Overview
ISO 68-2:2023 defines the basic and design profiles for ISO inch screw threads (UN and UNR). It is part of the ISO 68 series and applies specifically to inch fastening screw threads. The standard establishes the geometric form, key dimensional formulae and root/crest contour requirements used by designers, manufacturers and inspection personnel when working with UN (internal and external) and UNR (external only) threads.
Key topics and technical requirements
- Thread types covered: UN (internal and external) and UNR (external threads only).
- Basic profile: a common profile for internal and external threads; dimensions are derived from the fundamental triangle height H and pitch P. Example formulae included in the standard:
- H = 3/(2n) = 0.86602540 / n (where n = threads per inch)
- P = 1/n
- Design profile: separate profiles for internal and external threads with defined thread heights and root radii:
- External thread design height h = 17H/24 = 0.61343466 / n
- Root radius R = H/6 = 0.14433757 / n
- Root and crest contour rules:
- UN threads: a flat root contour is specified by default but fully or partially rounded roots (unspecified size) are permitted because of the threading process.
- UNR threads: require a defined rounded root contour. The root radius must be not less than 0.10825318·P (H/8) and for fully rounded roots not more than 0.14433757·P (H/6).
- Internal threads: flat root contour; may be fully or partially rounded and cleared beyond the P/8 flat width.
- Dimension rounding and consistency: dimensions are rounded per ASME B1.30 practice; the standard includes inch and converted metric tables (informative).
Practical applications and users
ISO 68-2:2023 is used where consistent inch thread geometry is required, including:
- Fastener manufacturers and suppliers specifying UN/UNR thread form and root shapes
- Mechanical designers and CAD/CAM engineers creating parts with inch screw threads
- Tooling and thread cutting tool makers selecting tool profiles and root relief
- Quality, inspection and metrology teams verifying thread dimensions and conformity
- Standards committees and procurement professionals referencing thread geometry for product specifications
The standard helps improve interchangeability, reduce tool wear (rounded roots), and increase fatigue strength of external threads.
Related standards
- ISO 68 series (other parts covering metric and other thread families)
- ISO 5408 - Screw threads vocabulary (normative reference)
- ASME B1.30 - Rounding and calculation practices referenced for dimensional rounding
Keywords: ISO 68-2, inch screw threads, UN threads, UNR threads, basic profile, design profile, thread root radius, fastening screw threads, thread dimensions.