I have a strange thread size...
I have a strange thread size...
(OP)
A 25.6-15 Special Form External Left Hand thread. The 25.6 is in millimeters, which as I understand it a 1" thread. It is simply called out as 25.6 instead of 25.4 to make the conversion easier. So basically, I have a 1"-15 left hand thread. My question is: does anyone have any dimensions and tolerances for this type of thread? Pitch diameter, major diameter, minor diameter? I have never seen or heard of this thread size before. I am unable to use a more common thread due to design restrictions.





RE: I have a strange thread size...
Mike Halloran
Pembroke Pines, FL, USA
RE: I have a strange thread size...
What is meant in your post by "special form"? Is it something other than a 60 degree symmetrical threadform? Straight or tapered?
It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.
RE: I have a strange thread size...
Ornerynorsk: I found this:
http://www.gomog.com/allmorgan/whitworth.html
Scroll down to the bottom, and there is a "re-naming" of sorts that allows easy conversion in one's head between metric and imperial units. I was not aware of this practice before doing an internet search yesterday, but once I read it I assumed that 25.6-15 was a metric designation for a 1"-15 thread. It is not uncommon to find a metric designation for an inch thread in my work.
RE: I have a strange thread size...
You may want to take a look at ASME B1.1 on the left hand thread aspect, I don't think it's too complex but I don't have it to hand.
Using 25.6 is an approximation that link makes to simplify the math for fractional sizes. I'm not sure it has any place in actually specifying threads in an engineering context. Are you saying a part you're working with is specified this way?
Mixing terminology in the way you have specified the threads seems like a recipe for trouble.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: I have a strange thread size...
RE: I have a strange thread size...
Never "convert" (by math) a metric thread to an ANSI (US or inches) bolt. They must be regarded as unique and very specific "designations" of a fastener that includes thread spacing, shape, OD and root diameter, material and tolerances, wrench (head) sizes and tolerances, etc.
Only slightly exaggerating, "converting measured values" - ESPECIALLY with a "convenient-to-remember" (almost right) trick to make the changeover from metric to ANSI values - is like going to your Toyoto dealer to order a fender for your GMC pickup and telling the parts clerk: "I don't know what the VIN number is, but it's yellow and has 4 wheels and is almost 122 inches long."
Trying this WILL get you (and your mechanics and millwrights and parts suppliers and customers!) in big trouble.
RE: I have a strange thread size...
RE: I have a strange thread size...
RE: I have a strange thread size...
RE: I have a strange thread size...
That's because it fits no current standard, nor any other standard dating back to at least 1943...it's some kind of bastard hybrid invented out of whole cloth by the person who drew it up and wrote the nonstandard thread callout. If you are in the U.S., you could ASSume that it's to be based on the ANSI thread standards, with non-standard major diameter defined (in mm) and thread pitch called out in turns per inch. It at least is a starting point, and you should be able to use the formulas in (e.g.) the Machinery's Handbook to calculate pitch diameter and minor diameter. Tolerances are going to be the headache, as they always are for specials. Good luck.
RE: I have a strange thread size...
RE: I have a strange thread size...
So long as the thread form is 60° and the tolerances aren't too far off from standard UN (pick your thread class carefully just in case) you may well get away with a left hand UNS thread.
However, you're at the point of it being so 'special' that you really might want to look at the ASME B1.1 standard not second or third hand information.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: I have a strange thread size...
Is it possible that the thread callout is 26.5 x 1.5, and not 15?
It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.
RE: I have a strange thread size...
I am pretty sure it is "-15" and not "-1.5," but when I get some time later today I will check it again on the comparator.
RE: I have a strange thread size...
RE: I have a strange thread size...
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RE: I have a strange thread size...