Custom Thread Basics 5

[renderedchild]

OK, I'll start this off with an apology, as I know this is a basic issue. I will also state that I have searched, both here and google with no real success. I suspect I can find the answer in the machinery handbook, however, I am off site and cannot get to one easily. (Is there an online version?)

I am currently working on a design where 2 pieces will thread together. Because of packaging restrictions the threads will have to be "custom". I've decided on 3.125" X 12 TPI X .75" deep thread. Now my questions:

How do I design the tolerance between the male and female threads? Once the tolerance between the two is figured out, how do i notate that on a drawing?

I assume I can't just point to the male thread with a 3.125-12 - 2A dimension and 3.125-12 - 2B on the female and expect the machinist to get the clearances right so everything fits. Surely it is more complicated than that.

Like I said, I've done searches here and on google with no real results. I suspect what I'm asking is pretty much common knowledge, plus I'm sure i'm not using the best search terms. So sorry for that. This is my first design with "custom" threads, obviously, so any links on the subject are appreciated.

Thanks in advance!

 

[MadMango]

I don't know about links, but a good start would be to refer to the Machinery's Handbook. It contains a whole chapter on threads and threading.

"Art without engineering is dreaming; Engineering without art is calculating."

Have you read faq731-376 to make the best use of these Forums?

 

[renderedchild]

"I suspect I can find the answer in the machinery handbook, however, I am off site and cannot get to one easily. (Is there an online version?)"

As I stated, I'm off site and can't get to one easily/quickly. I'm kinda hoping this is a fairly simple answer that someone here can supply faster/easier than getting to a book.

 

[CorBlimeyLimey]

I assume I can't just point to the male thread with a 3.125-12 - 2A dimension and 3.125-12 - 2B on the female and expect the machinist to get the clearances right so everything fits. Surely it is more complicated than that.

Unless you are using a really rniky-dink machine shop, don't understimate a machinists abilities. They should have access to MH or similar.

See also

http://www.efunda.com/DesignStandards/screws/unified.cfm?start=575&finish=687

 

[ctopher]

I agree with the others about having the machinery handbook with you.

Try these for reference:

http://en.wikipedia.org/wiki/Unified_Thread_Standard

http://www.sae.org/technical/standards/AS1338

http://catalog.asme.org/home.cfm?CATEGORY=CS&ORDERBY=productyear&TaxonomyItemID=3044

Chris
SolidWorks/PDMWorks 08 3.1
AutoCAD 08
ctopher's home (updated Aug 5, 2008)
ctopher's blog
SolidWorks Legion

 

[KENAT]

Do you want to do it properly or half A$$ it as I see on most drawings with special threads?

What thread form are you interested in?

If you are talking about a unified thread (since you mention class 2A & B) form then consult ASME B1.1-2003 it gives you all the information you need.

...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.

The fact that you specify thread depth as well as pitch is confusing me a little.

Based on your OP, if you can't get to machineries I doubt you have B1.1 to hand.

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.
EXAMPLES: …

(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

http://www.boltplanet.com/Un/un.asp?txtseries_page=UNS

can help calculate the diamters.

Section 6.5 talks about threads with unified form but different tolerances.

If you aren’t looking at unified form then you probably want to get the relevant spec for whatever form you are considering.

Please excuse any typos but you do get what you pay for;-)


KENAT, probably the least qualified checker you'll ever meet...

 

[CorBlimeyLimey]

I suspect I can find the answer in the machinery handbook, however, I am off site and cannot get to one easily

The fact that you specify thread depth as well as pitch is confusing me a little.

The .75" thread depth is, I'm sure, referring to the threaded portion length.

 

[KENAT]

Umm, yeah CBL I think you're right. Really should have read that little bit more closely. I was thinking male thread for some reason and in my mind made the .75 a .075.

Still, the rest of what I put above is correct, I hope!

KENAT, probably the least qualified checker you'll ever meet...

 

[MadMango]

3.125-12 is a common size, so I think calling out "3.125-12 UN-2A" or 2B would suffice. I think the UNS designation is not required. Depending on the purpose and environment of the threads though, it could be required to use the UNS designator.

"Art without engineering is dreaming; Engineering without art is calculating."

Have you read faq731-376 to make the best use of these Forums?

 

[KENAT]

You're right MM. I should have looked at Table 1 or 2 in B1.1.

3.1250-12 UN is one of the 12 TPI series.

KENAT, probably the least qualified checker you'll ever meet...

 

[eromlignod]

Actually 3.125-12 is not a special (UNS) thread. It is part of the standard 12UN contant-pitch thread series. Any machine shop should be able to produce this thread with the simple callout:

3.125-12 UN-2A

Hope this helps.

Don
Kansas City

 

[renderedchild]

You guys are awesome!

Thanks for the help here, I knew I had to be making this more complicated than I needed to be.

Your assumption that I wanted to use Unified standard is correct.

.75" is referring to the thread depth, i don't know why i threw that in the OP. Sorry for the confusion.

Next time I will be sure to take my machinist handbook whenever I work off site. The one time you leave it at home...

I hadn't run across efunda.com, thanks for that link.

Thanks for the link to the bolt calculator; I think I saw that link in another thread here just before I posted this topic, I'm guessing it was you that had posted it. Shame on me, I didn't scroll down past the stresses and get to the sizes. Also thanks for those standards, as I'm sure I'll be doing this sort of thing again in the future.

Again, Thanks thanks thanks, you guys saved me a headache.

 

[KENAT]

Hmm, this almost inspires me to right a FAQ on 'special' unified threads.

"Step 1, verify it is indeed a special";-)

KENAT, probably the least qualified checker you'll ever meet...

 

[CorBlimeyLimey]

"Step 2, learn the difference between right and write."

 

[KENAT]

Man what the *(&(*& is wrong with me the last few days. I can't spell/answer the right (write;-)) question/use correct grammar for the life of me.

KENAT, probably the least qualified checker you'll ever meet...

 

[eromlignod]

There is a really nice table in the MH called "Standard Series and Selected Combinations" for unified threads. In my 23rd Ed. it's on p. 1498, at the very end of the unified section.

It basically lists every standard UNC, UNF, UNEF and UN (and common UNS) thread from #0 to 4" (22 pages). It gives the major, minor and pitch diameters for internal and external threads of each fit class. Anymore it's virtually the only chart I use.

Don
Kansas City

 

[KENAT]

"Table 3, Standard Series and Selected Combinations - Unified Screw Threads" page 1736 in the 27th edition.

So...

Step 1: Verify you truely need a special 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 is 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 Machinery's Handbook.

Step 2: Some 'specials' are less special than others. Resources such as Machinery's Handbook may 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: For specials you must specify more than just the Nominal ID, pitch and class, see ASME B1.1...

Step 4: The following link will help you calculate the required information. As with anything (especially anything free) on the internet for critical applications you should verify its accuracy.



KENAT, probably the least qualified checker you'll ever meet...

 

[renderedchild]

That sounds good to me.

I suggest adding the link CorBlimeyLimey posted to the efunda.com UN standard chart.

http://www.efunda.com/DesignStandards/screws/unified.cfm?start=575&finish=687

Also, where will that FAQ go? Is this the correct place? There are so many categories and sub catagories, its hard to know where to post or look for answers.

 

[KENAT]

Funny, I was just thinking the same thing. It could go here, or over in Welding, Bonding & Fastener engineering or as regards correctly specifying maybe Drafting Standards, GD&T & Tolerance Analysis.

It would also be relevant to other threads such as Machines & Machining engineering; QC, Inspection & Testing engineering.

It is possibley to cross link FAQs (I think I recall) so that would fix most of that problem.

Good call on the efunda link.

Busy right now I'll see if I can tidy it up and submit it soon though (faster than my idea of a faq on picking CAD systems anyway).

KENAT, probably the least qualified checker you'll ever meet...

 

[mrainey]

This program is a fast alternative to the book and calculator approach to making calculations for special screw threads of all types.

http://closetolerancesoftware.com/METhreadPal.html

Software For Metalworking

http://closetolerancesoftware.com