CNC Feed Rate Calculation for FANUC: SFM, RPM, IPM

By Nishikant Xalxo | CNC Machining Expert | Updated: January 10, 2025 | 10 min read

Feed rate calculation is where theory meets practice. After breaking 3 end mills in my first month, I learned these formulas the hard way. Here's how to calculate speeds and feeds correctly for FANUC CNC mills.

The Three Formulas You Must Know

1. Spindle Speed (RPM) 
RPM = (SFM × 4) ÷ Tool Diameter

Where SFM = Surface Feet per Minute (material dependent)

2. Feed Rate (IPM) 
IPM = RPM × Feed per Tooth × Number of Flutes
3. Metric Conversion 
SFM = RPM × π × Diameter (inches) ÷ 12
Feed per Tooth = IPM ÷ (RPM × Flutes)

Recommended SFM Values (Real Shop Data)

Material Carbide SFM HSS SFM Notes
Aluminum 6061 800-1200 300-400 Use high RPM, aggressive feeds
Steel 1018 400-600 150-250 Moderate speeds, watch chip color
Steel 4140 250-400 100-150 Slower, use coolant
Stainless 304 200-350 80-120 Work hardens! Keep feed up
Titanium 6Al-4V 150-250 N/A Carbide only, flood coolant

Feed Per Tooth Recommendations

Rule of thumb: Start conservative, then increase 10% until chatter begins. Back off 15% for optimal feed.

Real Calculation Example

Scenario: 1/2" 4-flute carbide end mill in Aluminum 6061 
Tool: 0.5" diameter, 4 flutes
Material: Aluminum (use 1000 SFM)
Feed per tooth: 0.003"

RPM = (1000 × 4) ÷ 0.5 = 8000 RPM
IPM = 8000 × 0.003 × 4 = 96 IPM

FANUC program:
G90 G54 G00 X0 Y0
S8000 M03
G01 X2.0 F96.0

FANUC-specific Considerations

G95 (Feed per Revolution): Use for tapping!

G95 G84 X0 Y0 Z-0.5 R0.1 F0.05 (0.05" per rev = proper tap feed)

G96/G97 (Constant Surface Speed): For turning operations on mill-turn machines.

Common Mistakes

  1. Too aggressive in corners: Use G62 automatic corner override or reduce feed manually
  2. Wrong units: Always verify G20 (inch) vs G21 (metric) matches your intent
  3. No coolant: Aluminum needs coolant to prevent chip welding
  4. Dull tools: Reduce feed 20-30% for worn tools to prevent breakage
About the Author: Nishikant Xalxo runs FANUC mills at 8000+ RPM daily. He optimized cycle times 23% through proper feed rates. Questions about specific materials? nxdecore@gmail.com

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