Thread Systems

Thread Systems

A thread system is a set of basic thread proportions which is scaled to different screw sizes to define the thread geometry. Ex: ISO Metric (M), Unified (UNC, UNF, UNEF, UNS ) National pipe taper (NPT), British standard Whitwirth (BSW).

The only 2 major screw thread systems that used in the world today:

  1. The ISO Metric
  2. The Unified

In the countries other than the USA & Canada, the most common screw thread in use today are ISO. USA and Canada, still use the Unified, however, both are slowly but surely moving to the ISO. It is estimated that approximately 60% of screw threads in use in the North America today are still Inch based.

Threads are produced either by cold forming / cold rolling, hot forming / cold rolling or the cut-thread process. The most common manufacturing method for standard fasteners up to 1’’ / 25mm is the cold forming / cold rolling process whereby both the head and the thread are produced with the material in the cold state. The heads for bolts over this size are produced by the hot forging process, however, the threads are still cold rolled when size permits. Cut-threads are produced mainly to satisfy special requirements – such as low volume replacement parts and larger fasteners – over 1’’ / 25mm.

ISO Metric

ISO Metric are designated by letter M followed by the values of the nominal diameter and the pitch expressed in millimeters ; separated by the sign ‘’X’’. ex: M10 x 1.50

Metric coarse threads do not need to have the pitch specified. The mere absence of the pitch indicates that the thread is a coarse thread series. In all other cases, the thread pitch need to be specified. Ex: M10 x 1.25

Refer to the standard thread chart

Unified (Inch)

Unified form threads are designated by abbreviations such as UNC (coarse), UNF (fine), UNEF (extra fine), UNS (special) and UN (constant pitch) with the thread major diameter preceding the designation. Ex: 5/8 UNC , 5/8 UNF

Threads in the UNC,UNF,UNEF categories have a fixed number of threads per inch (TPI) for a specified diameter ex: ½ UNC has 13 TPI, ½ UNF has 20 TPI, ½ UNEF has 28 TPI.

Refer to the standard thread chart


UNC (Coarse)

Is the most commonly used and preferred thread for general purpose inch fasteners. Because of the inherent coarse pitch, the threads are deeper than fine threads and are easier to assemble without cross threading.

UNF (Fine)

Thread have a  slightly higher breaking load capacity than UNC because of the leaser thread depth and a lager tensile stress area for fasteners of the same identical material and diameter. The smaller lead angle allows for fine adjustment where needed.

UN (constant pitch)

Series thread with 4, 6, 8, 12, 16, 20, 28, & 32 TPI, offer a wide range of diameter-pitch combinations for those applications where the standard UNC,UNF, UNEF are not suitable.

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