Master the mathematics of machining: From basic RPM/IPM formulas to advanced chip thinning and material-specific strategies.
"Speeds and Feeds" is the universal language of CNC machining, but the terms are often confused. At its core, machining is about balancing two opposing forces: Heat and Pressure.
💡 The Golden Rule:
If the tool burns up, decrease Speed (RPM). If the tool breaks (snaps), decrease Feed.
Skip the math. Use our optimized calculators for specific operations and materials:
Revolutions Per Minute
SFM (Surface Feet per Minute): The constant speed at which the cutting edge moves across the material. This is a property of the material/tool combination (e.g., Carbide in Aluminum = 1200 SFM).
3.82: A constant derived from (12 / π). It converts feet to inches.
Inches Per Minute
IPT (Inches Per Tooth): Also known as Chip Load. The thickness of the chip removed by each cutting edge. Essential for tool life.
Surface Feet Per Minute
Use this to reverse-calculate the speed you are running to check if it's within tool manufacturer recommendations.
CNC materials are classified into 6 ISO groups, each with a distinct color code and machining characteristics.
Examples:
1018, 4140, 1045, A36
Characteristics:
Long continuous chips, high heat generation. Requires thermal coating (TiAlN).
Typical SFM (Carbide):
350 - 800 SFM
Examples:
303, 304, 316, 17-4 PH
Characteristics:
Work hardening, high cutting forces, built-up edge. Keep feed constant — don't dwell!
Typical SFM (Carbide):
150 - 350 SFM
Examples:
Grey Iron, Ductile Iron
Characteristics:
Short chips (powder), highly abrasive. Wears tools via abrasion. Run dry (no coolant).
Typical SFM (Carbide):
250 - 600 SFM
Examples:
6061-T6, 7075, Brass, Bronze
Characteristics:
High speed, gummy material. Needs polished flutes (ZrN or uncoated) to prevent sticking.
Typical SFM (Carbide):
1000 - 3000+ SFM
Examples:
Ti-6Al-4V, Inconel 625/718, Hastelloy
Characteristics:
Extreme heat generation, poor thermal conductivity. Heat stays in tool.
Typical SFM (Carbide):
40 - 150 SFM (Slow!)
Examples:
Hardened D2, A2 (45-65 HRC)
Characteristics:
Requires extremely rigid setup. Negative rake angles. Light cuts, high speed.
Typical SFM (Carbide):
100 - 400 SFM
When the Radial Width of Cut (WOC) is less than 50% of the tool diameter, the actual chip thickness is thinner than the programmed Feed Per Tooth (IPT).
This means your tool is rubbing instead of cutting, generating excess heat and premature wear. to fix this, you must increase your feed rate.
RCTF = 1 / √(1 - (1 - (2 × WOC / Dia))²)
Multiply your programmed IPM by this factor to maintain proper chip thickness.
Example
10% Radial Stepover
Tool Diameter: 0.500"
Feed Rate Increase Required
(If programmed 20 IPM, run 33 IPM)
| Symptom | Likely Cause | Solution |
|---|---|---|
| Tool Built-Up Edge (BUE) | Material welding to flute. Common in Aluminum/Stainless. | Increase RPM (Heat), Increase Coolant concentration, Check coating. |
| Chatter (Vibration) | Lack of rigidity, harmonics. long stick-out. | Reduce RPM (10-20%), Increase Feed (stabilizes cut), Reduce stick-out. |
| Rapid Flank Wear | Excessive speed (RPM). Friction heat. | Decrease RPM, check coolant supply. |
| Tool Chipping / Breaking | Excessive mechanical load (Feed). Runout. Recutting chips. | Decrease Feed (IPM), Inspect runout, improve chip evacuation. |
| Poor Surface Finish | Feed too high, tool deflection, BUE. | Increase RPM, Decrease Feed (for finish pass only), Take lighter depth of cut. |