FANUC 15i-MB helical interpolation: why the same F value can produce different axial speeds
The original 15i/150i-MB manual distinguishes circular tangential feed from spatial-path feed. Calculations for the same hypothetical geometry show the two velocity components and why HTG semantics must be checked.
Contents
In helical interpolation, circular and axial motion occur together. The same F value can specify circular tangential velocity or spatial velocity that includes axial motion. Overlooking this distinction when comparing programs can make a semantic difference look like a machine speed fault.
Limited to the 15i/150i-MB and the cited manual edition
This article follows Section 4.5, printed pages 51–52, of the public mirror of the FANUC B-63784EN/01 programming manual. In this edition, the HTG bit (#2) of parameter 1401 selects the feed interpretation: 0 specifies circular tangential velocity; 1 specifies actual spatial-path velocity. This explains the meaning when reading the configuration, not a recommendation to change the parameter. The number must not be copied across series.
Separate the velocity components geometrically
Consider a constant-radius arc in the XY plane synchronized with motion of the Z axis perpendicular to that plane. Assume constant helix pitch and feed per minute F (mm/min). Let arc length be Lc and Z displacement Lz; the spatial path length is L＝√(Lc²＋Lz²). This is a constant-speed geometric model, excluding acceleration/deceleration and machine speed limits.
| Interpretation of F | Circular tangential velocity | Axial velocity |
|---|---|---|
| Circular tangential velocity | F | F×Lz÷Lc |
| Spatial-path velocity | F×Lc÷L | F×Lz÷L |
Arithmetic comparison at the same F
Assume Lc＝40 mm and Lz＝30 mm, giving L＝50 mm. If F＝1000 mm/min:
- With circular tangential feed, the circular component is 1000 mm/min, the axial component 750 mm/min and the resultant speed 1250 mm/min.
- With spatial-path feed, the circular component is 800 mm/min, the axial component 600 mm/min and the resultant speed 1000 mm/min.
These values verify the formulas only; they are not recommended cutting conditions. The greater the axial displacement relative to arc length, the more the two interpretations affect velocity distribution.
What to check before comparing programs
- Confirm the manual edition, active HTG semantics and units of F instead of comparing program numbers alone.
- Calculate arc length and axial displacement from the actual path. Check the required axis velocities separately, not only the resultant feedrate.
- Then examine acceleration/deceleration, speed limits and tool/process constraints in the actual operation. A speed below the simplified model does not automatically indicate a fault.
This article addresses one axial component in ordinary helical interpolation. It does not equate that function with extensions capable of coordinating more axes. Further information is available in the OPTION section.