Incremental vs Absolute Positioning in G-Code (G90 vs G91) Explained

Incremental vs Absolute Positioning in G-Code (G90 vs G91) Explained

One of the most fundamental — and most misused — concepts in CNC programming is the difference between absolute and incremental positioning. Controlled by G90 and G91, this affects how your machine interprets movement commands.

Misunderstanding this concept leads to:

• Lost part zero
• Overcuts
• Collisions
• Unpredictable toolpaths

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🧭 G90 — Absolute Positioning

G90
G0 X50 Y25

The machine moves to X=50, Y=25 from the active origin (usually part zero or machine zero).

✅ Always goes to a specific point in space.

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🔁 G91 — Incremental Positioning

G91
G0 X10 Y-5

Moves +10 mm in X and –5 mm in Y from current position.

✅ Think in terms of steps or deltas.

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📊 Comparison Table

Feature	G90 (Absolute)	G91 (Incremental)
Reference Point	Fixed origin (e.g. part zero)	Current position
Code Interpretation	To coordinate	By movement amount
Safer for complex parts	✅	❌ (can accumulate error)
Preferred for loops	❌	✅

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🛠️ Real Example: Drilling Holes in a Line

🔸 With G90:

G90
G0 X0 Y0
G81 Z-10 R1 F100
X20
X40
X60
G80

Each X is an absolute coordinate from origin.

🔹 With G91:

G91
G0 X0 Y0
G81 Z-10 R1 F100
X20
X20
X20
G80

Each X move is +20 mm from previous — total movement accumulates.

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📌 Best Use Cases

G-Code	Use Case
G90	Precise contouring, safe return paths
G91	Drilling grids, repetitive moves, loops

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⚠️ Common Mistake: Forgetting Mode

This will go wrong:

G91
G0 X10 Y0
...
G0 X50 Y0 ← Not 50 mm from zero, but 50 mm from current!

Always set mode explicitly (G90 or G91) to avoid confusion.

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🔄 Switching Modes Mid-Program

Programs often toggle between modes:

G90 ; Start with absolute
...
G91 ; Do an incremental drill loop
M98 P1000 L5
G90 ; Return to absolute

Good practice: always reset to G90 before program end.

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🧪 Parametric Example: Incremental Looping

G91
#100=0
WHILE [#100 LT 5] DO1
  G0 X20
  G81 Z-10 R1 F100
  #100=#100+1
END1
G90

This drills 5 holes spaced 20 mm apart using incremental logic.

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🔧 G90/G91 with Z-Axis

• G90 Z-10 → go to Z = -10 from zero
• G91 Z-10 → move down 10 mm from current Z

⚠️ Always be cautious — with Z-axis, an incorrect mode may cause crashes.

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🧠 Machine vs Work Coordinate System

• G90/G91 work within the current G54–G59 coordinate system
• If using machine coordinates (G53), G90 mode is required

G53 G0 Z0 ; Go to machine Z zero (absolute only)

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✅ Summary

Understanding G90 vs G91 is critical for safe, efficient, and accurate CNC programming:

• Use G90 for precision, safety, and clarity
• Use G91 for loops, repetitive moves, and compact code
• Always reset to G90 after incremental operations
• Document mode switches to avoid ambiguity

By mastering positioning logic, you can prevent programming errors, reduce crashes, and build bulletproof CNC routines.