Page 684 - Yamawa_European_Catalogue_YEU21

11. Differences between UN and UNJ threads

Intro

The minor diameter of both the external and internal screw threads of a UNJ thread are larger than that of a UN thread. This design is to
enhance the bending and shearing strength of an external thread as per the diagrams below.
SP
UNJ screw thread (MIL-S-8879, AS8879 and ISO3161) is one of the Unifi ed screw threads standard and was established for fastening parts of
commercial and military aircrafts with threaded components called “Air-fastener”. UNJ threads have only one combination of 3A class external
threads and 3B class internal threads per size and both are the smallest tolerance for Unifi ed threads to ensure that air fasteners are securely
fastened for excessive loads.
The external screw threads of a UNJ have rounded root with radius of specific tolerance for added strength. Yamawa (YMW) taps
SL
manufactured for UN threads can be used to produce internal UNJ threads per MIL-S-8879, AS 8879 and ISO 3161.
Internal threads require a minor diameter bore larger than those recommended for UN threads. This larger minor diameter prevents
interference of external and internal threads within the tolerance for UNJ standards.
Tapping conditions must be highly accurate to produce an internal screw thread within a 3B class of thread. Thread results can be infl uenced
by tapping conditions such as feed mechanism of tapping, machine, etc.
PO
Tapping machines with synchronized tapping attachment are strongly recommended.

ST

ROLL

CARBIDE

LONG
Note:
External threads shall be Unified
form, in accordance with ASME
HAND B1.1 (Class 3A), altered at the
TAPS root so that the flanks of the
adjacent threads are joined by
a continuous smoothly blended
curve tangent to the flanks. The
EG (STI) root radius tangent to the flanks
shall be within specific values
determined by the AS8879 norm.
SPECIAL
THREADS,
GAUGES
(1) Feature of UNJ threads:
Larger minor diameter (larger core diameter) and rounded root make the external screw threads of a UNJ stronger than a UN thread. Internal
UNJ threads require a minor diameter bore larger than those recommended for UN threads while staying within the tolerance of the minor
THREAD diameter for UNJ standards.
MILLS
(2) Example:
Limit size for minor diameter of Unifi ed internal threads
1/4-20UNC (2B) Min: 0.1960” (or 4.979 mm) - Max: 0.2070” (or 5.257 mm)
DIES 1/4-20UNJC (3B) Min: 0.2013” (or 5.114 mm) - Max: 0.2121” (or 5.387 mm)

CENTER
DRILLS

Technical
info

682