G90 and G91: Absolute and Incremental Positioning

G90 and G91 decide what the numbers in a move mean.

• G90 (absolute): coordinates are positions measured from the work zero set by the active work offset, such as G54. G1 X10 goes to X10, wherever the tool starts.
• G91 (incremental): coordinates are distances from the tool's current position. G1 X10 moves 10 in +X from wherever the tool is now.

Both are modal: the mode stays active until the other code replaces it. Almost every CAM program runs in G90 and sets it in the first few lines.

When to use G91

• Repeated patterns. A shape written in incremental moves can be cut anywhere: position the tool in G90, then run the same G91 moves. Subprograms often use this to repeat a feature.
• Moving relative to where the tool is. For example, stepping down another 0.5 mm, or backing away from a wall.
• Going home safely. G91 G28 Z0 is the classic way to send Z straight up to the G28 reference position, because in G91 Z0 means "don't move Z" for the intermediate point.

The risk: staying in G91

The usual G91 mistake is forgetting to switch back. After a G91 block, every later coordinate is still read as a distance, so a G0 Z10 meant as "go to 10 above the part" becomes "move up 10 from here", and the next X and Y moves land in the wrong places. Put G90 back as soon as the incremental moves are done, and set G90 at the start of every program.

Example: position absolutely, cut a square incrementally

The program moves to X10 Y10 in G90, then switches to G91 and cuts a 20 mm square with four relative moves. It switches back to G90 before retracting.

(G90 and G91 example: position in absolute, then cut a square in incremental)
G21 G17 G90 G54
T1 M6 (6 mm flat end mill)
S12000 M3
(G90: X10 Y10 is a position measured from the work zero)
G0 Z10
G0 X10 Y10
G0 Z2
G1 Z-1 F200
(G91: each move below is a distance from where the tool is now)
G91
G1 X20 F600
Y20
X-20
Y-20
(Back to G90 before the retract, so Z10 is a height again)
G90
G0 Z10
M5
M30

Line by line:

• G0 X10 Y10 in G90 goes to the position X10 Y10.
• G91 switches to incremental. G1 X20 F600 moves 20 mm in +X, to X30.
• Y20, X-20 and Y-20 move up, left and down by 20 mm, back to the start.
• G90 before G0 Z10, so Z10 is a height above the work zero. Left in G91, the tool would move up 10 mm from Z-1, to Z9.

How CutViewer shows G90 and G91

CutViewer tracks G90 and G91 and draws each move from the right start point. The active distance mode is shown with the other modes for the current line. Canned drilling cycles work in G91 too: X and Y step from the current position, R is measured from the initial Z and Z from the R plane.