Special Metric Screw Threads
Special Metric Screw Threads
(OP)
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!
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!





RE: Special Metric Screw Threads
Thanks again!
RE: Special Metric Screw Threads
Metric Fine
http://www.gewinde-normen.de/iso-feingewinde-2.htm...
RE: Special Metric Screw Threads
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.
RE: Special Metric Screw Threads
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.
RE: Special Metric Screw Threads
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.
RE: Special Metric Screw Threads
"2nd choice" does not automatically make a thread "special".
My apologies.
RE: Special Metric Screw Threads
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”
RE: Special Metric Screw Threads
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?
RE: Special Metric Screw Threads
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Special Metric Screw Threads
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.
RE: Special Metric Screw Threads
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Special Metric Screw Threads
http://www.carrlane.com/catalog/index.cfm/29425071...
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&bl...
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
RE: Special Metric Screw Threads
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Special Metric Screw Threads
Unfortunately ASME is very clear on that matter.
KENAT
Great idea, just make into general / titleblock note
RE: Special Metric Screw Threads
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.