CNC / machinist calculator

Thread Mill Flank Overcut Compensation Calculator

A thread mill's cutting profile sits in the plane perpendicular to the tool axis, but the edge sweeps a helix around the thread, so the flanks come out overcut and the thread gauges loose at the pitch diameter even when the program is "correct". Enter the thread dimensions, pitch and cutter, and this calculator solves the compensated orbit radius and the pitch diameter offset to program instead of the CAM default.

Thread type

Major diameter (in)

Pitch diameter (in)

Minor diameter (in)

Pitch (in)

Cutter diameter (in)

Cutter tip form

Tip flat width (in)

Initial orbit radius (CAM default): -
Compensated orbit radius: -
Radial compensation: -
Diametral compensation: -
Fusion 360 automatic PDO: -
Compensated PDO (use this): -

Method credit

The flank overcut compensation method and solver on this page are adapted from threadmill by Andrew Downing (source on GitHub, MIT License). Our port is pinned test-for-test to his original solver's outputs.

Saved setups

Saved in this browser only. Export to move setups between machines.

How it works

Picture the cutter tooth at the moment it passes the center plane of the thread: there its profile matches the thread form exactly. But cutting also happens above and below that plane, where the helix has already advanced axially. The tooth profile is fixed perpendicular to the tool axis, so at those contact points it removes more flank material than the ideal thread form allows. That extra removal is the flank overcut, and it grows with the ratio of cutter diameter to thread diameter and with pitch.

CAM systems generally ignore this. Fusion 360's automatic pitch diameter offset simply aligns the cutter's pitch diameter with the thread's pitch diameter, which is why a thread milled with the automatic PDO gauges loose on the pitch diameter wires even though the major or minor diameter measures fine. The effect is worst for internal threads cut with a cutter close to the hole size, where the overcut can be well over a tenth of a millimeter on diameter.

The solver models the axial overcut as a series in the sweep angle with coefficients C2 and C4 built from the thread pitch radius and the orbit radius, finds the worst contact angle by Newton-Raphson, converts that axial error to a radial correction through the flank angle, and iterates the orbit radius until it settles. Internal threads orbit tighter than the CAM default, external threads orbit wider; either way the flanks back off to the ideal form. The cutter tip form (flat, corner radius or sharp) shifts the cutter's effective pitch diameter, so it changes the starting orbit too.

Rc = (PD - cutter PD) / 2 (internal), (PD + cutter PD) / 2 (external); C2 = R × Rc / (2 r0); C4 = R × Rc / (24 r0) + R² Rc² / (8 r0³); overcut dZ = theta P/(2 π) - C2 tan(a) theta² + C4 tan(a) theta⁴ at the Newton-solved worst angle; dr = dZ / tan(a); Rc comp = Rc - dr (internal), Rc + dr (external)

Worked example

A female M14x2 thread cut with an 11.7 mm cutter (0.13 mm tip flat): pitch diameter alignment gives an orbit radius of 1.3069 mm, but solving the flank overcut pulls it in to 1.2017 mm, a diametral compensation of 0.2103 mm. The Fusion 360 automatic PDO of 2.2913 mm becomes 2.0810 mm.

Frequently asked questions

Why does a thread mill overcut the flanks at all?

The cutter's profile is ground in the plane perpendicular to the tool axis, but the cutting edge sweeps a helix around the thread. Away from the center plane the helix has moved axially while the profile has not, so the flanks get cut deeper than the ideal thread form.

Why does my thread gauge loose even though the PDO looks right?

An automatic pitch diameter offset only aligns the cutter's pitch diameter with the thread's pitch diameter. It does not account for the helical flank overcut, so the pitch diameter ends up oversize for internal threads and undersize for external ones, and a thread wire or go gauge shows it.

When is the compensation big enough to matter?

It grows quickly as the cutter diameter approaches the thread diameter and as the pitch gets coarser. A 11.7 mm cutter in an M14x2 internal thread needs roughly 0.2 mm of diametral compensation, while a small cutter in a large thread needs almost none. Tight-class threads feel it first.

What do the flat, rounded and sharp tip options change?

The tip form sets where on the cutter the tooth width equals half the pitch, which is the cutter's effective pitch diameter. A flat width, a corner radius or a theoretical sharp point each give a slightly different cutter pitch diameter, so the starting orbit radius and the reconstructed PDO shift with it.

Is this the same as the thread milling feed adjustment?

No. The feed adjustment scales the programmed feed because the tool center and the cutting edge travel different size circles. This tool corrects the tool path geometry itself, the orbit radius and pitch diameter offset, so the flanks are not overcut. A good program needs both.

Does this replace cutting a test thread?

It gets the first part much closer, but tool deflection, runout and wear still move the result. Treat the compensated PDO as the starting point, gauge the thread, and trim the offset by the measured error like you would with any finishing pass.

Related calculators

Sources

Every formula on this page is shown and sourced. See how we verify.

These calculators are for planning and as a starting point. Recommended speeds and feeds are published starting values that vary with your specific tool, coating, machine rigidity, workholding and coolant. Always start conservative, listen to the cut, and follow your tool maker data sheet.