Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Mechanical engineering general discussion FAQ


How Do I Specify Unified Thread Form Custom Threads? by KENAT
Posted: 17 Sep 08 (Edited 2 Mar 09)

Step 1:  Verify you truly need a 'special' (custom) thread and cannot use one of the standard sizes.  There are many more standard sizes than just the typical hardware sizes you find at the store or in a hardware catalogue.  Advantages of using standard sizes are that mating nuts/bolts, taps, dies, cutters, gages etc. are more likely to be readily available.  Look in ASME B1.1 or a general resource such as efunda http://www.efunda.com/DesignStandards/screws/screwun.cfm or Machinery's Handbook. ("Table 3, Standard Series and Selected Combinations - Unified Screw Threads" page 1736 in the 27th Edition; page 1716 in the 26th Ed.; page 1498 in the 23rd Ed.)

Step 2:  Some 'specials' are less special than others.  Resources such as Machinery's Handbook give values for certain select 'special' combinations.  Also check vendors catalogues for mating parts in the required thread, for instance many small adjuster screws with matching bosses are available in 1/4-80 or even 1/4-100 sizes.

Step 3:   Correctly specify the required thread on the drawing/product documentation.  

'Special' unified threads are not specified using the normal UN, UNC, UNF etc. which refer to classes of standard sizes.  Instead 'special' unified threads using the standard tolerances but custom diameters and/or thread pitches are designated UNS.  

Quote (ASME B1.1-2003 6.1):

...The thread series symbols for the UN thread form are ... and UNS for any other intermediate and larger size diameter-pitch combination having tolerances to unified formulation.
For such custom threads you must specify more than just the Nominal ID, pitch and class, see ASME B1.1.  

Quote (ASME B1.1-2003):

6.4 Method of Designating UNS Threads

  UNS threads are special combinations of diameter and pitch with tolerance to unified formulation.
  UNS threads have the basic form of designation set out above, supplemented always by the limits of size.

(a)     ¼-20 UNS-3A
         MAJOR DIAMETER 0.2500 – 0.2428
         PD 0.2229 – 0.2201
         MAX. MINOR DIAMETER 0.205

 (b)    .495-20 UNS-3A
         MAJOR DIAMETER 0.4950 – 0.4869
         PD 0.4625 – 0.4593
         MAX. MINOR DIAMETER 0.441

(c)    1.200-10 UNS-2B
         MINOR DIAMETER 1.092 - 1.113
         PD 1.1350 – 1.1432
         MIN. MAJOR DIAMETER 1.200

Section 6.5 of ASME B1.1-2003 talks about threads with unified form but different tolerances as well as multiple start threads and special forms not covered by other thread specifications.

Step 4:  Assuming standard Unified form & tolerance the following link will help you calculate the required diameter information.  http://www.boltplanet.com/Un/un.asp?txtseries_page=UNS  Thread diameters are at the bottom of the pop-up.  As with anything (especially anything free) on the internet, for critical applications you should verify its accuracy.

Note: Dimensional information for the old No. 0000, No. 000 and No. 00 thread sizes may be found in Appendix V of ASME B18.6.3-2003.

Back to Mechanical engineering general discussion FAQ Index
Back to Mechanical engineering general discussion Forum

My Archive


Research Report - How Engineers are Using Remote Access
Remote access enables engineers to work from anywhere provided they have an internet connection. We surveyed our audience of engineers, designers and product managers to learn how they use remote access within their organizations. We wanted to know which industries have adopted remote access, which software they are using, and what features matter most. Download Now
eBook - Managing the Context of Product Complexity Using the Digital Twin
Keeping track of changes to complex products is difficult—think Aerospace & Defense equipment, new generations of commercial aircraft, and software-based automobiles. A new way to managing the digital context of the physical product is required and the answer is the Digital Twin. This ebook explores the opportunity available for Operations and Maintenance for the Digital Twin. Download Now
White Paper - Trends in Industrial Filtration
Substantial progress has been made in filtration technologies in recent years. New filter media materials, designs and processes have led to filters that are more efficient, reliable, compact and longer lasting. This white paper will discuss the various trends that are impacting operational responsibilities of MROs today and the resources that are available for staying up-to-date on the latest filtration solutions. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close