Restarting a CNC program incorrectly remains one of the most common causes of expensive machine crashes. Machines inherit modal states, offsets, and compensation from previous blocks, creating dangerous motion when restarted below safe logic.
This recovery bible explains professional restart discipline.
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SECTION 1 — WHY RESTARTS FAIL
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Programs assume earlier commands executed.
Restart skips:
G43.
Offset calls.
Plane selection.
Machine logic becomes inconsistent.
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SECTION 2 — SAFE RESTART BLOCK
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Professional rebuild:
G90 G17 G40 G49 G80
G94
G54.
Machine becomes predictable again.
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SECTION 3 — TOOL LENGTH DANGER
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Restart below compensation:
Tool plunges instantly.
Always restart above activation block.
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SECTION 4 — CANNED CYCLE TRAPS
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Forgotten drilling cycles continue automatically.
Always cancel:
G80.
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SECTION 5 — INCREMENTAL MODE TRAP
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G91 inherited.
Unexpected travel occurs.
Force:
G90.
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SECTION 6 — SAFE MACHINE RETRACT
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Before restart:
G53 Z0.
Clear workspace completely.
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SECTION 7 — PROFESSIONAL PROVE-OUT
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Single block.
Reduced rapid override.
Dry run.
Experienced operators never skip verification.
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FINAL TAKEAWAY
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Restart discipline prevents more crashes than programming skill alone.
Professional machining controls machine memory before motion.
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