G43 and G43.4 are two of the most important G-codes in precision CNC machining, yet they serve completely different purposes. G43 applies traditional tool length compensation, allowing the CNC to offset the Z-axis position based on the measured tool length stored in an H value. This is used on nearly every 3-axis and 4-axis machine. G43.4, however, activates Tool Center Point Control (TCP), which is required for true simultaneous 5-axis machining. While G43 only compensates in Z, G43.4 compensates in all axes, constantly recalculating the tool tip position through rotary transformations as A, B, or C axes move.
1. What G43 Actually Does
G43 reads the tool length stored in an H offset and shifts the tool down accordingly. For example:
T12 M06
G43 H12 Z50.
This simply says: “Move the spindle so the tool tip ends up at Z50 using tool length offset #12.”
This works perfectly on 3-axis and basic 3+2 machining because no rotary-axis-based geometric deformation occurs.
2. What G43.4 Does (Tool Center Point Control)
G43.4 is much more advanced. It tells the CNC:
“Keep the tool tip exactly on the programmed XYZ point regardless of rotary axis movement.”
This requires constant recalculation of the tool vector and pivot length during A/B/C rotations.
Without G43.4, 5-axis paths will gouge surfaces or miss positions.
Example:
G43.4 H12
G01 X100. Y40. Z-22. A30. C110.
The control automatically computes how the rotary axes change the tool tip location and adjusts XYZ accordingly.
3. Real 5-Axis Example (Without TCP vs With TCP)
WITHOUT G43.4 — incorrect cutting:
G43 H8
G01 X50. Y20. Z-10. A45.
The tool tip will lift off the surface because rotation changes the pivot point.
WITH G43.4 — correct cutting:
G43.4 H8
G01 X50. Y20. Z-10. A45.
The machine adjusts X/Y/Z in real time to keep the cutter exactly on the surface.
4. Common Real Errors When G43.4 Is Missing
- Surfaces cut too shallow or too deep
- Gouging near steep walls
- 5-axis contours showing visible “steps”
- CAM paths not matching actual motion
- Rotary-table parts being machined out of tolerance
This is because G43 alone cannot compensate for the geometry shift caused by rotary movement.
5. Pivot Length (Kinematic Center) Example
Every TCP machine has a pivot length — the distance from rotary pivot to spindle gauge line.
If pivot length is measured incorrectly, G43.4 motion becomes unstable.
Fanuc example where pivot length must be calibrated:
Parameter 19700 or 19701 (machine-dependent)
If TCP seems “wobbly”:
- Recalibrate probe
- Re-measure pivot length
- Verify machine geometry
6. Haas Equivalent
Haas does not use G43.4. Instead:
- DWO/TCP = G234
- Combined with G43 H#
Example:
G234
G43 H12
G01 X85. Y30. Z-14. A25. C60.
Identical function to G43.4 in Fanuc.
7. When to Use G43 vs G43.4
Use G43 for:
- 3-axis machining
- Drilling, tapping, facing
- 3+2 positioning
- Simple fixtures
- Non-rotary work
Use G43.4 for:
- True 5-axis simultaneous machining
- Mold surfacing
- Impellers, blisks, turbines
- Undercuts and organic surfaces
- High-precision aerospace parts
8. Final Summary
G43 is mandatory for basic tool length compensation, while G43.4 is essential for accurate 5-axis machining. If you run 5-axis paths without TCP, you will experience surface errors, gouges, and geometrical drift. When properly calibrated, G43.4 delivers perfect tool-tip accuracy through continuous real-time kinematic calculations and is a critical feature for advanced machining in 2025 and beyond.
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