Burned edges, fuzzy cuts, snapped bits — nine times out of ten the culprit is chip load: how much material each cutting edge removes per revolution. Too low and the bit rubs and overheats; too high and you deflect or break the tool. Here are practical starting ranges, the formula, and a free calculator that does the math for you.
The formula
Feed rate (IPM) = RPM × number of flutes × chip load
Rearranged: Chip load = feed rate ÷ (RPM × flutes). Most CNC routers run happiest at ~75% of maximum spindle RPM — set that first, then solve for feed.
Chip load starting ranges (inches per tooth)
Typical starting points for solid-carbide spiral bits. Always confirm against the tool manufacturer's chart for your exact series, and treat these as a starting window to tune from.
| Material | 1/8" bit | 1/4" bit | 3/8" bit | 1/2" bit |
|---|---|---|---|---|
| Softwood / plywood | 0.004–0.006 | 0.009–0.011 | 0.015–0.018 | 0.019–0.021 |
| Hardwood | 0.003–0.005 | 0.008–0.010 | 0.014–0.016 | 0.018–0.020 |
| MDF / particle board | 0.004–0.007 | 0.011–0.013 | 0.015–0.018 | 0.020–0.023 |
| Soft plastic | 0.003–0.005 | 0.007–0.010 | 0.010–0.012 | 0.012–0.016 |
| Hard plastic / acrylic | 0.002–0.004 | 0.006–0.009 | 0.008–0.010 | 0.010–0.012 |
| Aluminum (with lubricant) | 0.002–0.003 | 0.003–0.005 | 0.004–0.006 | — |
Worked example
Cutting hard maple with a 1/4" 2-flute compression bit at 18,000 RPM, starting chip load 0.009":
18,000 × 2 × 0.009 = 324 IPM — then tune: chips should be small solid flakes, not dust (rubbing) and not thick splinters (overloading).
Five rules of thumb
- Dust means too little chip load — speed up the feed or slow the spindle.
- Single-flute bits let slower machines reach proper chip load — the go-to for plastics.
- Plastics melt before they burn — if edges re-weld, increase chip load immediately.
- Aluminum wants lubricant — a mist system with Coolube plus an O-flute bit prevents chip welding.
- Depth of cut changes everything — past 1× diameter deep, reduce chip load ~25%; past 2×, ~50%.
Skip the math
Our Feed Rate Calculator builds these values in — pick your material, bit series and diameter, and it returns your feed rate. Then grab the right tool:
- Hardwood & plywood bits — compression and spiral
- Aluminum cutting bits — O-flutes and finishers
- Hard plastic & acrylic bits
Full walkthrough of the calculation: Calculating Feed Rate. Questions about a specific material or machine? Ask us — same-day answers.
