G94 and G95 are two of the most misunderstood but critically important CNC feed modes. G94 commands feed per minute (mm/min or in/min), which is predominantly used in milling. G95 commands feed per revolution (mm/rev), which synchronizes tool feed with spindle rotation—essential for turning, threading, boring, and rigid material machining. Understanding these two modes determines tool life, chip formation, surface finish, cycle time efficiency, and machine safety. In modern CNC shops, incorrect feed selection is one of the top causes of broken tools and poor part quality, especially when switching between turning and milling centers.
1. What G94 Actually Does
G94 runs feed in linear speed units—mm/min or in/min—regardless of spindle speed change.
Example:
G94 F250
This means the axis feeds at 250 mm/min even if RPM increases or decreases.
When spindle speed changes under CSS (G96), chip load fluctuates. That is why G94 is rarely used for turning.
Best suited for:
- Milling
- Drilling without synchronized chip load
- 3-axis contour milling
- Face milling in aluminum
The operator must monitor chip load manually.
2. What G95 Actually Does
G95 synchronizes feedrate with spindle rotation:
Feed = F × RPM
Example:
G95 F0.25
Means 0.25 mm per revolution—constant chip thickness regardless of RPM changes.
Essential for:
- Turning
- Boring
- Threading
- Grooving
- Deep feed tool engagement
This mode ensures chip load stability and predictable finish.
3. Real Turning Example
G95 ensures consistent material removal:
G95
F0.22
G01 X25.0 Z-45.0
Chip thickness remains constant across the cut, improving tool life dramatically.
4. Milling Example (Where G95 Is Dangerous)
If you switched to G95 in milling unintentionally:
F0.20 Z-10
At 4,000 rpm:
Feed = 800 mm/min
At spindle slowdown to 1,500 rpm:
Feed = 300 mm/min
This variability can destroy the tool or burn material.
5. G94 vs G95 When Using G96 CSS (Constant Surface Speed)
CSS changes spindle speed automatically.
Using G94 = fluctuating chip load
Using G95 = constant chip load
This is why pro shops recommend:
CSS ➜ Use G95 always.
6. G94 vs G95 in Threading
Threading (G32, G76) always requires G95 because pitch must equal feed per rev.
Using G94 will result in:
- pitch drift
- oversize threads
- tool breakage
- alarms
7. Real Multi-Operation Program Switch Example
N10 G96 S180 M03
N20 G95 F0.25
(Cut OD turning)
After turning:
N200 G94
(Fixture positioning and rapid feed operation)
Switching modes intentionally is standard practice.
8. Surface Finish, Tool Life & Heat Generation
G95 advantages:
- predictable cutting force
- higher tool life
- safer finishing
- accurate diameter control
G94 advantages:
- smoother 3-axis motion in milling
- CAM-generated feed transitions
- predictable linear feed rate
9. The Number One Mistake Shops Make
Technicians forget mode states when changing tools.
Example:
Lathe drilling with live tools:
G95 active → CAM code imports → linear drilling behaves erratically.
Industry rule:
Before tool change, explicitly specify mode:
G94 or G95 before motion.
10. Professional Rules for 2025 CNC Shops
- Turning? Always G95.
- Facing with CSS? G95.
- Threading? Must be G95.
- Milling pockets? Use G94.
- Contour milling? Use G94.
- Drilling steel? Either OK, but G95 yields better chip consistency.
11. Real Troubleshooting Table
| Symptom | Likely Cause | Fix |
|---|---|---|
| Tool burns corners in turning | Using G94 | Switch to G95 |
| Thread pitch inconsistent | Using G94 | Switch to G95 |
| CAM feed erratic | Using G95 in milling | Switch to G94 |
| Tool chatter on OD finish | G94 | G95 improves stability |
12. Summary
G94 controls feed as speed over time, ideal for milling. G95 synchronizes feed with spindle rotation, ideal for turning and CSS machining. Professional CNC programmers explicitly switch between modes to stabilize tool load, protect tooling, and produce consistent finishes. Mastering G94 vs G95 is a defining capability for elite CNC machining performance in 2025 and beyond.
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