G71 is the most widely used rough turning cycle on Fanuc-controlled CNC lathes. It automates the entire rough-cutting process by repeatedly cutting along a defined profile until the part reaches a predetermined near-net shape. G71 handles stock removal, depth control, retract clearance, and cutting sequence automatically, making it a powerful tool for efficient turning. It supports both OD and ID roughing and works with extremely complex profiles including tapers, radii, grooves, and blended shapes.
1. Two-Block G71 Format (Modern Fanuc)
G71 uses two blocks:
Block 1 — Roughing conditions
G71 U(depth) R(retract)
Block 2 — Finishing profile conditions
G71 P(start) Q(end) U(finish X) W(finish Z) F(feed)
Example:
G71 U2.0 R0.5
G71 P100 Q200 U0.3 W0.15 F0.25
Meaning:
- U2.0 → depth of cut per pass = 2 mm
- R0.5 → retract amount = 0.5 mm
- P100/Q200 → profile from N100 to N200
- U0.3 → finishing allowance in X
- W0.15 → finishing allowance in Z
- F0.25 → roughing feedrate
2. Real External Roughing Example
N100 G00 X40. Z2.
N110 G01 Z-35.
N120 G01 X60.
N130 G01 Z-70.
N200
G71 U2. R0.5
G71 P100 Q200 U0.4 W0.2 F0.3
This cycle will remove all material while following the exact geometric profile.
3. Internal Boring Roughing Example
N300 G00 X32. Z2.
N310 G01 Z-45.
N320 G01 X50.
N330 G01 Z-70.
N400
G71 U1.5 R0.4
G71 P300 Q400 U0.30 W0.15 F0.20
This produces a smooth bored profile with accurate stock allowance for finishing.
4. G71 Roughing with Radii & Tapers
G71 supports profiles containing:
- Radii (G02/G03)
- Tapers
- Compound angles
- Blended geometry
Example profile:
N500 G01 X55. Z-20.
N510 G03 X65. Z-30. R10.
N520 G01 Z-60.
N600
G71 U2. R0.5
G71 P500 Q600 U0.3 W0.15 F0.25
The cycle automatically roughs everything including the radius.
5. Common G71 Mistakes and Fixes
1. Alarm: “Profile not monotonic”
The X values must move consistently (OD decreasing, ID increasing).
Fix: Ensure profile lines move in the correct direction.
2. “Interference in G71 cycle”
Profile loops back or crosses itself.
Fix: Check the geometry around grooves and radii.
3. Wrong retract causing collisions
R value too small.
Fix: Increase R retract for clearance.
4. Wrong finish allowance
U/W values incorrect.
Fix: Match finishing tool requirements.
6. Real Shop Tips for Perfect G71 Roughing
- Use G50 to set safe max spindle speed.
- Always leave enough stock (U/W) for the finish tool.
- Use G96 CSS mode for stable cutting load.
- Avoid extremely deep U cuts—causes chatter.
- Insert breakage is reduced by lowering feed on the first few passes.
Example:
G96 S220
G71 U1.8 R0.5
G71 P150 Q250 U0.25 W0.15 F0.22
Perfect for tough steels like 4140 or 17-4 PH.
7. Finishing After Roughing (Typical Sequence)
After G71, use:
- G70 finishing cycle
- with the same P/Q profile
Example:
G70 P100 Q200
Produces clean, final geometry.
8. Summary
G71 is a critical CNC turning cycle for efficient roughing of external and internal profiles. It handles stock removal automatically, follows complex geometry, and prepares the part for a final finishing operation. Used correctly, G71 dramatically improves productivity, accuracy, and tool life—making it a must-have skill for every CNC lathe programmer in 2025.
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