FANUC jerk control: why a tangent line-to-arc connection can still require deceleration
The 30i-A line-to-arc jerk-control rules distinguish continuous velocity direction, an acceleration jump and jerk. A tangent path may still require deceleration, and acceleration-change units must not be confused with jerk units.
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CAM shows an exactly tangent line-to-arc connection, yet the program slows there. This need not indicate a tangent discontinuity: a smooth travel direction addresses velocity direction only. A sudden curvature change can still produce an acceleration jump.
The function constrains more than feed F
The FANUC 30i/31i/32i-A user manual B-63944EN/03, §20.3.1, pages 671–673 (public mirror of the original manual) explains that per-axis acceleration-change control reduces speed according to the permissible change and requires both jerk-control and AI contour-control options. The section's “permissible acceleration change amount” is in mm/s²; it must not be treated directly as mathematical jerk in mm/s³.
A tangent junction can have an acceleration jump without a velocity jump
An original model travels along a horizontal line in +X and then enters a tangent arc curving toward +Y, with radius 25 mm. Assume constant speed 3600 mm/min, or 60 mm/s. Normal acceleration is 0 on the line and v²/R=60²/25=144 mm/s² on entering the arc. Tangent direction agrees at the junction, but Y-axis acceleration changes from 0 to 144.
If an idealized acceleration-change limit at the junction is 80 mm/s², speed must satisfy v≤√(80×25)=44.721 mm/s, about 2683.282 mm/min. This is an independent mechanics calculation, not advice to enter 80 into a machine or a prediction of the CNC's final actual speed.
Amount of change and rate of change are different measures
| Quantity | Meaning in this example | Unit |
|---|---|---|
| Acceleration a | Required to change velocity direction on the arc | mm/s² |
| Acceleration change Δa | The line-to-arc change 0→144 | mm/s² |
| Average jerk Δa/Δt | Assuming the change takes 12 ms, the value is 12000 | mm/s³ |
The 12 ms transition is assumed only to explain units; it is not this series' interpolation period or an actual transition time. Comparing 12000 directly with an acceleration-change threshold mixes physical quantities.
Separate geometry from dynamic constraints
Offline analysis can ask two questions: is there a direction corner at the junction, and is curvature the same on both sides? Tangency addresses only the first. A line has curvature 0, whereas a finite-radius arc has curvature 1/R, so the second quantity still jumps. Designing a transition with gradually increasing curvature can mathematically reduce this jump, but permission to alter the part contour is a separate issue.
The manufacturer notes that changes calculated from CNC interpolation data may differ from theoretical values. The ideal bound therefore explains a deceleration trend but does not replace checking the actual path, override and control mode. Neither “the arc radius is fairly large” nor “the tangent is continuous” alone proves that deceleration is unwarranted. Related resources are in the OPTION section.