FANUC G51.1 mirroring: why reflecting one axis and two axes affects arc direction differently
Using the 0i-F descriptions manual, point coordinates and signed triangle area explain single- and dual-axis mirroring. Geometric reflection, arc-direction handling and physical spindle rotation remain distinct to avoid reversing the program twice.
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When producing left/right symmetric parts, having the CNC mirror the path and also reversing arc direction in CAM output can alter an otherwise correct path. First establish whether the postprocessor or the machine's programmable mirror-image function performs the mirroring.
Control semantics stated in the 0i-F documentation
FANUC B-64602EN/01 Descriptions, §5.47, printed page 195 (public mirror of the original manual) uses G51.1 to specify the mirror axes and symmetry positions, and G50.1 to cancel mirroring on designated axes. When only one axis in the selected plane is mirrored, arc rotation direction, cutter-compensation offset direction and coordinate-system rotation angle are reversed. The following is limited to the XY plane of a 0i-F machining center configured with the function; it excludes superimposed external-signal mirroring.
Calculate position before examining orientation
Original example: the X symmetry position is a=5 mm, the Y symmetry position b=10 mm, and point P=(8,12) mm. Mirroring the X coordinate uses x′=2a−x. This is reflection about the line x=a, not simply negating every coordinate.
| Transformation | Calculation | New coordinates |
|---|---|---|
| Mirror the X coordinate only | (2×5−8,12) | (2,12) mm |
| Mirror both X and Y coordinates | (2×5−8,2×10−12) | (2,8) mm |
In planar geometry, mirroring both axes is equivalent to rotating 180° about (5,10). It changes position but preserves the path's clockwise or counterclockwise orientation. Mirroring only one axis reverses that orientation.
Three points can test whether orientation has reversed
Take A=(6,11), B=(8,11), C=(6,12). The signed area S=[(Bx−Ax)(Cy−Ay)−(By−Ay)(Cx−Ax)]÷2=+1 mm² indicates that A→B→C is counterclockwise. Mirroring X alone gives S=−1; mirroring both axes returns +1. This original geometric calculation can help check how many reversals have been applied to the postprocessed contour.
If the original unmirrored contour is used and G51.1 performs one reflection, a change in the displayed arc direction is not by itself a reason to manually swap G02 / G03 or left/right cutter compensation again. First compare final coordinates and contour orientation with the drawing and identify where the transformation occurs in the program chain.
Geometric orientation does not establish an equivalent cutting process
Even with the correct mirrored path, climb versus conventional milling must be assessed using the actual spindle direction, the side occupied by material, the tool and the setup. The geometric derivation proves neither automatic spindle reversal nor suitability of the original cutting parameters for the symmetric part. Retain the original contour, mirror center and final tool-center path together during validation to identify repeated mirroring or datum errors. Related fundamentals are available in the OPTION section.