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.