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Thread definitions (Root Dia, Core Dia, Min Minor Dia)Helpful Member! 

Guest (visitor) (OP)
5 Dec 01 13:06
Can anyone please give a proper definition of these three diameters on an external/bolt thread?  How many are equivalent?  What is a good reference for these sorts of things?

Thanks.
MadMango (Mechanical)
5 Dec 01 14:16
A great reference for these sorts of things would be the Machery's Handbook, which I think is up to 26th edition now.

Root, Core, Minor, Major... they can all be used as reference dimensions, depending on what you are trying to do.  If you want clearance holes, use Root.  If you are looking to pre-dill a tapped hole, then look at Minor, ect.

"Happy the Hare at morning for she is ignorant to the Hunter's waking thoughts."

Helpful Member!  vonlueke (Structural)
6 Dec 01 9:44
Fred3001 wrote:
> Can anyone please give a proper definition of these
> three diameters on an external/bolt thread?

For metric or imperial bolts? And for which thread profile, if you know it or if the question is for a specific thread profile? Some possible choices for thread profile are M, MJ, UN, and UNJ, with perhaps M being the most typical. Also, do you prefer numeric formula definitions or a word definition?
vonlueke (Structural)
7 Dec 01 3:11
"Root diameter," "core diameter," and "minor diameter" are three ambiguous names meaning "minor diameter." I can't tell you which of these three ambiguous names is most common worldwide, as we would have to find and download the ISO specification of standard nomenclature definitions to find out. I can only tell you the standard name used by ASME (USA) specifications for this quantity is "minor diameter."
  
"Minor diameter," with no modifier, means "nominal minor diameter." "Nominal" means "the commonly agreed-upon name of a size" or "in name only but not necessarily in actuality," and can sometimes simultaneously mean "approximate" or "approximate average."

ASME B1.13M-1995, ASME B1.1-1989, ASME B1.7M, etc., are very confusing (which implies poorly written, so even though they are a USA standard, I can't recommend them as a "good" reference), and I could not find an exact formula for "minimum minor diameter" for external threads. "Minimum minor diameter" would equal nominal minor diameter minus some thread tolerance based on the thread tolerance class of the particular thread profile, with thread tolerance class "6g" being typical for M bolts, e.g. I must defer to some thread expert who could clarify the exact "minimum minor diameter" formula, for, say, the ISO "M" thread profile with thread tolerance class 6g. Good luck.
vonlueke (Structural)
7 Dec 01 9:14
Even though, as mentioned above, I couldn't find an exact formula for you for "minimum minor diameter," the formulas for nominal minor diameter, in case you need them, for M, UN, and UNR external threads are defined as follows, where Dm = nominal minor diameter, Dn = nominal major diameter (bolt size), and p = thread pitch (bolt axial length per thread).  p = 1/N, where N = threads per unit length.

Dm = Dn - 1.226869p for M bolts.
Dm = Dn - 1.299038p for UN and UNR bolts.
vonlueke (Structural)
10 Dec 01 21:41
Fred3001:  ISO 5408, "Cylindrical Screw Threads, Vocabulary," says the standard nomenclature for minor diameter worldwide is "minor diameter."  ASME B1.7M, "Nomenclature and Definitions for Screw Threads," says "minor diameter" is the diameter of an imaginary cylindrical surface tangent to the very bottom of the root radii of an external thread or to the top of the crests of an internal thread, and says, "root diameter is a nonpreferred term for external thread minor diameter or internal thread major diameter." Therefore, "root diameter" and "core diameter" are ambiguous, nonstandard terms for external thread minor diameter.

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