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Spindle revolutions per minute
Enter RPM#
Cutting flutes on tool
Enter flute countin
Recommended chip load for material
Enter chip load⌀
Cutting tool diameter
Enter diameterRecommended Feed Rate
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CNC Feed Rate Formula Explained
Feed rate is the speed at which the cutting tool advances through the material. Getting feed rate right is critical — too slow wastes time and causes rubbing; too fast breaks tools or leaves poor surface finish.
Feed Rate (IPM) = RPM × Number of Flutes × Chip Load per Tooth
Example: 3,500 RPM × 4 flutes × 0.004" chip load = 56 IPM
Surface Speed (SFM) = (RPM × Tool Diameter × π) ÷ 12
RPM from SFM = (SFM × 12) ÷ (π × Tool Diameter)
Surface Speed (SFM) = (RPM × Tool Diameter × π) ÷ 12
RPM from SFM = (SFM × 12) ÷ (π × Tool Diameter)
Recommended Chip Load by Material
- Aluminum (HSS/Carbide): 0.003"–0.010" per tooth
- Mild Steel: 0.001"–0.004" per tooth
- Stainless Steel: 0.001"–0.003" per tooth
- Hardened Steel: 0.0005"–0.002" per tooth
- Wood/MDF: 0.010"–0.030" per tooth
- Plastic/Acrylic: 0.005"–0.015" per tooth
⚠️ Start conservative. These are calculated starting points. Always verify with tool manufacturer recommendations and adjust based on actual cutting performance, tool wear, and machine rigidity.
Frequently Asked Questions
What is feed rate in CNC machining?
Feed rate in CNC machining is the speed at which the cutting tool moves through the workpiece, measured in inches per minute (IPM) or millimeters per minute (MMPM). It works together with spindle speed (RPM) and depth of cut to determine material removal rate, surface finish quality, tool life, and cutting forces. Optimal feed rate balances machining efficiency against tool wear and part quality.
What is chip load and why does it matter?
Chip load (also called feed per tooth or chip thickness) is the amount of material removed by each cutting edge per revolution. It is the most fundamental parameter in milling — too low causes rubbing, heat, and work hardening; too high causes chipping, tool breakage, and poor finish. Each tool manufacturer provides recommended chip loads by material and tool diameter in their cutting data charts.
How do I calculate RPM from surface feet per minute (SFM)?
RPM = (SFM × 3.82) ÷ Tool Diameter (inches). The factor 3.82 = 12 ÷ π. Example: 600 SFM with a 0.5" end mill = (600 × 3.82) ÷ 0.5 = 4,584 RPM. SFM is the surface speed recommendation from the tool manufacturer based on material — common values are 800–1,200 SFM for aluminum and 100–300 SFM for steel.
What feed rate should I use for aluminum on a CNC router?
For aluminum on a CNC router with a 2-flute, 1/4" carbide end mill, a typical starting point is: RPM 18,000, chip load 0.002"–0.003", giving a feed rate of 72–108 IPM. CNC routers typically run faster RPM than mills due to high-speed spindles. Use climb milling, ensure good chip evacuation with air blast, and use toolpaths with small radial engagement (less than 50% stepover) for best results.
How do I convert feed rate from IPM to MMPM?
To convert inches per minute (IPM) to millimeters per minute (MMPM): MMPM = IPM × 25.4. Example: 56 IPM × 25.4 = 1,422 MMPM. To convert back: IPM = MMPM ÷ 25.4. Most CNC controllers can be set to work in either unit system — just ensure all parameters are consistent.
What causes poor surface finish in CNC milling?
Common causes of poor CNC surface finish: (1) Feed rate too high — increase chip load reduces surface quality; (2) RPM too low for the material; (3) Worn or chipped cutting tool; (4) Tool deflection from excessive depth of cut or long tool overhang; (5) Machine vibration or chatter; (6) Dull insert or coating breakdown; (7) Inadequate coolant or chip evacuation. For finishing passes, reduce feed rate 30–50% and use a light radial depth of cut.
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