Standard rule of thumb is 50% of the horizontal feed rate.
Calculated Horizontal Feed Rate:–
Recommended Plunge Rate:–
function calculatePlungeRate() {
// Get input values
var rpm = parseFloat(document.getElementById('spindleSpeed').value);
var flutes = parseFloat(document.getElementById('numFlutes').value);
var chipLoad = parseFloat(document.getElementById('chipLoad').value);
var factor = parseFloat(document.getElementById('plungeFactor').value);
// Validation
if (isNaN(rpm) || rpm <= 0) {
alert("Please enter a valid Spindle Speed.");
return;
}
if (isNaN(flutes) || flutes <= 0) {
alert("Please enter a valid Number of Flutes.");
return;
}
if (isNaN(chipLoad) || chipLoad <= 0) {
alert("Please enter a valid Chip Load.");
return;
}
if (isNaN(factor) || factor <= 0) {
alert("Please enter a valid Plunge Percentage.");
return;
}
// Calculation: Feed Rate = RPM * Flutes * Chip Load
var feedRate = rpm * flutes * chipLoad;
// Calculation: Plunge Rate = Feed Rate * (Percentage / 100)
var plungeRate = feedRate * (factor / 100);
// Display Results
var resultContainer = document.getElementById('plungeResults');
var feedDisplay = document.getElementById('displayFeedRate');
var plungeDisplay = document.getElementById('displayPlungeRate');
// Determining unit label context (generic units/min)
var unitLabel = " units/min";
feedDisplay.innerHTML = feedRate.toFixed(2) + unitLabel;
plungeDisplay.innerHTML = plungeRate.toFixed(2) + unitLabel;
resultContainer.style.display = 'block';
}
How to Calculate Plunge Rate for CNC Machining
In CNC machining and routing, the Plunge Rate is the speed at which the router bit or cutting tool moves vertically (Z-axis) into the material before it begins its horizontal cut. Setting the correct plunge rate is crucial for preventing tool damage, avoiding burn marks on the material, and ensuring accurate cuts.
The Core Formulas
While horizontal Feed Rate is calculated based on the chip load, RPM, and number of flutes, the Plunge Rate is typically derived as a percentage of that Feed Rate.
1. Calculate Horizontal Feed Rate:
Feed Rate = Spindle Speed (RPM) × Number of Flutes × Chip Load
2. Calculate Plunge Rate:
Plunge Rate = Feed Rate × Plunge Factor (%)
Understanding the Variables
Spindle Speed (RPM): The rotational speed of the tool. This is usually determined by the material hardness and tool diameter.
Number of Flutes: The number of cutting edges on the bit. A 1-flute bit clears chips faster, while a 4-flute bit provides a smoother finish but generates more heat.
Chip Load: The thickness of the material removed by each cutting edge during one revolution. This is the most critical factor for tool life.
Plunge Factor: Since cutting vertically is harder on the tool than cutting horizontally (due to poor chip evacuation), the plunge rate is usually reduced.
Safe Plunge Rate Percentages
The "Plunge Factor" varies depending on the material and the type of entry:
Soft Woods / Plastics: 50% of the feed rate is standard.
Hard Woods: 40-50% of the feed rate.
Aluminum / Soft Metals: 20-30% of the feed rate is recommended to prevent the tool from welding to the material.
Ramping Moves: If your machine can "ramp" into the material (move diagonally rather than straight down), you can often increase the plunge rate closer to 100% of the feed rate.
Why Not Plunge at 100%?
End mills and router bits are designed primarily to cut from the side (radially). While many are "center cutting" (designed to drill), the geometry at the bottom of the tool is not as efficient as a dedicated drill bit. Plunging too fast creates immense heat and pressure because chips have nowhere to go, leading to work hardening in metals or burning in wood.
Using the calculator above helps you find a safe starting point. Always listen to the machine; a high-pitched scream usually indicates the feed or plunge is too slow (rubbing), while a low-pitched bogging sound indicates it is too fast.