Kaimera Industries
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KM-T-04 / CNC · Thread Milling

Thread Mill PDO Calculator

Bore straight out of the minor-diameter window, then the Pitch Diameter Offset from your cutter’s crest flat. Unified limits from ASME B1.1 formulas, metric from ISO 965-2 6H.

Live

A free thread mill calculator for CNC programming. Pick a thread size and class, enter your cutter's crest flat, and it returns the pre-thread hole size from the ASME B1.1 / ISO 965 minor diameter window, the pitch diameter offset (PDO) to type into CAM, and the resulting percent of thread engagement. Works for inch UN threads and metric threads.

Thread Mill PDO

BORE FROM MINOR WINDOW · PDO FROM CREST FLAT

Thread

Major dia
0.1380in
3.5052 mm
Pitch
0.03125in
0.79375 mm
Threads / in
32
TPI

The numbers CAM asks for when defining the thread itself — pitch drives the helix, major diameter sets the profile. The PDO on the right is entered separately as the offset.

Thread mill

Crest flat comes off the tool packaging or the manufacturer’s page. It is the only tool number that changes the PDO — cutter diameter is used here purely for the clearance check, since CAM derives the orbit from it on its own.

Bore

MID-BAND
interference0.1177 – 0.1214 in classclearance
Pitch Diameter Offset — type this into CAM
0.0281in
at bore 0.1083 · pitch dia 0.1195 · flat 0.0020
PDO range in class
0.0262 – 0.0299in
the in-class range — slider can exceed it
Pitch dia window
0.1177 – 0.1214in
2B
Minor dia window
0.1042 – 0.1116in
drill must land in here
Thread engagement
73%
~75% is the usual target
Quote the bore with the PDO, always. PDO is measured from the modeled bore wall, so the same offset on a different hole size cuts a different thread. That is the classic way to scrap a part — a PDO proven on one hole reused on another comes out oversize and the no-go gauge drops straight in. If you change the drill, recompute here.

How this number is built

PDO = PD + 0.649519·P − 1.732·F − bore
    = 0.1195 + 0.0203 − 0.0035 − 0.1083
    = 0.0281 in

The tool’s tip flat sits at the cut major diameter, but pitch diameter is set by the flanks — whose sharp apex lies 0.866·F beyond that flat. That is the whole reason a textbook “major minus bore” value cuts threads that gauge tight, and why the flat has to be in the arithmetic rather than dialed in over three test parts.

Sources: PDO is defined by Autodesk Fusion’s Thread operation as a diametral depth from the modeled bore, with the cut depth being half that value and tool compensation derived from the selected tool diameter. Unified thread windows are computed from ASME B1.1 formulas (internal 2B/3B); metric windows are the verified ISO 965-2 6H values carried over from the Thread Dimension Reference. The 3B minor maximum uses the widely-published ~0.93 ratio and is flagged in the readout as an approximation.

Verified against a real setup: 6-32 at bore 0.1065 with a 0.002 in crest flat returns PDO 0.0280 at the 2B pitch-diameter minimum — matching a proven shop value. Cutting-tool wear, deflection on long reaches, and material springback all shift the result; the PDO window shown is how much room you have to correct before the thread leaves class.

§ Common Questions

Frequently asked

What is pitch diameter offset in thread milling?
Pitch diameter offset is the radial distance the thread mill steps away from the hole centerline to cut the thread to the correct pitch diameter. It depends on the cutter's crest flat and effective diameter, not just the nominal thread size.
What drill size do I use before thread milling?
Use a hole inside the minor diameter window for the thread class. Thread milling lets you sit near the top of that window, which reduces cutter load while still hitting acceptable thread engagement.
Is 75% thread engagement necessary?
Rarely. 60–70% engagement holds nearly the same strength as 75% with far less tool load and breakage risk, because the fastener usually fails before a properly engaged thread strips.

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