Why G91 Incremental Mode Is Responsible For Hidden CNC Crashes (2026 Expert Programming Guide)

G91 incremental positioning is extremely powerful — and extremely dangerous when forgotten. Many CNC crashes happen because the machine remained in incremental mode from a previous operation or manual input.

Operators often believe the machine malfunctioned when the real problem is modal state confusion.

This guide explains why G91 causes hidden crashes and how professionals control incremental motion safely.

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1) G90 vs G91 Reality
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G90:
Absolute positioning.

Machine moves to exact coordinate.

G91:
Incremental positioning.

Machine moves relative to current position.

Example:

Current X = 200.

Command:

G91 X100

Machine moves to X300.

If program expected G90 → disaster.

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2) The Classic Incremental Crash
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Previous setup used incremental jogging.

Operator loads new program.

Program assumes G90.

First rapid:

G00 X150

Machine moves +150 from unknown location.

Fixture collision occurs instantly.

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3) Restart Danger
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Restarting after alarm:

Machine left in G91.

Skipped safe start block.

Unexpected travel occurs.

Restart only before modal reset section.

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4) Tool Change Movement Disaster
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Incremental mode active during tool positioning.

Machine moves relative instead of absolute.

Unexpected spindle movement toward clamps.

Always force G90 after tool change.

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5) Probe Cycle Risk
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Probe macros may temporarily activate G91.

Improper macro exit leaves machine incremental.

Next move unpredictable.

Validate macro completion.

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6) Professional Safe Start Block
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Elite shops always begin with:

G90
G17
G40
G49
G80

Modal reset removes inherited state risk.

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7) Manual Jog Influence
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Operators sometimes jog incrementally before running program.

Control retains state.

Program inherits risk.

Always verify positioning mode before cycle start.

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8) Automation Considerations
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Lights-out machining demands deterministic positioning.

Incremental inheritance breaks automation reliability.

Explicit modal control is mandatory.

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9) Quick Checklist
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Unexpected movement?

Check:

• G90 active?
• Restart location safe?
• Probe macro completed?
• Manual jog performed earlier?

Incremental state is often the answer.

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10) Final Takeaway
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G91 is not dangerous.

Forgetting it is.

Control modal state — control machine motion.