Thread Callout for UNS with altered pitch diameter

[tpederzani]

Hi all,

I have a custom external thread, a .500-56 and I want to know what to call it. Per the calculations in ASME B1.1, the pitch diameter should be .4876/.4848, but we're specifying .4840/.4870.

I think that, per ASME B1.1-2003 section 6.5.1 "Method of Designating Thread Having Tolerance Not to Unified Formulation" I should call it this:

[ul][li]½-56 Unified Form SPL-EXT
Major diameter .496 - .492 SPL
Pitch diameter .4840 - .4870 SPL[/li][/ul]

The ambiguous part of 6.5.1 is this:

If a standard series thread is altered in any respect other than revised pitch diameter limits for a special length of engagement, the modification of crests, or the adjustment of the limits of size to accommodate coating, as shown above, it is designated in accordance with the following:

Thanks for your help!

 

[KENAT]

Hmm, standard does read a little odd. Tempting to say you should follow "6.6 Method of Designating threads Having Special Length of Engagement" because it covers special pitch diameters.

However, if length of engagement doesn't come into your case then seems questionable if this applies.

On the bright side, you've already researched it more than most folks with custom threads so you're beyond my UNS FAQ: faq404-1480

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[tpederzani]

Thanks for the reply!

I have also used the convention in section 6.5.3, too, but I think I used it incorrectly. In every case, I've just been ignoring the length of engagement -- partly because I haven't come up with it, and also because the threads are typically used across a bunch of related, but slightly different, products, so the thread length differs.

I've had brushes with other, more custom, threads in the past (108 TPI, "Sharp V"), so I've given the subject some thought.

I'll stick with my initial suggestion for now, then.