switching Between G90 & G91

%
O3000 (ABSOLUTE VS INCREMENTAL)
G90 G00 X0 Y0 Z50
G01 X50. Y25. F300
G91 X10. Y10.
G90 X60. Y35.
M30
%

Code	Explanation
G90	Moves to X0 Y0 Z50 from origin
G91	Moves +10 X, +10 Y from current position
G90	Returns to absolute mode

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📌 5. Haas Example — Incremental Drilling Pattern

G90 G00 X0 Y0 Z5
G81 Z-15. R2. F150
G91 X20
G80
G90

• Drills first hole at X0
• Then moves +20 mm and drills again
• Switches back to G90 to end safely

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📌 6. Siemens Example — TRAORI and G90/G91 Use

TRAFOOF
G90 L X50 Y0 Z10
G91 L X20
G90 L X70 Y20

Siemens retains motion modes in 3D transformations — TRAORI active moves are always relative, even with G90 declared.

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📌 7. Heidenhain Example

L X+50 Y+25 (Absolute)
L IX+10 IY-5 (Incremental)

• L = linear motion
• IX/IY prefix defines incremental shift

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📌 8. Common Mistakes

Mistake	Result
Forgetting to return to G90	Tool moves relative → crash
Mixing absolute and incremental in same cycle	Wrong hole pattern
Incremental retract without G90 reset	Rapid move to wrong position

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📌 9. Safe Programming Practices

• Always define G90 in your safe start block.
• Use G91 only in short, controlled loops.
• Cancel G91 with G90 immediately after use.
• Comment lines where G91 is activated.

Example:

G90 G17 G40 G80 G21
...
(Drilling pattern)
G91 (Incremental loop start)
X20 (Next hole)
G90 (Return to absolute mode)

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📌 10. Macro Example — Pattern with G91 Loop

#100=0
G90 G00 X0 Y0
WHILE[#100LT5]DO1
G91 X20
G81 Z-10. R2. F120
G80
#100=#100+1
END1
G90

This creates five holes, each spaced 20 mm apart, using incremental motion in a macro loop.

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📌 11. Future (2025–2030)

• AI safety modes — automatic G91 detection before motion.
• Simulation-driven checks — warning pop-up if G91 left active.
• Hybrid coordinate modes — blending absolute and incremental control in 5-axis machining.

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✅ Conclusion

Mastering G90 and G91 is critical for safe, predictable CNC programming.
By clearly controlling absolute and incremental modes, you prevent crashes, ensure dimensional accuracy, and maintain professional programming standards.