FANUC normal direction control: why halving an arc radius can make the C axis limit feedrate
The 0i-F machining-center normal direction control chapter relates C-axis angular travel to arc length and speed. Distinguishing corner orientation from continuous arc following explains small-radius feed limits and configuration prerequisites.
Contents
If the tool slows abruptly on a small arc at the same F value, insufficient program-block transfer is not the only possible cause. Under normal direction control, the tool's orientation axis must follow changes in the contour tangent. A smaller arc demands a faster change of orientation per unit path length.
Distinguish direction control from radius compensation
The 0i-F machining-center manual B-64604EN-2/01, §6.13, pages 231–234 (public mirror of the original manual) selects normal direction control with G41.1 / G42.1 and cancels it with G40.1. The decimal suffix “.1” must not be omitted: this function differs from G41 / G42 cutter compensation. This edition requires the relevant helical-cutting option. The orientation C axis must be a rotary axis, and its workpiece-coordinate angle must correspond to the actual tool orientation.
Calculating speed during an arc
Page 233 gives the C-axis speed relationship during an arc: ω=F×ΔC/L, where F is path feedrate in mm/min, ΔC is axis angular travel in degrees, and L is arc length in mm. When the C axis would exceed its maximum cutting speed, feedrates of the other axes are limited accordingly. The following assumes an ideal XY arc without tool compensation or activation of the small-arc ignore rule, with the C axis continuously maintaining the normal direction.
For an original 90° arc example, F=900 mm/min. At tool-center path radius r=15 mm, L=πr/2=23.562 mm and the C axis turns 90°, giving ω=900×90/(15π/2)≈3437.747°/min. Increasing the radius to 30 mm doubles arc length and halves the required C-axis speed.
| Tool-center arc radius | Required C-axis speed | With an assumed C-axis limit of 3000°/min |
|---|---|---|
| 15 mm | 3437.747°/min | Corresponding path feed limit is about 785.398 mm/min |
| 30 mm | 1718.873°/min | This constraint alone does not require reducing 900 mm/min |
3000 is an assumed comparison value, not a parameter recommendation. The first row gives maximum F=3000×(15π/2)/90≈785.398 mm/min. The displayed arc length 23.562 is only an approximation; calculations use the unrounded π expression. Actual feedrate may be lower after acceleration/deceleration or other axis constraints are considered.
A corner-orientation move does not use the arc formula
The manufacturer separately describes orientation moves inserted between blocks and following during an arc. If slowing occurs at a polyline junction, first determine whether an orientation move is in progress. If it occurs on continuous small arcs, examine ΔC/L and actual C-axis capability. These consume time for different reasons; a general description of the C axis as fast or slow is insufficient.
For validation, compare controlled paths at the same feedrate but different radii and observe C-axis changes while keeping other conditions constant. Do not simply increase axis speed limits to eliminate slowing; the arc geometry itself may demand greater angular velocity. This article explains only the specified mechanism. Other multi-axis resources are available in the OPTION section.