Special Metric Screw Threads

[mdambros]

I am currently in the process of updating an older drawing, and it has an internal thread callout of M7 x .75-6H. Under the thread callout it also calls out the minimum Major Diameter, Pitch Diameter limits, and Minor Diameter limits. It's designated according to section 7.6 Designaton of Special Threads in ASME B1.13M-2005.

My question is this, is this screw thread really a special thread? In ASME B1.13M-2005 Table 6 titled ISO 261 Diameter/Pitch Combinations From Which the Standards Were Selected the M7 x .75 is shown in Col. 2. Is this a standard thread size then? What makes a special metric screw thread? Is it a non-standard combination of nominal diameter and pitch? For instance M7 x .5?

The main reason I ask is, as I update this drawing, do I remove the superfluous thread information and have the callout just read M7 x .75-6H?

Thanks in advance!

 

[mdambros]

Something else I just remembered is that if it is really a special thread, the first line of the callout should read M7 x .75-6H - SPL correct?

Thanks again!

 

[juergenwt]

M7 x 0.75 6H
Metric Fine

http://www.gewinde-normen.de/iso-feingewinde-2.html

 

[CheckerHater]

This is unfortunate reality of life that not all standards are always universally understood on the shop floor.

Sometimes you have to place redundant information on your drawings in order to get the part you want. It is possible that design source got tired of being ask the same questions over and over again. Or maybe the shop was trying to substitute “wrong” thread for something else.

Little story from my own experience. Theoretically if you call NPT thread you expect to get standard engagement by default. When sending drawings to outside shop we were getting back threads cut too deep or too short, whatever they pleased. We ended up placing notes like “Unless otherwise specified NPT thread to have standard engagement”, “Check with L1 gage”, etc. Totally wrong from purist point of view, but the only way to finally get better parts.

So, if manufacturer keeps asking “is it “M7” M7, or is it some other M7?” you usually don’t have time to change the world to your liking. Just throw some redundant note on your drawing.

 

[pmarc]

Per table 4 of B1.13 M7 thread is special thread and its selection "shall be limited to unusual requirements". See paragraph. 4.4.

Does it automatically mean that thread callout must specify Major Diameter, Pitch Diameter limits, and Minor Diameter limits in addition? Well, paragraph 7.6 says: "Special diameter-pitch threads, developed in accordance with this Standard, shall be identified by the letters SPL following the tolerance class. Below the designation shall be specified the limits of size for major diameter, pitch diameter, and minor diameter."

My understanding has always been "shall" = "must", therefore if I wanted to follow the standard, I would leave the designation, even though it looked supefluous to me.

 

[CheckerHater]

Pmarc,
As far as I could see the book shows M6.5 x 1 as an example of "Special diameter-pitch threads, developed in accordance with this Standard".
Makes sense since M6.5 is not shown in Table 6
Where exactly does it say that M7 thread is "special"? Being "second choice" doesn't automatically make it special, just second choice.

 

[pmarc]

You are right, CH.
"2nd choice" does not automatically make a thread "special".
My apologies.

 

[CheckerHater]

Nothing to apologize for.
I am always curious if I missed something, or there is a good reason to do certain things certain way.
I think some time ago ISO and ASME were not fully agreeing on their first- and second choices, so one man’s “second choice” could be another man’s “special”

 

[mdambros]

Thanks for all the replies!

CH, that was my interpretation as well. I read through section 4 of ASME B 1.13M-2005, and I see where it says that the screw threads in regular type are used for "unusual requirements where the selections in accordance with paras 4.1, 4.2, and 4.3 cannot be used". I don't see anywhere where it would classify those screw threads in regular type as special screw threads.

Thank to juergenwt as well. I think you are correct with you reply that the callout should be:

M7 x .75 - 6H

It appears to be a standard metric fine thread.

So, what makes a special metric screw thread then? Would it be like the example you mentioned in the ASME standard CH? An odd size nominal diameter like the M6.5? So what makes this special thread so special?

 

[KENAT]

If it's similar to UNS then non standard diameter and/or pitch combinations can be the reason for a special.

Posting guidelines faq731-376

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

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

 

[juergenwt]

If you specify M7 x .5 6H than that is what you should get. Period! Nothing else is needed.
Designating it as M7 x .5 6H makes it METRIC FINE.
For a Metric Standard like M10 - that is all that is required. Unless you need it to be a certain tol. class like 6H for some particular reason.
The more info you try to put into it - the more you will confuse the customer. It is counterproductive!
Excample: This thread.

 

[KENAT]

juergenwt - actually per the metric ASME thread spec you have to specify pitch regardless. You don't get to assume coarse pitch if you just put M10 or similar.

Posting guidelines faq731-376

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

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

 

[juergenwt]

Kenat - thank you for pointing that out. I believe ANSI also allows the pitch to be listed for coarse threads. All other Standards ISO, DIN, JSI etc. call for NO pitch listing for coarse threads.

http://www.carrlane.com/catalog/ind...C3B285357474A2D020609090C0015312A36515F554A5B

I believe ASME and ANSI did this to help US technicians who are used to see the number of threads per inch listed.
By doing so they caused nothing but confusion for the US manufacturers.
Metric thread is designed so the coarse thread can be used in over 90% of all cases. Metric Fine is not kept in stock by the majority of shops. It is rarely used.
If you see a pitch listed - you know you are dealing with a Metric Fine. By listing the pitch on all you are now forcing the craftsman to look in a book or table to see whether he is using a metric fine.

https://www.misumiusa.com/blog.aspx?id=2396&blogid=86

I know some people ill say:"What is wrong with listing the pitch all the time"? But once you start working with Metric Threads you will soon see the advantage of not listing the pitch on standard coarse.

http://mdmetric.com/tech/tict.htm

 

[KENAT]

Juerganwt - I believe I've mentioned before that we get around this by having a standard note that metric threads without picth specified are standard coarse pitch (or words to that effect).

Posting guidelines faq731-376

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

 

[CheckerHater]

Juerganwt
Unfortunately ASME is very clear on that matter.
KENAT
Great idea, just make into general / titleblock note

 

[juergenwt]

Kenat that is true and unfortunate for all. ASME is the only one that had to go this way. Now an American reading a print from the rest of the world and reading something like M16 x 15 can "assume" they missed a decimal point and he is looking at a M16 x1,5 (pitch) when in reality he is looking at M16 (coarse) x 15mm long.
On the other hand a worker in the rest of the world looking at an American print reading something like M16 x 2 has to wonder if he is looking at a Metric Fine or worst a Metric coarse 20mm long (missing O).
Regardless - both parties will now need additional info.
Speaking from experience on the shop floor I can say one thing: Every print we received from Germany, Japan, England or Italy required clarification. Once the workers understood that threads without a pitch listing are always COARSE - no more problem. Solution: Lock up all Metric Fine Taps and Dies. The usage of Metric Fine was next to "zero".
Let's hope this can be straightened out.