Feeds Speeds and Chip Load for Hobby CNC
Why Feeds and Speeds Matter
Run a router bit too slow and it rubs instead of cuts — generating heat, burning the wood, and dulling the bit fast. Run it too fast and the load exceeds what the bit can handle — chatter, deflection, or outright breakage. The sweet spot is chip load: the right amount of material removed per cutting edge per revolution.
The Chip Load Formula
Chip load = Feed rate (mm/min) / (RPM × number of flutes)
Chip load is measured in mm — the thickness of the chip each cutting edge removes per revolution.
Example: 2-flute bit at 18,000 RPM, 1,800mm/min feed rate: 1800 / (18,000 × 2) = 0.05mm chip load
Target Chip Loads by Material
| Material | Target Chip Load (mm) | Notes |
|---|---|---|
| Softwood (pine, poplar) | 0.05–0.10 | Forgiving, wide range works |
| Hardwood (oak, maple) | 0.03–0.07 | Too low = burning |
| MDF | 0.04–0.08 | Very forgiving, high dust |
| Plywood | 0.04–0.08 | Watch for face tearout |
| Acrylic | 0.05–0.12 | Needs O-flute, keep it up |
| HDPE | 0.06–0.12 | Waxy — go faster not slower |
| Aluminum (6061) | 0.01–0.03 | Lubrication essential |
These are starting points. Adjust based on what you hear and see.
Solving for Feed Rate
Once you know your target chip load, RPM, and flute count — solve for feed rate:
Feed rate = Chip load × RPM × flutes
Example: Target 0.06mm chip load, 18,000 RPM, 2-flute bit: 0.06 × 18,000 × 2 = 2,160 mm/min
Depth Per Pass and Stepover
Depth per pass (DOC — depth of cut): For hardwood with a 1/4in 2-flute bit, 1.5-2mm per pass is a reasonable starting point. For softwood or MDF, 2-3mm. This is the axial engagement.
Stepover: The amount the bit moves laterally between passes during pocketing. Typically 40-50% of bit diameter for roughing, 5-10% for finishing passes. 40% of a 6.35mm (1/4in) bit = 2.5mm stepover.
Rule of thumb: Depth of cut rarely exceeds 1× bit diameter for hardwood, 1.5× for softwood.
What Bad Cuts Look/Sound Like
Burning: Feed rate too low, RPM too high, or dull bit. The chips are too thin — friction instead of cutting.
Chatter / vibration: RPM too high for the cutting load, or bit deflecting. Lower RPM or reduce DOC.
Rough surface: Too aggressive DOC or stepover. Add a light finishing pass.
Bit breakage: Feed too fast, DOC too deep, or incorrect bit type for material. Reduce aggressiveness.
Fuzzy exit edge (tearout): Going against the wood grain, or upcut bit on face veneer. Switch to downcut or compression bit for final pass.
Speeds for Common Hobby CNC Setups
Shapeoko 4 / Carbide Compact Router (65W, 10k-30k RPM)
| Material | Bit | RPM | Feed (mm/min) | DOC |
|---|---|---|---|---|
| Pine/Poplar | 1/4" 2F upcut | 16,000 | 2,500 | 2.5mm |
| Hardwood | 1/4" 2F upcut | 18,000 | 1,500 | 1.5mm |
| MDF | 1/4" 2F upcut | 16,000 | 3,000 | 2.5mm |
| Acrylic | 1/4" O-flute | 18,000 | 1,200 | 1.5mm |
LongMill MK2 / Makita RT0701C (1.25HP, 10k-30k RPM)
| Material | Bit | RPM | Feed (mm/min) | DOC |
|---|---|---|---|---|
| Pine/Poplar | 1/4" 2F upcut | 16,000 | 2,800 | 2.5mm |
| Hardwood | 1/4" 2F upcut | 18,000 | 1,400 | 1.5mm |
| MDF | 1/4" 2F upcut | 16,000 | 3,200 | 2.5mm |
Tuning in Practice
Start conservative. Run a short test pass and listen:
- Clean, quiet cutting with visible chips: good, you can push feed rate 10-20%
- High-pitched screaming or whine: reduce RPM or feed
- Dull thudding or chatter: increase RPM or reduce DOC
- Burning smell: immediately increase feed or reduce RPM
Check chips after the pass. Wood should produce actual chips — not fine dust. Fine dust means chips are too thin — increase feed rate.