Fanuc Parameter 8901 Better -
“Better” also requires:
Mode 2 allows for nanometer-level interpolation. It processes high-density CAM output (points every 0.001mm) faster than Mode 1. This prevents "data starvation," where the machine stutters because it can't read the G-code fast enough.
For high-precision mold making or aerospace components where surface finish and dimensional accuracy are paramount.
On a 3-axis mill, axes are typically:
To select Z as the sync master:
Common mistake: Setting 8901 = 3 is invalid (3 decimal = binary 11 → bits 0 and 1 both 1 → selects axis 4).
Correct mapping:
Actual FANUC 8901 values: | Setting | Axis selected | |---------|----------------| | 0 | 1st axis (X) | | 1 | 2nd axis (Y) | | 2 | Not used (invalid) | | 3 | Not used | | 4 | 3rd axis (Z) | | 5 | 4th axis (A/B) |
Thus for a standard VMC tapping in Z: Set 8901 = 4 → that is “better” than default 0. fanuc parameter 8901 better
Problem: A DMG Mori CMX 1100V with default 8901=0 produced erratic thread pitch on M6x1.0 holes deeper than 20mm. Pitch error varied by 0.05mm.
Diagnosis: Servo lag in X axis (non-moving during tapping) was being unnecessarily referenced. The sync loop included X, causing phase shift at high spindle speeds.
Solution: Changed 8901 from 0 → 4. No hardware change.
Result: Thread pitch variation dropped to 0.005mm. Cycle time unchanged. Tapping speed increased from 1500 to 4000 rpm without breakage. For high-precision mold making or aerospace components where
A: Possibly. Axis hunting (oscillation at the quadrant change) is often caused by 8901=1. Switching to 8901=2 engages S-curve acceleration, which usually eliminates quadrant marks.
The bits are assigned as follows:
| Bit | Function | |------|------------| | #7 (ROTx) | 1 = Axis specified by bits #6, #5, #4 is a rotation axis | | #6, #5, #4 | Axis number (1 to n) of the rotation axis | | #3 (ROTx) | 1 = Rotation axis uses shortcut positioning (0 to 360°) | | #2 (ROTx) | 1 = Rotation axis uses linear positioning (no shortcut) | | #1 (ROTx) | Reserved (usually 0) | | #0 (ROTx) | 1 = Automatic rotation axis assignment for tool change |
Note: Some documentation shows bit 3 = RAB (Rotation Axis by shortest path), bit 2 = RLA (Rotation Axis Linear). On a 3-axis mill, axes are typically: