Feeds, Speeds, and Chip Load for Hobby CNC

The Goal: Making Chips, Not Dust A CNC router bit should be cutting — shearing material in clean chips. If it's producing fine dust, you're either spinning too fast or moving too slowly (or both). Dust means the material is being rubbed and burned rather than cut. Proper chip load produces visible chips and gives you longer bit life, better surface finish, and safer operation. -- What Is Chip Load? Chip load is the thickness of material removed per flute per revolution. It's the fundamental parameter that all feed/speed calculations are based on. Formula: Feed Rate (mm/min) = RPM × Number of Flutes × Chip Load (mm) Or rearranged: Chip Load = Feed Rate ÷ (RPM × Number of Flutes) Target chip loads by material: -- Worked Example: 1/4in 2-Flute Upcut in Hardwood Target chip load: 0.05mm Bit: 2 flutes RPM: 18,000 (Makita router speed 3) Feed Rate = 18,000 × 2 × 0.05 = 1,800 mm/min This gives you a starting feed rate of 1,800mm/min. -- Depth of Cut (DOC) and Stepover Depth of cut (DOC): How deep each pass goes. A common starting point is 50% of bit diameter for slotting cuts. 1/4in bit → 3mm DOC 1/8in bit → 1.5mm DOC Stepover: How much the bit moves sideways per pass (for pocket/surface operations). Common: 40-50% of diameter for roughing, 5-15% for finishing passes. -- Diagnosing Problems Burning/charring: RPM too high relative to feed, or bit is dull. Increase feed rate or decrease RPM. Rough surface / tearout: Feed rate too high or DOC too aggressive. Reduce feed rate 20%. Chatter (vibrating/chattering sound): Feed rate too low, DOC too deep, or workpiece not secured. Increase feed or reduce DOC. Bit deflection (wandering path): Feed rate too aggressive for the bit diameter/length. Reduce DOC or feed. Broken bit: Feed too aggressive, DOC too deep, or hit a void/knot. Start conservative. -- RPM Reference: Common Hobby Routers -- Quick Starting Points Use these when you don't want to calculate from scratch: These are starting points. Always run the first pass slowly and listen — adjust from there.

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