Thread Milling Calculator
Calculate helical interpolation feeds, speeds, and thread geometry for CNC thread milling operations.
About this calculator
This calculator sizes a CNC thread-milling operation: given the target thread's Major Thread Diameter and Thread Pitch plus the thread mill cutter's diameter, spindle speed, chip load, and flute count, it computes the helical feed rate to program, how long one helical pass takes, and the resulting thread geometry. Thread milling cuts a thread by orbiting a rotating end-mill-style cutter in a helical path -- a circular XY motion combined with a steady Z-axis advance equal to one thread pitch per revolution -- rather than a single-point tap cutting the whole thread profile in one linear pass. Spindle RPM and Chip Load drive the raw cutting-edge feed rate the way they would for any milling operation, but that raw number has to be compensated for helical interpolation: the CNC controller programs the velocity of the tool's center as it orbits, while the target chip thickness is set at the cutting edge, which travels a different-radius circle than the center path does. Thread Depth and Minor Diameter follow the standard 60-degree thread-form approximation (depth is roughly 0.6134 times the pitch), and Threads per Inch is simply the reciprocal of Pitch.
This calculator covers INTERNAL thread milling only -- boring a thread into a hole, where the tool orbits inside the bore and the helix diameter it travels is Major Diameter minus Tool Diameter. External (OD) thread milling, where the tool orbits around the outside of a boss or shaft, uses the opposite geometry (helix diameter equals Major Diameter plus Tool Diameter) and is not what this tool computes -- do not use these results for an external thread-milling setup. Tool Diameter must also stay smaller than Major Thread Diameter for an internal setup to be physically possible (the cutter has to fit inside the hole it's threading); this calculator will not let Tool Diameter reach or exceed Major Thread Diameter. What this calculator does NOT account for: cutting-edge wear, radial engagement and chip-thinning effects specific to circular interpolation at very small helix diameters, coolant and chip-evacuation limits, or machine dynamic and servo response limits, all of which often force a slower real-world feed rate than the theoretical value calculated here.
Inputs
Results
Helical Feed Rate
6 IPM
How to Use This Calculator
- Enter the major thread diameter and thread pitch (or TPI) for the required thread specification.
- Set the thread mill cutter diameter, spindle RPM, chip load per tooth, and flute count.
- Review helical feed rate, time per threading pass, thread depth, and minor diameter.
- Program a single helical interpolation pass using the calculated feed rate and thread depth.
How the result changes with Major Thread Diameter
| Major Thread Diameter | Helical Feed Rate |
|---|---|
| 0.25 | 0.2 IPM |
| 0.38 | 0.2 IPM |
| 0.75 | 12 IPM |
| 1.25 | 16.8 IPM |
What each input means
- Major Thread Diameter
- Nominal thread major diameter.
- Thread Pitch
- Thread pitch (1/TPI). For 20 TPI, enter 0.05.
- Thread Mill Diameter
- Diameter of the thread milling cutter.
- Spindle RPM
- Spindle speed for the thread milling operation.
- Chip Load
- Recommended chip load per tooth for thread milling.
- Number of Flutes
- Number of cutting edges on the thread mill.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersMajor Thread Diameter = 0.5, Thread Pitch = 0.05, Thread Mill Diameter = 0.375, Spindle RPM = 4000 = 6 input(s) provided
- Calculate Helical Feed Rate6 = 6
- Calculate Time per PassTime per Pass0.066 = 0.066
- Calculate Thread DepthThread Depth0.0307 = 0.0307
Engine last updated . Checked against 1 independently-derived test — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why is there a compensation factor applied to the basic chip-load feed rate formula?
Because the CNC controller programs the velocity of the thread mill's center as it orbits the hole, but the target chip thickness is actually set at the cutting edge, which sweeps a circle at the larger major-diameter radius rather than the smaller center-path radius. Standard thread-milling practice compensates the nominal chip-load feed rate by the ratio of (Major Diameter minus Tool Diameter) divided by Major Diameter to get the correct programmed feed rate.
Does the thread mill's Number of Flutes change the thread's Minor Diameter?
No -- Number of Flutes only affects the Helical Feed Rate output, since more flutes means more cutting edges passing per revolution and the feed rate scales up proportionally; it has zero effect on Minor Diameter, Thread Depth, or Threads per Inch, all of which are governed entirely by Thread Pitch and Major Thread Diameter.
How is Threads per Inch related to the Thread Pitch input?
Threads per Inch is simply the reciprocal of Thread Pitch (TPI = 1 / Pitch), so entering a pitch of 0.05 inches gives exactly 20 TPI. Raising Thread Pitch always lowers Threads per Inch across this calculator's entire input range, since fewer, wider-spaced threads fit in an inch as the distance between them grows.
Why would the Helical Feed Rate be very low if the tool diameter is close to the major diameter?
When the thread mill's diameter approaches the thread's major diameter, the helix the tool center orbits shrinks toward zero radius, and the compensated feed rate shrinks along with it, since there's very little radial clearance left to actually interpolate a circular path. In practice, thread mills are always chosen meaningfully smaller than the major diameter they cut, so this low-feed-rate region sits outside normal tooling selection rather than describing a realistic setup.
Does raising Spindle RPM always increase the Helical Feed Rate?
Yes, at any fixed tool and thread geometry: Helical Feed Rate scales directly and linearly with Spindle RPM, since more spindle revolutions per minute means each flute passes through the cut more often, so the programmed feed rate must rise proportionally to maintain the same chip load per tooth.
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