CNC Plunge Rate Calculator
Plunge rate is the Z-axis feed used when an end mill drops straight into the work, and getting it right protects both the tool and the spindle. This calculator derives the lateral feed rate from spindle RPM, flute count, and the per-tooth chipload your tool maker specifies, then applies your chosen plunge percentage to give a safe Z feed. It also reports the surface (cutting) speed so you can sanity-check RPM against material guidance. Chipload is tool and material specific, so it is a user-editable input rather than a guessed figure.
Plunge rate formula
Lateral feed F = RPM * flutes * chipload
Plunge feed = F * (plunge percent / 100)
Surface speed = pi * diameter * RPM
Feed per revolution = flutes * chipload
Surface speed uses the same length unit as the diameter you enter. Convert to surface feet per minute by dividing by 12 if your diameter is in inches, or to meters per minute by dividing by 1000 if in millimeters.
Working with plunge rate
- Plunge percentages of 25 to 50 percent of lateral feed are common to protect the tool tip.
- Helical or ramped entry lets you raise the effective plunge feed safely.
- Always use the chipload range published by your specific tool maker for the material.
- Center-cutting end mills are required for any straight plunge into solid stock.
- Check surface speed against the recommended cutting speed for your material and coating.
CNC plunge rate: frequently asked questions
What is plunge rate in CNC machining?
Plunge rate is the speed at which the cutting tool moves straight down into the workpiece along the Z axis, measured in units per minute. Because the center of an end mill cuts poorly, plunge rate is usually set lower than the lateral feed rate to avoid breaking the tool or overloading the spindle.
How is feed rate calculated from chipload?
Feed rate equals RPM times the number of flutes times the chipload (feed per tooth): F = RPM x flutes x chipload. Plunge rate is commonly set as a fraction of this lateral feed, often 25 to 50 percent, because the tool tip is less efficient than the flutes on the side.
What chipload should I use?
Chipload depends on tool diameter, material, and tool material. Manufacturers publish recommended feed-per-tooth ranges for each cutter. Because the correct chipload is tool and material specific, this calculator takes it as a user-editable input rather than guessing a value.
Why set plunge rate lower than feed rate?
When a tool plunges straight down, the cutting happens near the tool center where edge speed approaches zero, so chip evacuation is poor and cutting forces concentrate. Reducing plunge rate, ramping, or helical entry reduces the chance of tool breakage and spindle stall.
How does surface speed relate to RPM?
Surface speed (cutting speed) is the speed of the cutting edge relative to the work: SFM = pi x diameter x RPM, with diameter in feet, or surface meters per minute with diameter in meters. It is reported here so you can check it against the recommended cutting speed for your material and tool.
Official sources
- National Institute of Standards and Technology: NIST, machining metrology.
- American Society of Mechanical Engineers: ASME, machine tool standards.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.