Part 1: How to Calculate Cutting Speed (RPM)
When machinists ask "how to calculate cutting speed," they are usually trying to figure out the correct Spindle Speed (RPM) for a given tool diameter and material. The baseline metric is Surface Feet per Minute (SFM) or Meters per Minute (m/min).
Tooling manufacturers provide the SFM rating because it represents the actual speed at which the cutting edge is traveling over the material. Smaller tools must spin faster to achieve the same SFM as larger tools.
The Imperial RPM Formula
RPM = (SFM × 3.82) ÷ Tool Diameter
- SFM: Surface Feet Per Minute (from manufacturer charts)
- 3.82: A constant derived from (12 inches / π)
- Diameter: The tool's cutting diameter in inches
Example: Cutting 6061 Aluminum (recommended SFM = 800) with a 0.5-inch end mill.
RPM = (800 × 3.82) ÷ 0.5 = 6,112 RPM.
The Metric RPM Formula
RPM = (Vc × 1000) ÷ (π × Diameter)
- Vc: Cutting Speed in meters per minute (m/min)
- Diameter: The tool's cutting diameter in millimeters
Part 2: How to Calculate Feed Rate
Once your spindle speed is set, the next step is calculating the Feed Rate—the linear speed the machine moves the tool through the workpiece. This connects directly to Chip Load (Chipload per Tooth, or IPT/FPT).
Insufficient feed causes the tool to rub and quickly dull due to heat buildup. Excessive feed will lead to tool breakage.
The Feed Rate Formula
Feed Rate = RPM × Flutes × Chip Load
- RPM: The spindle speed calculated in Part 1
- Flutes: Number of cutting edges on the tool
- Chip Load: Recommended thickness of material removed per tooth (e.g., 0.003")
Example: Using the 6,112 RPM from above with a 3-flute end mill, and a recommended chip load of 0.004".
Feed Rate = 6,112 × 3 × 0.004 = 73.34 Inches Per Minute (IPM).
Adjusting for Radial Engagement (Chip Thinning)
The basic formula assumes you are cutting straight slots (100% radial engagement) or taking a profile pass greater than 50% tool diameter. If your Stepover (Ae) is less than 50% of the tool's diameter, your actual chip thickness decreases significantly compared to the programmed feed rate. This geometric phenomenon is called Radial Chip Thinning, meaning you must drastically increase your feed rate to maintain the correct chip thickness, or the tool will rub and fail prematurely.
Stop Doing The Math By Hand
Remembering constants, accounting for metric/imperial conversions, and keeping a physical binder of material SFM ratings is an outdated workflow. Modern machining relies on digital, instantly accessible databases to generate accurate speeds and feeds.