CNC Speeds and Feeds Calculator
Understanding Speeds and Feeds in CNC Machining
In the world of CNC milling, "Speeds and Feeds" refer to two distinct but interconnected variables that determine how a cutting tool interacts with the material. Getting these numbers right is the difference between a perfectly finished part and a snapped tool or a fire on the machine bed.
1. Spindle Speed (RPM)
Spindle speed is the rotational speed of the cutting tool, measured in Revolutions Per Minute (RPM). It is calculated based on the Surface Feet per Minute (SFM), which is a constant determined by the material being cut (e.g., Aluminum, Steel, or Wood) and the tool material (Carbide vs. HSS).
The Formula:
RPM = (SFM × 12) / (π × Tool Diameter)
2. Feed Rate (IPM)
Feed rate is the speed at which the cutting tool moves through the material laterally, measured in Inches Per Minute (IPM). This is dependent on how many "teeth" or flutes the tool has and how much material each tooth should carve out in a single pass.
The Formula:
IPM = RPM × Number of Flutes × Chip Load (IPT)
Key Definitions
- SFM (Surface Feet per Minute): The linear speed at the outermost edge of the tool. Harder materials require lower SFM.
- Chip Load (IPT): The thickness of the material slice taken by a single flute. Also called "Inches Per Tooth". Too low a chip load causes "rubbing," which generates heat and dulls the tool. Too high a chip load will break the tool.
- Flutes: The cutting edges on a mill. Common end mills have 2, 3, or 4 flutes.
Real-World Example
If you are milling 6061 Aluminum with a 1/4″ (0.250) 3-flute carbide end mill:
- Typical SFM for Aluminum: 600-1000 (Let's use 800)
- Typical Chip Load for 1/4″ tool: 0.002″
- Calculation: RPM = (800 * 12) / (3.1415 * 0.25) = 12,223 RPM
- Calculation: IPM = 12,223 * 3 * 0.002 = 73.3 IPM
Always listen to your machine. If you hear a high-pitched scream, your speed might be too high (RPM). If you hear a heavy vibration or "chatter," you may need to adjust your feed rate or depth of cut.