Printed from Kezuriba (kezuriba.net/en/nc/g91/)

G91

Machining center, LatheModal, group 03

Incremental command

⌨On machining centers

G91 G01 X_ Y_ F_

Coordinates are written as travel from the current position. The same G91 X20. reaches a different end point if the start point differs
Accidents often happen when G91 is carried into the next operation. Explicitly write G90 at the start of each operation

Diagram: Writing the same move in absolute (G90) and incremental (G91)

The same path from the origin through ①→②→③ is written in G90 as positions from the origin and in G91 as moves from the current position. In G91, axes that do not move need not be written.

◎On lathes: (Not used in system A. Incremental values are written with U/W)

The same number means something different on lathes (FANUC-type G-code system A).

Siemens ISO T: in system A, G91 cannot be used, and incremental X/Z is commanded with U/W. In systems B/C, G90/G91 are absolute/incremental

📚Sources

The meaning shown agrees in at least two sources (agreement confirmed). Explanations are written in our own words.

Mitsubishi Electric — The Basics of CNC Machining Programming (for Machining center) e-Learning material (PDF) mitsubishielectric.com
Siemens AG — SINUMERIK 802D sl/840D/840D sl/840Di/840Di sl/810D Programming Manual ISO Milling, 04.2007 Edition (6FC5398-7BP10-0BA0) cache.industry.siemens.com
Haas Automation, Inc. — Mill Operator's Manual – 17 Mill G-Codes (online version) haascnc.com
National Institute of Standards and Technology (NIST), USA — The NIST RS274NGC Interpreter – Version 3 (NISTIR 6556, 2000) nvlpubs.nist.gov
Siemens AG — SINUMERIK 802D sl/840D/840D sl/840Di/840Di sl/810D Programming Manual ISO Turning, 04.2007 Edition cache.industry.siemens.com
Mitsubishi Electric — The Basics of CNC Machining Programming (for Lathe) e-Learning material (PDF, BNP-C8027-688) mitsubishielectric.com