Calculate the required feed rate (mm/min) for CNC rigid tapping.
Enter the rotational speed of the spindle.
Enter the distance between threads (e.g., 1.5 for M10x1.5).
Calculated Feed Rate
0 mm/min
Formula: RPM × Pitch
function calculateTapFeed() {
var rpmInput = document.getElementById('spindleSpeed');
var pitchInput = document.getElementById('threadPitch');
var resultBox = document.getElementById('resultOutput');
var resultValue = document.getElementById('feedResult');
var formulaDisplay = document.getElementById('formulaDisplay');
var rpm = parseFloat(rpmInput.value);
var pitch = parseFloat(pitchInput.value);
if (isNaN(rpm) || isNaN(pitch) || rpm <= 0 || pitch <= 0) {
alert("Please enter valid positive numbers for both Spindle Speed and Thread Pitch.");
resultBox.style.display = "none";
return;
}
// Calculation: Feed Rate (mm/min) = RPM * Pitch (mm)
var feedRate = rpm * pitch;
// Formatting the output
// Most CNC controllers accept integers or up to 1-2 decimal places.
// We will show up to 2 decimal places if necessary.
var formattedFeed = Math.round(feedRate * 100) / 100;
resultValue.innerHTML = formattedFeed + " mm/min";
formulaDisplay.innerHTML = rpm + " RPM × " + pitch + " mm = " + formattedFeed + " mm/min";
resultBox.style.display = "block";
}
Understanding Metric Tap Feed Rate
In CNC machining, specifically during rigid tapping cycles, synchronizing the spindle speed with the linear feed of the tool is critical. Unlike a drill bit which cuts material continuously, a tap must follow a precise helical path to cut threads without stripping them or breaking the tool.
The Feed Rate determines how fast the tap descends into the hole in millimeters per minute (mm/min). If this rate does not perfectly match the rotation of the tap, the threads will be damaged.
Note: This calculator assumes your machine is configured for Feed Per Minute (G94). If your machine is set to Feed Per Revolution (G95), the feed rate is simply equal to the pitch of the thread.
The Tapping Formula
The logic behind calculating the feed rate for metric taps is straightforward physics. For every single revolution of the spindle, the tap must advance into the material by a distance exactly equal to the Thread Pitch.
Spindle Speed (S): Measured in RPM (Revolutions Per Minute).
Thread Pitch (P): Measured in mm (millimeter per thread).
Example Calculation
Let's say you are tapping an M8 x 1.25 hole and you want to run the spindle at 500 RPM.
RPM: 500
Pitch: 1.25 mm
Calculation: 500 × 1.25 = 625
Your Feed Rate is 625 mm/min.
Standard Metric Coarse Pitches Reference
When programming your CNC machine, you often need to look up the standard pitch for a specific metric diameter. Below is a quick reference table for common metric coarse threads:
Metric Size
Standard Pitch (mm)
Recommended Drill Size (mm)
M3
0.5
2.5
M4
0.7
3.3
M5
0.8
4.2
M6
1.0
5.0
M8
1.25
6.8
M10
1.5
8.5
M12
1.75
10.2
M16
2.0
14.0
Tips for Successful CNC Tapping
Verify Mode: Ensure your CNC controller supports rigid tapping (often G84 cycle). Older machines without encoder feedback on the spindle may require a floating tap holder (tension-compression holder).
Decimal Precision: While the math is exact, some CNC controllers round values. It is safer to use an integer RPM that results in an integer Feed Rate (e.g., using 1000 RPM with a 1.25 pitch = 1250 mm/min) to avoid rounding errors.
Lubrication: Tapping requires excellent lubrication. Use a rich coolant mix or specific tapping oil (Moly-D) to prevent the tap from welding to the material.
Clearance Plane: Ensure your R-plane (retract plane) is high enough to allow chips to clear but close enough to minimize cycle time.