I have a strange thread size... 4

[billthebutcher]

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.

 

[MikeHalloran]

You should be able to get the numbers from whoever is making the tap and die for you.


Mike Halloran
Pembroke Pines, FL, USA

 

[ornerynorsk]

You are assuming 25.6 is 1 inch ?????? Most threads actually measure UNDERSIZE from nominal. It was unclear in your original posting, are you designing this thread or is this an existing design (someone else's)? If someone else's, they will need to supply you with all of the pertinent data.

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.

 

[billthebutcher]

Mike: No one is making a tap or die for us. I am making drawings based off of incomplete information.

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.

 

[KENAT]

faq404-1480 covers some of the aspects of custom UN threads - if that's what you actually have/need.

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.

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[billthebutcher]

KENAT: Thank you. Yes, I am working off of an old drawing that specifies a "25.6-15" thread. And I just had the bright idea of actually measuring the major diameter of the part that has been sitting in front of me for months - it measures 1.004, so maybe I am just an idiot.

 

[racookpe1978]

NO, No, NO.

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.

 

[1gibson]

Pretty thorough bank of thread information here:

http://www.gewinde-normen.de/en/

 

[btrueblood]

That's not a metric nor standard (inch) thread designation. It has the nominal diameter called out in mm, and (apparently) the threads per turn (in inches?). Metric thread callouts are deliberately done in opposite fashion to inch threads, to avoid confusion. i.e standard threads dia(in) x (threads/inch), metric dia(mm) x pitch(mm/thread).

 

[billthebutcher]

btrueblood: The 25.6 appears to be in mm, while the 15 appears to be turns per inch. There is a note on the drawing (and many drawings that I work with) that all screw threads are represented in inches. I have confirmed that 25.6 is the major diameter and the 15 represents the turns per inch. I just can't find the specs for a thread of that type.

 

[btrueblood]

"I just can't find the specs for a thread of that type. "

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.

 

[billthebutcher]

Ok, thanks btrueblood. The part in question is a quick change barrel for a machine gun, which has its origins in the 1930s. Perhaps I should be looking at some older standards?

 

[KENAT]

30's quick change, that could be fun. Maybe back then because the rarity of Metric in the US they used hybrid designation? You could try national standards but given that the unified standard is pretty much just he national stds re-written to cover the UK & US plus modified tols I'd be surprised.

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.

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[ornerynorsk]

It's military! That changes the ballgame. I'd be surprised to see a standard 60deg threadform on a quick change barrel. Many of these are of an acme or buttress form. Had an old DEWAT Bren LMG, a MkI, if I remember, that I had picked up on a lark years ago. It's got an acme or a modified acme on the quick change, if memory serves.

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.

 

[billthebutcher]

Well, I think I probably over-simplified my original post. There is an ACME thread on a separate piece that threads on to the barrel. This piece allows the barrel to be mated to the receiver. The threads that mate this separate piece to the barrel are the wierd ones outlined in my original post. I have no experience with the BREN, but I am told it is similar. I believe both quick-change barrels are copied from an Fabrique Nationale FND, which is basically a Browning BAR with a quick-change barrel that was produced in the 1930s. I have seen the acme-threaded piece from the FND and it is visibly very similar to the one from the weapon I am working on.

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.

 

[billthebutcher]

Nope, it's definitely 15 threads/inch.

 

[dgallup]

There are times when manufactures will intentionally use non standard thread sizes to either prevent assembly with similar but incompatible parts or to block others (like you) from making replacement parts. If the size is not standard the thread form may not be either. You are probably going to have to reverse engineer everything about this thread.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[billthebutcher]

dgallup: I think you are right. We came to the realization today that we are just going to have to reverse engineer the thread features. Oh well!