G187 is the Haas CNC command that controls the machine’s motion smoothing, accuracy level, and corner-rounding behavior during high-speed machining. It functions as a tolerance filter that adjusts how aggressively the control blends linear and circular toolpath segments. By modifying the motion algorithm, G187 directly affects surface finish, machining speed, tool pressure, and dimensional accuracy. Understanding this command is essential for achieving optimal results in both roughing and finishing operations on Haas machines in 2025.
1. What G187 Actually Does
G187 adjusts:
- Tolerance (E value) → allowable contour deviation
- Smoothing mode (P value) → rough / medium / finish
- Corner rounding behavior
- Feedrate smoothing
- Toolpath blending
- Axis acceleration curves
In short:
G187 controls how “tight” or “loose” the machine follows the programmed geometry.
2. Basic Syntax
G187 P# E#
Where:
- P0 → Rough Mode
- P1 → Medium Mode
- P2 → Finish Mode
- E → Max allowed deviation from programmed path (in mm)
Example:
G187 P2 E0.002
Ultra-precision finishing mode.
3. Roughing Example (High Material Removal)
G187 P0 E0.050
Effects:
- Faster motion
- Larger path blending
- Increased productivity
- Lower accuracy (±0.05 mm deviation)
Perfect for roughing pockets or adaptive toolpaths.
4. Finishing Example (High Accuracy, Best Surface Finish)
G187 P2 E0.003
Effects:
- Tight path control
- Excellent surface finish
- Slower axis motion
- Better dimensional accuracy
Used for:
- Aerospace surfaces
- Molds
- Dies
- Precision cavities
5. Medium Accuracy (General Milling)
G187 P1 E0.015
This is Haas’s default “balanced” mode for general work.
6. Verifying Active Tolerance Mode
To check which tolerance mode is active:
- Look at diagnostics → “HSM Filter”
- Look at active E-value
- Many CAM post processors force G187 modes automatically
7. Using G187 With 3D Surfacing
For 3D toolpaths:
- Rough → P0 E0.050
- Semi-finish → P1 E0.015
- Finish → P2 E0.002
This ensures:
- Smooth 3D surfaces
- Reduced faceting
- Better scallop consistency
8. Real Example — Full Pocket Program With Proper Tolerance Control
(High-speed rough)
G187 P0 E0.040
(Adaptive roughing toolpath)
(Semi-finish)
G187 P1 E0.015
(Contour / raster toolpath)
(Finish pass)
G187 P2 E0.003
(Finishing walls / floors)
Using the correct sequence dramatically improves part quality.
9. Reducing Corner Overcutting
During high-speed moves, corners may round off if tolerance is too high.
Fix:
G187 P2 E0.001
This forces accurate, sharp edges.
10. Using G187 for Aluminum vs Steel
Aluminum:
- High-speed is common → P0 or P1
- Larger E-values OK for roughing
Steel / Tool Steel:
- Smaller tolerance required
- Use P2 E0.005 for precision finishing
11. CAM Integration
Fusion 360, Mastercam, HyperMill, SolidCAM all output smoothing settings that map to:
- High smoothing → G187 P0
- Standard → G187 P1
- Precision → G187 P2
But adding G187 explicitly in G-code guarantees correct behavior.
12. Common Problems & Expert Fixes
Problem: Surface looks faceted or wavy
– E-value too large → reduce E
– Use P2 for finishing
Problem: Machine feels “slow”
– Using P2 in roughing
– Reduce accuracy mode → P0 or P1
Problem: Corners not sharp
– E too large → set to 0.005 or smaller
Problem: CAM and machine behave differently
– CAM smoothing conflicts → override with G187
13. Advanced Use — Override CAM Smoothing
If CAM outputs unwanted smoothing:
Insert:
G187 P2 E0.001
before finishing toolpath.
This forces:
- Exact geometry
- Sharp corners
- Perfect surfaces
14. Summary
G187 is a powerful tolerance and smoothing control that determines how accurately Haas CNC machines follow programmed geometry. It directly influences speed, accuracy, surface finish, and machine behavior. By controlling tolerance modes intelligently—roughing, medium, and finishing—you can dramatically improve machining performance and gain professional-level control over Haas high-speed motion algorithms in 2025.
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