Tool Length Offset (G43)

A tool length offset tells the CNC how far each tool's tip projects from the spindle gauge line, so tools of different lengths all cut to the same programmed Z positions. FANUC-style controls apply it with G43.

Each tool assembly in a machining centre has a different length from the spindle gauge line to its cutting tip. The measured length is stored in the control's offset table, and the program activates it, typically with G43 and an H number on FANUC-style controls, with G49 cancelling it. The control then shifts Z positions by that amount so that every tool works from the same part zero. Lathes use tool offsets in both X and Z for the same purpose.

Lengths are measured on the machine with a contact or laser tool setter, offline with a tool presetter, or by touching each tool off on a reference surface. Controls usually separate a geometry offset, the measured length, from a wear offset, a small adjustment that operators use to hold dimensions as tools wear.

Calling the wrong offset number, entering a value with the wrong sign or forgetting to activate the offset can drive the tool into the part or fixture, so careful verification is part of every setup. Measurement accuracy is affected by cleanliness of tapers, runout and thermal growth of the spindle. Tool data transferred from presetters by tag or network reduces manual transcription errors.

Key points

Compensates for different tool lengths along the spindle axis
Activated with G43 and an H number on FANUC-style controls, cancelled by G49
Geometry offsets hold the measured length; wear offsets hold small adjustments
A wrong offset number or value can cause a collision

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Related terms

Terms that refer to Tool Length Offset (G43)

Machine Coordinate System (Machine Zero)CNC & Precision Machining