UNC Threads vs. UNS Threads

UNC = Unified National Coarse thread.

UNF = Unified National Fine thread

UNS = Unified National Superfine thread (I think, and which is comparatively rare.)

An engineering handbook such as Machinery's or Mark's will give further detail.

My 2000 Machinery's Handbook gives UNC (Unified National Coarse), UNF (Unified National Fine) and UNEF (Unified National Extra Fine). UMS (Unified Miniature Screw) is also available. I see no definition of UNS but it is listed in the tables, for example 10-32UNF, 10-36UNS and even 0.390-27UNS which leads me to believe it is Unified National Special.

In general the specification lists the Nominal Diameter followed by the Threads Per Inch and then the Thread Series (UNC,UNF,Etc.) Some also add the class of Thread Fit and designate internal or External threading.

.750-10 UNC-2A is a 3/4 inch, 10 threads per inch (which would be coarse for that size) ordinary screw fit (machine screw fit, not loose or tight for tolerancing purposes), external thread (Bolt or Screw)

Depending on the size Coarse and Fine will occur at different Threads Per Inch as well.

-T

My understanding is the same as yours except that I believe UNS is Unified Screw.

Unified screw threads come in UNC, UNF etc as well as constant pitch series. (6,8,12,16,20,28 thread series).

Also, I believe that the word "national" does not feature in the standard. The Unified thread series was agreed between Britain, Canada and the USA to achieve screw thread interchangeablity between the three countries.

I am jealous, my Machinery's Handbook dates back to the early '70's

If my memory serves me correctly, the abbreviations in the American system are:

UNC United National Course

UNF United National Fine

UNEF United National Extra Fine

UNS United National Special.

No, the two thread systems are not the same. The major diameters may coincide, but the number of threads per inch will always be different for the same major diameter.

Thk, everyone for the help

UNC United National Course - Unified Coarse

UNF United National Fine - Unified Fine

UNEF United National Extra Fine - Unified Extra fine

UNS United National Special - Unified Screw

Found an old photostat copy of the BS standard - here is an extract. Apologies for the quality.

Hi,

See ASME B1.1 § 3.1.2 where it is said that UNS is the special series which are not included in the standard series.

Standard series are:
UNC: Coarse,
UNF: Fine,
UNEF: Extra-Fine

Try this link - it's to the Naval Ship's Technical Manual Chapter 75, Fasteners. It a good reference for ALL fastener related information. The link will take you to the everyspec search for NSTM Chapter 75 on the everyspec.com website. The second link is to the home page for everyspec.com.

http://www.everyspec.com/USN/NAVSEA/download.php?spec=S9086-CJ-STM-010.005629.pdf

http://www.everyspec.com/

My 2004 edition quotes

"The United States Standard formerly used in the United States

was replaced by an American Standard having the same thread

form as the former standard and a more extensive series of pitches,

as well as tolerances and allowances for different classes of fits.

This American Standard was revised in 1949 to include a Unified

Thread Series, which was established to obtain screw-thread interchangeability

among the United Kingdom, Canada, and the United

States.

The Standard was revised again in 1959. The Unified threads

are now the standard for use in the United States and the former

American Standard threads are now used only in certain applications

where the changeover in tools, gages, and manufacturing has

not been completed. The differences between Unified and the

former National Standard threads are explained on pages 1725 and

1732

in the Handbook.

As may be seen in the table on Handbook page 1735, the Unified

Series of screw threads consists of three standard series having

graded pitches (UNC, UNF, and UNEF) and eight standard series

of uniform (constant) pitch. In addition to these standard series.

There are places in the table beginning on Handbook page 1736

where special threads (UNS) are listed. These UNS threads are for

use only if standard series threads do not meet requirements. "

Hope this helps.

Stephen

Whenever we see a UNS thread, it generally has a modified pitch diameter to allow for plating buildup.

Just checked my Machinery's Handbook 21st edition, it states:

Designating UNS threads: - UNS screw threads which have special combinations of diameter and pitch with tolerance to Unified formulation have the basic form designation set out first followed always by the limits of size.

Its brief, but that is all it says.

Unified National Coarse Thread and Unified National Fine Thread

These are the only two imperial thread designations used ( in the U.S.A. and Canada )

before Metric designations were introduced.

No these two threads are different, UNS threads are applicable only after 1 inch size. UNS is Unified National special threads having 8 threads per inch (TPI) irrespective of dia of the threads. It is also known as 8UN or 8 pitch series.

UNS threads were developed for makings threads by rolling. To limit the number of dies for different pitch, single die of 8 TPI were used for making thread of any diameter size above one inch dia. These threads are in industrial use at many critical services without much problems.

Thk for your info

hi guys,

just wanna check whether UNS and UN threads are interchangable. I currently have 1-1/2 8 UN threads tapped on my equipment which needs to be re-tapped. However, all my suppliers can only provide 1-1/2-8UNS taps.

it the same

Just came across this post as I was looking for a UNS thread. Which is Unified National Special.

Historic American National Standards Institute-American Bearing Manufacturers Association threads

for bearings have been since adapted within Unified Inch Screw Thread Form (ANSI/ASME B1.1)

identified as 'UNS'.

Here is a thread chart for the designations specs.

I needed it I was using a Nook Ball Screw and the Nook flange (FLG6751) for the Ball Screw Nut has a UNS-0.662-32 Thread that the Flange threads onto

Yes. Kempe's Engineers' Yearbook can. Any edition.

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