G87 is a specialized CNC milling cycle used for back boring operations—machining that takes place on the back side of a hole, where the tool must pass completely through the part and then expand to cut a feature that is not accessible from the front side. This cycle is essential for creating back-side counterbores, hidden recesses, undercuts, valve seats, precision hydraulic ports, and internal pockets that cannot be machined from the outside. G87 automatically handles the required motion sequence: pass-through travel, tool expansion, controlled back boring, retraction, and safe exit.
1. What G87 Actually Does
A typical back boring operation requires:
- Travel through the existing hole
- Expand or offset the tool
- Machine the backside feature
- Collapse the tool
- Retract safely through the hole
G87 automates all of these movements in a safe and controlled cycle.
2. Basic Syntax (Fanuc-Style)
G87 X# Y# Z# R# Q# P# F#
Where:
- X/Y → Hole center
- Z → Back boring depth (behind rear surface)
- R → Front-side retract plane
- Q → Feed in / feed out positions
- P → Tool expansion amount or offset
- F → Feedrate
Example:
G87 X40. Y20. Z-55. R2. Q6. P3. F120
Meaning:
- Move through the hole
- Expand tool by 3 mm
- Bore to Z –55
- Retract, collapse, and exit safely
3. Real Machine Example — Back Counterbore on Through-Hole
Hole already drilled Ø10 mm.
Goal: back-side counterbore Ø16 mm × 4 mm deep.
Example:
G00 X30. Y25.
G87 X30. Y25. Z-22. R1. Q5. P6. F90
P6 = 6 mm radial offset = tool expands to cut Ø16 mm.
4. Back-Side Valve Seat Machining Example
For hydraulic valve plates where the seat is inside the channel:
G87 X60. Y18. Z-40. R2. Q4. P2. F100
This machines the tapered seat on the hidden surface.
5. Using Tool Expansion Mechanisms
Most back boring tools support:
- Mechanical pivoting blades
- Spring-loaded expansion
- Hydraulic expansion heads
- Custom ground offset bars
G87 assumes tool expands automatically when feed is applied.
6. Multi-Hole Back Boring Pattern Example
G90 G54
G87 X10. Y20. Z-30. R1. Q6. P3. F120
X35. Y20.
X60. Y20.
G80
Perfect for aerospace and hydraulic manifold components.
7. Handling Thin Materials
For thin plates, reduce tool expansion (P value):
G87 X42. Y16. Z-12. R1. Q3. P1.5 F150
This prevents chatter and plate distortion.
8. Deep Back Boring (Long Tool Reach)
If the tool must reach deep through a long channel:
- Use slow Q feed
- Increase retract plane R
- Minimize tool expansion P
- Use through-tool coolant if possible
Example:
G87 X30. Y55. Z-75. R2. Q3. P2. F80
9. Back Spotface Example (Extremely Common)
To create a flat seating face behind a flange:
G87 X48. Y12. Z-15. R1. Q4. P5. F120
This is one of the most widely used industrial applications.
10. Common Problems & Professional Fixes
Problem: Tool hits hole wall while retracting
– P too large
– Hole misaligned
– Use G68 rotation to compensate
Problem: Vibration during back boring
– Reduce feed
– Reduce expansion
– Use shorter extension bar
Problem: Poor finish
– Use wiper insert
– Reduce Q feed
– Increase Z finish feed F
Problem: Chips accumulate behind part
– Add through-coolant or M-code flush cycle
11. G87 vs Manual Programming
Without G87, manual back boring requires:
- Long peck entries
- Custom offsets
- Tool expansion commands
- Many retract + collapse motions
G87 reduces this to a single cycle call.
12. Summary
G87 is a powerful cycle that automates one of the most difficult machinable features: back-side boring inside a hole. It handles pass-through motion, tool expansion, controlled boring, collapse, and safe retraction automatically. For aerospace, hydraulic, and high-precision manufacturing in 2025, mastering G87 is essential for producing hidden features that cannot be accessed from the outside of the part.
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