AbarTech

Free to use, provided as-is. No warranty or guarantee of accuracy or fitness for purpose — an engineering aid, not a substitute for checking fit against a ring gauge or witness nut. AbarTech Ltd accepts no liability for outcomes arising from its use.

Thread Root Relief — Dual-Index Calculator

For threading with a cutter whose nose radius is smaller than the standard root radius, without over-cutting the root depth.

How to use this calculator →

DEV AbarTech / Mark Cragg REV 1.1 Jun 2026 UNITS mm ANGLE CONV. deg

Thread Definition

Thread family

Pitch P (mm)

Major Ø (mm)

Half angle θ

—

Fundamental height H

—

Pitch Ø

—

Minor Ø (flat-root, basic)

—

Tool & Root Profile

Ideal root radius R_i (mm)

auto: 0.144 × P (ISO 68-1 fully-rounded root)

Actual insert nose radius R_a (mm)

Depth to root, ideal tool

—

Depth if R_a fed to match flank (single pass)

—

Excess over ideal

—

Cutting a single pass with the smaller insert to the correct flank position over-cuts the root past the design depth — this is what the dual-index pass in section 03 avoids.

Dual-Index Root Relief

Axial separation between starts

—

Start angle separation

—

Both passes feed to (X depth)

—

Method	Start 1 (Q)	Start 2 (Q)	Notes
Simple (offset second pass)	—	—	Easiest to program. Groove is correct shape, shifted ~half the separation off true pitch line — irrelevant for nut/flank fit only.
Centred (split about nominal)	—	—	Keeps the pitch line exactly on the programmed centreline. Use if axial/lead accuracy matters.

Both passes are fed to the same X depth shown above — the depth that the ideal radius tool would reach. Only the start angle differs between the two passes.

When threading on a CNC lathe with a general-purpose insert, the insert nose radius is frequently smaller than the standard root radius for the thread being cut. If the tool is fed in until the flanks reach the correct pitch diameter, it also cuts deeper into the root than the thread standard intends — weakening the thread at its root. This calculator works out the exact dual-index offset that fills out the root profile correctly without over-cutting. Developed by AbarTech Ltd, Dyserth, North Wales — in-house CNC turning and milling using Hurco machines.