Plunge Rate Calculator

CNC Plunge Rate Calculator

Recommended Plunge Rate:

function calculatePlungeRate() { var rpm = parseFloat(document.getElementById('spindleSpeed').value); var flutes = parseFloat(document.getElementById('numFlutes').value); var chipLoad = parseFloat(document.getElementById('plungeChipLoad').value); var unit = document.getElementById('unitType').value; if (isNaN(rpm) || isNaN(flutes) || isNaN(chipLoad) || rpm <= 0 || flutes <= 0 || chipLoad <= 0) { alert("Please enter valid positive numbers for all fields."); return; } var plungeRate = rpm * flutes * chipLoad; var resultElement = document.getElementById('plungeResultValue'); var wrapper = document.getElementById('plungeResultWrapper'); var description = document.getElementById('plungeDescription'); wrapper.style.display = "block"; if (unit === "imperial") { resultElement.innerHTML = plungeRate.toFixed(2) + " IPM"; description.innerHTML = "Calculated at " + chipLoad + " inches per tooth."; } else { resultElement.innerHTML = plungeRate.toFixed(0) + " mm/min"; description.innerHTML = "Calculated at " + chipLoad + " mm per tooth."; } }

---

Understanding CNC Plunge Rates

In CNC machining, the plunge rate is the speed at which the cutting tool moves vertically downward into the workpiece (the Z-axis). It is distinct from the feed rate, which typically refers to horizontal movement (X and Y axes).

The Plunge Rate Formula

The calculation for plunge rate follows the fundamental machining formula for feed speed, but uses a specific "plunge chip load" which is generally much lower than the horizontal chip load:

Plunge Rate = RPM × Number of Flutes × Plunge Chip Load

Why Plunge Rates Differ from Feed Rates

Most end mills are not designed to move vertically like a drill bit. Because they have "end teeth" rather than a continuous spiral point, plunging creates significant stress on the tool and the spindle. Here is why you should adjust your plunge rate:

• Chip Clearance: When plunging straight down, chips have nowhere to go but up the flutes, which can lead to clogging and tool breakage.
• Tool Design: Not all end mills are "center-cutting." Non-center-cutting tools cannot plunge straight down at all and require a ramp or helix entry.
• Heat Buildup: Plunging generates concentrated heat at the tip of the tool, which can dull carbide quickly.

Practical Example

Suppose you are using a 1/4″ 2-flute end mill in plywood at 18,000 RPM. Your horizontal chip load might be 0.005″, but for a straight vertical plunge, you might reduce that to 0.002″ per tooth.

Calculation: 18,000 RPM × 2 Flutes × 0.002″ = 72 Inches Per Minute (IPM).

Plunge Rate Tips for Beginners

1. The 50% Rule: If you are unsure of the plunge chip load, a common industry starting point is to set your plunge rate to 50% of your horizontal feed rate.
2. Use Ramping: Whenever possible, use a "Ramp" or "Helical" entry instead of a straight plunge. This allows the tool to use its side cutting edges, which are much more efficient.
3. Listen to the Machine: A high-pitched screaming or heavy vibration during a plunge usually indicates the rate is too high or the RPM is too fast for that vertical descent.

Frequently Asked Questions

Can I plunge with a compression bit?

You can, but you must be very careful. Compression bits are notorious for poor chip clearance during plunges. It is highly recommended to ramp into the material with compression bits to avoid burning.

What is "Peck" drilling?

Peck drilling is a technique where the tool plunges a small distance, retracts to clear chips, and then plunges deeper. If you must plunge deeply, use a peck cycle with a conservative plunge rate.