UNC Unified Coarse America Threads

 

Threaded fasteners, bolts, nuts, screws and fastenings in UNC

unified thread

 

.....d......................TPI.........Pmajor diametereffective diameter minor  diameter   external  threads minor  diameter  internal threads
1/4"200.0500 0.2500 0.21750.18870.1959
5/16"180.05560.31250.27640.24430.2524
3/8"160.06250.37500.33440.29830.3073
7/16"140.07140.43750.39110.34990.3602
1/2"130.07690.50000.45000.40560.4167
9/16"120.08330.56250.50840.46030.4723
5/8"110.09090.62500.56600.51350.5266
3/4"100.10000.75000.68500.62730.6417
7/8"90.11110.87500.80280.73870.7547
1"80.12501.00000.91880.84660.8647
1 1/8"70.14291.12501.03220.94970.9704
1 1/4"70.14291.25001.15721.07471.0954
1 3/8"60.16671.37501.26671.17051.1946
1 1/2"60.16671.50001.39171.29551.3196
1 3/4"50.20001.75001.62011.50461.5335
2"4 1/20.22222.00001.85571.72741.7594
2 1/4"4 1/20.22222.25002.10571.97742.0094
2-1/2"40.25002.5000   
2-3/4"40.25002.7500   
3"40.25003.0000   
3-1/4"40.25003.2500   
3-1/2"40.25003.5000   
3-3/4"40.25003.7500   
4"40.25004.0000   

 

 

 

Americans experienced the same problem from lack of thread standardization that Britain did. The challenge was taken up by William Sellers, a member of an eminent family of American engineers Williams grandfather had made the plates with which the Continental Congress printed its currency. To William, we owe the colour “machinery grey.” When others were decorating their machinery, he insisted on painting his a uniform light grey, in order not to obscure the functions of the parts. Sellers specified a thread form and a graded series of nuts and bolts that used it.

In 1864 the Franklin Institute committee recommended the adoption of Seller’s system of screw threads. The thread form became known as the “Franklin thread,” or, more commonly “Seller's thread,” and later as the “United States Standard Thread.” In May 1924 it was designated the “American Standard Thread.”

The main difference between Seller's and the Whitworth's thread for that predated it is that the tops and bottoms of the threads (the crests and roots) are flattened. The flattened roots was a bad choice for Sellers. Such angular joins in metal concentrate stress, and the process of manufacture results in high stresses at the roots of threads anyway. The result is cracks and broken fasteners.

This problem was not so noticeable in Seller’s day for two reasons. One was that most machinery was stationary and the weight of a bolt rarely mattered. If a bolt broke it could be replaced with a larger one. The second reason was that thread roots tend to be rounded anyway as the tools that make the bolts become worn.

The Unified thread form was developed after World War II by representatives of Great Britain, Canada, and the United States of America, to prevent recurrence of the wartime difficulties in supplying fasteners and tools in both British Standard Whitworth and US Standard configurations. In 1949 the American National Standard Series was replaced with the Unified Inch Standard Series. In the end there were three base reasons identified for the change. The first reason was to provide interchangeability with Canada and United Kingdom. The second reason was to allow for interchangeability in the growing global marketplace. The third reason was to correct certain thread production difficulties. 

 

 

 

 

 

Threaded fasteners are commonly used aside from the obvious strength benefits a fastener properly threaded or a bolted joint is much stronger than unthreaded alternatives they are also reversible, meaning they can be taken apart after they are assembled. They are cost-efficient and available in a wide range of sizes.  

Because threaded fasteners like bolts, nuts, and washers can be used many times in a single application, they can add extra weight to a product. Additionally, they often need retightening when located in awkward places. They are susceptible to vibrations, and may need safety wire or other added features to further hold them in place and enable proper performance.

 

There is a wide array of threaded fastener types,          

Unified national coarse (UNC)

Unified national fine (UNF)

Unified national extra fine (UNEF)

UNJC and UNJF threads

UNR threads

UNK threads

Constant-pitch threads

 Unified National Coarse Threads

 UNC threads are the most common general fastener thread. Their fit is deeper and more generic than that of a fine thread, allowing for easy removal. Generally, they have a higher tolerance for manufacturing and plating, and do not need cross threading to assemble.

 

Unified National Fine Threads

UNF threads have better torque-locking and load-carrying ability than UNC threads because of their larger minor diameter. Because of their more specific fit, they have tighter tolerances, finer tension adjustment, and can carry heavier loads. They are most commonly found in the aerospace industry.

 

United National Extra Fine Threads

 

UNEF threads are finer than UNF threads; they are used in applications with tapped holes in hard material, thin threaded walls, and tapped holes in thin material. As with UNF threads, UNEF threads are common in the aerospace industry.

 

UNJC and UNJF Threads

 

There are two types of “J” threads: external and internal. External UNJC and UNJF threads have a larger root radius than the corresponding part (either UNC, UNR, UNK, or UNF threads). The larger root radius results in a larger tensile area than the corresponding thread, and smaller stress concentration—bolts that carry heavy loads usually use “J” threads.

 

UNR and UNK Threads

 

A UNR external thread is the same as a UNC thread, only the root radius is rounded. There is no internal UNR thread. UNK threads resemble UNR threads, but the root radius and minor diameter require inspection.

 

Constant-Pitch Threads

 

These threads come in a variety of diameters to fit a given application—bolts with diameters of 1 in. and above commonly use pitches of 8, 12, or 16 threads per inch.

 

UNIFIED COARSE THREAD