Thread Milling Calculator: Planar Feed Correction
Correct a peripheral feed to the circular tool-center feed used for internal or external thread milling. This is a geometric feed correction, not a cutting-data recommendation.
Independent orbit-time check — worked checks and practical limits.
Calculate and check
Default values are examples. Replace them with verified inputs for your job. Changing a unit or mode does not convert existing inputs.
Why the feed changes on a circular path
The cutter center and the contact point travel around different radii. In this simplified planar model, an internal path has center diameter D − d; an external path has center diameter D + d, where D is the chosen thread cutting diameter and d is the cutter cutting diameter. Centerline feed = peripheral feed × (D ∓ d)/D.
For internal threading, the tool must be smaller than the thread diameter. As the two diameters approach one another, the centerline feed becomes much lower than the peripheral feed. Use the effective diameters specified by the cutter supplier and CAM strategy; a catalog shank diameter is not a substitute for cutting diameter.
Example: 20 mm thread and 8 mm cutter
With a peripheral feed of 400 mm/min, the internal circular centerline feed is 400 × (20 − 8)/20 = 240 mm/min. For the external case with the same nominal geometric inputs, it is 400 × (20 + 8)/20 = 560 mm/min. Those results differ because the cutter center travels a smaller or larger circle, not because the material became easier to cut.
CAM and helical-path checks
This tool excludes axial helix travel, cutter profile corrections, radial engagement, lead-in arcs and controller-specific feed interpretation. For a coarse-pitch helix, inspect the full 3D path length and the CAM system’s feed convention. Some CAM systems already apply circular feed correction: applying it again can seriously change the intended cutting conditions.
Choose cutting speed and chip load from the actual thread mill manufacturer. Validate the postprocessor, tool hand, interpolation direction and thread size on a proven setup. No G-code is generated here.
Identify the feed convention before applying a correction
The input is a peripheral feed that has not already been corrected for circular centerline motion. A CAM feed box or posted F-word may use a different convention. Check the operation help and postprocessor documentation: multiplying an already corrected value by the diameter ratio applies the correction twice.
A double-correction check
Take D = 24 mm, cutter d = 6 mm and uncorrected peripheral feed 360 mm/min. The internal ratio is 18/24 = 0.75, so centerline feed is 270 mm/min. If CAM already output 270, correcting again gives 202.5 mm/min: a different, unintended command. For the external case the ratio is 30/24 = 1.25 and the result is 450 mm/min.
This tool has no thread-pitch input and does not calculate the axial component of a helical path. Use the cutter supplier and CAM’s full three-dimensional definition where helix travel matters. The diagram also does not define the effective diameter for a particular thread profile; obtain that from the actual tool documentation.
See thread milling versus tapping for process choice and pitch versus lead for multi-start threads. Formula reference: Harvey Performance circular interpolation.
Content updated October 19, 2026. Sources and numerical examples are explained above; examples are not production test results. Editorial method · Report a correction.
Verify the result using time around one planar orbit
An independent arithmetic check is to compare path length divided by feed. For the 20 mm internal example, the modeled peripheral reference travels π × 20 mm at 400 mm/min, while the tool center travels π × 12 mm at 240 mm/min. Both take 0.157080 min ≈ 9.4248 s per planar orbit. This is orbital motion around the feature, not one revolution of the cutting spindle.
If 400 mm/min were mistakenly entered as the centerline feed on the 12 mm path, that orbit would take only 5.6549 s. The equivalent peripheral feed in this planar model would be 400 × 20/12 ≈ 666.7 mm/min. The time comparison exposes the feed-convention error without relying on the calculator’s multiplier alone.
| What your starting value means | How to use this page |
|---|---|
| Uncorrected peripheral feed from the documented strategy | Enter it with the verified effective diameters; compare the planar centerline result. |
| CAM already supplies centerline feed | Do not apply the ratio again. Check against the original peripheral input if it is available. |
| Posted F-word with an unknown feed convention | Resolve the controller mode and postprocessor interpretation first; do not infer the meaning from the number alone. |
Where this verification stops
A helical tool-center path also travels axially. Its full path is longer than its planar projection, so the planar orbit-time check does not validate a three-dimensional feed command. Record thread lead (not just pitch for multi-start threads), the CAM feed convention and the postprocessor version. This calculator has no lead input and intentionally does not produce a helical feed correction or G-code.
Before machining, use the supplier-approved effective cutting diameters, verify the operation in CAM and follow the shop’s established proving procedure. Use the existing thread geometry worksheet to record the feed meaning and the independent check. A matching calculation verifies arithmetic, not thread form, clearance or machine safety.
Content and numerical examples reviewed October 19, 2026. Examples are not production test results.
References and verification
Original explanations and worked examples by MfgWorkbench. Sources below support the stated concepts or selected reference data; they do not certify every example or a production application.
For a correction, send the page URL, the affected value and supporting documentation through our contact page.
Print the feed-convention check
The free thread geometry worksheet (PDF, 2 pages) includes original internal/external center-path diagrams and fields for the CAM feed meaning, effective diameters and resulting ratio. Check it before applying a correction to an already-corrected value. See the example and 3D-helix limitations; the direct download has no email gate.