FANUC angular axis control: why actual X and Z can both move when the program changes only X
The angular-axis transformation in the 16i/18i/20i/21i-A connection manual is used to calculate travel and speed for a 30-degree mechanism. Program-space feed cannot simply be treated as actual feed-axis speed.
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On a machine with an inclined slide, programmed X and Z can remain Cartesian even though the physical motion axes are not orthogonal. A programmed X-only move can therefore move the actual Z axis too. This is part of coordinate compensation and does not necessarily mean that an extra command was executed.
Identify what is inclined
The FANUC 16i/18i/20i/21i-A connection manual B-63003EN-1/02, §1.11, pages 182–185 (public mirror of the original manual) describes angular axis control and arbitrary angular axis control. The ordinary form uses an inclined X axis and a Z axis, while programs are defined in orthogonal coordinates. The following uses that section's positive X/Z directions and assumes that the relevant two-axis transformation is active. It is not a rotation of an ordinary machine program as a whole.
Physical axis travel must cancel the cross component
Let the inclined X axis deviate from programmed X by θ. With programmed displacements Xp and Zp and physical axis travels Xa and Za, this geometry gives:
Xa=Xp/cosθ；Za=Zp−Xp·tanθ.
An original example uses θ=30°. All quantities have been converted to physical linear lengths, with diameter/radius programming differences already resolved. The 30° angle is for mathematical explanation, not a parameter recommendation.
| Programmed displacement Xp, Zp | Physical Xa | Physical Za |
|---|---|---|
| 20, 0 mm | About 23.094 mm | About −11.547 mm |
| 20, 10 mm | About 23.094 mm | About −1.547 mm |
In the first row, Xa projects onto Z by 23.094×sin30°≈11.547 mm, exactly canceled by Za, so the tool's programmed Z change remains zero. The second row cancels only enough to leave 10 mm. The table uses unrounded trigonometric values.
Speed constraints need the same transformation
During uniform motion purely along programmed X, a programmed speed vp=1200 mm/min requires physical X speed of about 1385.641 mm/min and physical Z speed of about −692.820 mm/min. F being lower than both axes' ratings does not prove that transformed speeds stay within their limits.
For an ideal calculation involving axis-speed constraints only, suppose physical X allows 1300 and physical Z allows 500 mm/min. Programmed X speed is then bounded respectively by 1300cos30°≈1125.833 and 500/tan30°≈866.025; the smaller 866.025 mm/min must be used. This is a geometric upper bound under assumed constraints, not the machine's actual acceleration/deceleration or speed-limiting algorithm.
Do not subtract unlike displayed coordinates to diagnose a fault
The referenced section distinguishes absolute/relative positions in programmed Cartesian coordinates from machine positions of the physical inclined mechanism. Before comparing records, identify their coordinate spaces and transform them. Directly subtracting the two X readings would mistake normal geometric differences for following error.
If a dedicated control state suppresses compensation of the perpendicular axis, the two-axis model no longer applies in full. This article provides no procedure for changing angles, enable parameters or signals; it analyzes a mechanism and mode already defined by the builder. Related resources are in the OPTION section.