G187 is a unique Haas CNC command that controls the balance between speed, accuracy, and surface finish. It allows programmers to dictate how aggressively the machine blends motion during contouring and 3D toolpaths. G187 is essential in high-speed machining because it directly affects corner rounding, toolpath smoothing, interpolation accuracy, and overall surface quality. By adjusting the P and E values inside G187, machinists can fine-tune performance for roughing, semi-finishing, or finishing.
1. What G187 Does
G187 adjusts the machine’s internal tolerance and smoothing behavior. It affects:
- How tightly the tool follows the programmed path
- How much the machine slows down before sharp transitions
- Surface finish quality on 3D geometries
- Overall cycle time
- Axis acceleration and blending limits
Lower tolerance = higher accuracy, slower motion.
Higher tolerance = faster motion, lower accuracy.
2. Syntax of G187
Basic format:
G187 P# E#
Where:
- P defines the motion mode
- P1 = High Accuracy (finishing)
- P2 = Medium Mode (general cutting)
- P3 = High Speed (roughing)
- E defines the allowed tolerance deviation
Example:
G187 P1 E0.003
Very high accuracy, minimal deviation.
3. Roughing Example (Fastest)
G187 P3 E0.050
This maximizes speed and allows generous blending, ideal for roughing with large stepovers.
Example roughing block:
G90 G54
T12 M06
G187 P3 E0.050
G01 X150. Y80. F6000
G01 Z-12. F2500
4. Semi-Finishing Example (Balanced)
G187 P2 E0.020
Used for medium-speed machining where accuracy matters but speed is still important.
5. Finishing Example (Highest Precision)
G187 P1 E0.003
Ideal for mold surfaces, pockets with tight tolerance, and thin-wall machining.
Finishing block:
G90 G54
T5 M06
G187 P1 E0.003
G05 P2 (if optional HSM smoothing enabled)
G01 X55.500 Y40.200 Z-2.600 F1800
6. Interaction with HSM and AI Modes
On newer Haas controls, G187 interacts with:
- AICC (High-speed smoothing)
- G187 factory tolerance limits
- Rotary smoothing settings during 4-/5-axis motion
Haas recommends:
- Using P1 with 3D finishing
- Avoiding P3 during delicate cuts
- Adjusting E based on CAM tolerance
7. When to Use Each Mode
Use P3 for:
- Roughing large pockets
- Adaptive clearing
- Non-critical geometry
Use P2 for:
- General contouring
- Semi-finish passes
- Medium complexity surfaces
Use P1 for:
- 3D surfacing
- Mold finishing
- Aerospace thin-wall machining
- Tight profile tolerance
8. Common Mistakes to Avoid
- Using P3 on finishing → causes waves in surfaces
- Using tiny E values on roughing → slow cycle time
- Forgetting to reset to P1 before final pass
- Combining aggressive CAM tolerances with large E values
9. Summary
G187 is one of the most powerful Haas commands for controlling machining performance. By fine-tuning motion tolerance using P and E values, a machinist can dramatically improve surface finish, reduce cycle time, and ensure stable high-speed cutting. Mastery of G187 is essential for shops doing advanced mold work, aerospace machining, or any application requiring tight dimensional accuracy.
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