FANUC Nano Smoothing: why a smoother surface does not establish CAD contour accuracy
The 30i-B Nano Smoothing mechanism infers curves from line segments. Original chord-height and independent error-budget examples distinguish surface smoothness from CAD geometric accuracy, including the limits of diagnosis 5000 and single-block stopping points.
Contents
Nano Smoothing processes a program that has already been discretized. Fewer visible segment marks can indicate smoother motion or geometric connections, but do not alone prove that the toolpath's distance from the original CAD surface meets drawing requirements.
What data reaches the controller?
The FANUC 30i/31i/32i-B common operator's manual B-64484EN/03, §4.13, pages 92, 94 and 97 (public mirror of the original manual) explains that Nano Smoothing infers a curve from short line segments and adjusts inserted points within a tolerance; it can also handle uneven point spacing. In this edition, program tolerance corresponds to parameter 19581, and diagnosis 5000 identifies whether the current block is actually being smoothed. The discussion assumes the applicable option and configuration; it recommends no parameter values.
First consider error already introduced by CAM discretization
Original planar example: approximate an arc of radius R=50 mm with a chord of length l. The midpoint sagitta is h=R−√(R²−(l/2)²).
| Chord length l | Sagitta h |
|---|---|
| 2 mm | About 0.010001 mm, or 10.001 μm |
| 1 mm | About 0.002500 mm, or 2.500 μm |
In this circular model, halving segment length reduces sagitta to approximately one quarter. General freeform surfaces also have changing local curvature; average segment length across an entire program is insufficient to assess error.
The same two endpoints can belong to different curves. Inferring shape from neighboring segments does not restore access to the original CAD geometry. Improving polyline connections and recovering the design surface are related but different objectives.
Two error bounds are not automatically the same bound
Consider a separate error-budget example. Suppose offline verification has established that the CAM path is no farther than 0.008 mm from the target curve, and the subsequently processed path no farther than 0.006 mm from the CAM path. The triangle inequality gives a conservative total bound of 0.014 mm. If allowable geometric deviation is only 0.010 mm, these two bounds do not prove compliance.
Errors may partly cancel, but cancellation must not be assumed without verification. Here 0.006 mm is an original, assumed-verified path-distance bound, not an interpretation of parameter 19581 as final toolpath or part error. Actual assessment must also distinguish following error, tool shape and cutting deformation.
Verify smoothing and accuracy separately
Check smoothing state for the currently executing block, rather than only whether an enable command appeared earlier. This edition also states that, while smoothing is active, single-block stops occur at adjusted inserted points, which need not be the original programmed points. A stopping point differing componentwise from the original coordinates does not alone establish a positioning fault.
For geometric checking, compare the final toolpath with the same CAD reference and retain the location and direction of maximum deviation. Separating whether smoothing is active from whether deviation is acceptable helps explain differences between surface improvement and dimensional results. See the OPTION section for the resource index.