How to Calculate Feed Rate in Cnc

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CNC Feed Rate Calculator

Imperial (Inches) Metric (Millimeters)

Calculated Feed Rate

function updateLabels() { var units = document.getElementById('units').value; var label = document.getElementById('label-chipload'); if (units === 'metric') { label.innerText = 'Chip Load (Feed Per Tooth – mm)'; } else { label.innerText = 'Chip Load (Feed Per Tooth – Inches)'; } } function calculateFeedRate() { var rpm = parseFloat(document.getElementById('rpm').value); var chipload = parseFloat(document.getElementById('chipload').value); var flutes = parseFloat(document.getElementById('flutes').value); var units = document.getElementById('units').value; var resultDiv = document.getElementById('cnc-result'); var output = document.getElementById('feed-rate-output'); var desc = document.getElementById('unit-description'); if (isNaN(rpm) || isNaN(chipload) || isNaN(flutes) || rpm <= 0 || chipload <= 0 || flutes <= 0) { alert('Please enter valid positive numbers for all fields.'); return; } // Formula: Feed Rate = RPM * Chip Load * Number of Flutes var feedRate = rpm * chipload * flutes; var unitText = units === 'imperial' ? 'Inches per Minute (IPM)' : 'Millimeters per Minute (mm/min)'; output.innerText = feedRate.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2}); desc.innerText = unitText; resultDiv.style.display = 'block'; }

How to Calculate Feed Rate in CNC

Understanding how to calculate the correct feed rate is critical for any CNC machining operation. Whether you are working with a milling machine, a router, or a lathe, the feed rate determines how fast the cutting tool moves through the material. Getting this number right ensures tool longevity, better surface finish, and prevents tool breakage.

The CNC Feed Rate Formula

The standard formula used in the industry to calculate feed rate is:

Feed Rate = RPM × Chip Load × Number of Flutes

Where:

  • RPM (Revolutions Per Minute): The speed at which the spindle and tool are rotating.
  • Chip Load (Feed Per Tooth): The thickness of the material that each individual cutting edge (flute) removes in one revolution.
  • Number of Flutes: The number of cutting edges on the tool (e.g., a 2-flute end mill or a 4-flute end mill).

Practical Example

Let's say you are using a 2-flute end mill with a recommended chip load of 0.002 inches per tooth. Your spindle is set to 12,000 RPM.

  • Step 1: Multiply RPM by Chip Load: 12,000 × 0.002 = 24
  • Step 2: Multiply that result by the number of flutes: 24 × 2 = 48
  • Result: Your Feed Rate should be 48 Inches Per Minute (IPM).

Why Chip Load Matters

The chip load is the most critical variable. If the chip load is too small, the tool will rub against the material rather than cutting it, generating excessive heat and dulling the tool. If the chip load is too large, you risk snapping the tool due to excessive pressure or stalling the spindle motor. Always refer to your tool manufacturer's "Speeds and Feeds" chart to find the recommended chip load for the specific material you are cutting (e.g., Aluminum, Hardwood, Acrylic, or Steel).

Tips for Better Machining

  • Material Type: Harder materials generally require slower feed rates and smaller chip loads than softer materials.
  • Rigidity: If your CNC machine is not very rigid (like many hobbyist desktop routers), you may need to reduce the feed rate or depth of cut to avoid chatter and vibration.
  • Listen to the machine: A healthy cut usually produces a consistent, crisp sound. High-pitched screaming often indicates the tool is rubbing (feed too slow), while deep chugging may indicate the tool is overloaded (feed too fast).

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