Does anyone have the wartime thread specifications please?

[Sutts]

I'm studying B-17 engineering drawings and finding references to threads I have no knowledge of.
Is anyone familiar with the thread standards of the time please? I'm looking specifically for thread major and minor dimensions.
The kind of refs I'm seeing are:

11/16-24 NS-2
1 15/16-16 NS-3
2 7/16-16 N-3
1-14 NF-3

There seems to be a common specification document for these threads which I haven't been able to locate: AN-GGG-S-126

Anyone out there that works on these old birds? Any help would be much appreciated.

Thanks!

 

[ThomasP]

I would suggest the Machinery's Handbook. It comes in various editions since 1914, with the current edition being the 30th, but you will need to locate an edition prior to about 1976 to find the thread specifications you are looking for. I think the 1976 edition was the 20th edition, so that edition or before, but after about 1930 should provide the information you are looking. Some of the sizes (nominal diameters and TPI) will probably be uncommon in use, so the other dimensions (major/pitch/minor diameters, lands, root forms, thread angles, tolerances, etc.) may not be in table form. If that is the case you will have to use the formula provided.

It has been a long time since I worked with any of the thread types you listed, but I think they are all part of the 'National' thread type, or modified variants of them. The 'N' indicating 'National', the 'F' indicating 'Fine' pitch, and the 'S' indicating 'Special'??

 

[ThomasP]

Possibly this will help:

https://www.govinfo.gov/content/pkg...VPUB-C13-e37fe604c6638ab929cfb4fe8af50ac2.pdf

 

[Sutts]

I would suggest the Machinery's Handbook. It comes in various editions since 1914, with the current edition being the 30th, but you will need to locate an edition prior to about 1976 to find the thread specifications you are looking for. I think the 1976 edition was the 20th edition, so that edition or before, but after about 1930 should provide the information you are looking. Some of the sizes (nominal diameters and TPI) will probably be uncommon in use, so the other dimensions (major/pitch/minor diameters, lands, root forms, thread angles, tolerances, etc.) may not be in table form. If that is the case you will have to use the formula provided.

It has been a long time since I worked with any of the thread types you listed, but I think they are all part of the 'National' thread type, or modified variants of them. The 'N' indicating 'National', the 'F' indicating 'Fine' pitch, and the 'S' indicating 'Special'??

Thanks for the great lead Thomas, very much appreciated. I'll definitely search out a copy of that publication.
I had no idea how complex this area was - lots of "special threads" used on the B-17. Why they couldn't stick to standard sizes I have no idea!

 

[Sutts]

Possibly this will help:

https://www.govinfo.gov/content/pkg...VPUB-C13-e37fe604c6638ab929cfb4fe8af50ac2.pdf

That's a lovely clear document Thomas - thanks. It covers the standard coarse and fine threads for sure. Not sure what to make of the "N-3" reference on my drawings though - coarse, fine or something else? Nothing is ever easy is it? It looks like a large proportion of my thread references are going to be "special" sizes so I'll see if I can find those in your other reference book. Thanks again for your assistance.

 

[ThomasP]

Any thread with the N as part of the designation will have a standard specific geometry and specification for a given pitch, modified by the class of fit. For threads of 1 inch and over that used either 8, 12, or 16 TPI (Threads Per Inch) and were not designated as either a Course, Fine, Extra Fine, or Special pitch, an N only was used in the designation. There were 3 classes of 'free' fit threads (-1,-2,-3) with class 1 having the greatest distance between internal and external thread surfaces and class 3 having the least distance. The class 1 might be used for assemblies where it was expected that dirt or corrosion would be a factor, and class 3 would usually be used where it mattered as to the centering of the part to a mating threaded part. Class 2 would be used where normal effect was specified. So a 2 7/16-16 N-3 would have the standard 16 TPI thread form modified by the class of fit which in this case is class 3. Whatever this bolt was used for it required a large load carrying ability and a fairly tight tolerance in locating one part to another.

 

[Sutts]

Any thread with the N as part of the designation will have a standard specific geometry and specification for a given pitch, modified by the class of fit. For threads of 1 inch and over that used either 8, 12, or 16 TPI (Threads Per Inch) and were not designated as either a Course, Fine, Extra Fine, or Special pitch, an N only was used in the designation. There were 3 classes of 'free' fit threads (-1,-2,-3) with class 1 having the greatest distance between internal and external thread surfaces and class 3 having the least distance. The class 1 might be used for assemblies where it was expected that dirt or corrosion would be a factor, and class 3 would usually be used where it mattered as to the centering of the part to a mating threaded part. Class 2 would be used where normal effect was specified. So a 2 7/16-16 N-3 would have the standard 16 TPI thread form modified by the class of fit which in this case is class 3. Whatever this bolt was used for it required a large load carrying ability and a fairly tight tolerance in locating one part to another.

Thanks for great info Thomas , I think I'm getting my head around it now. I hadn't grasped that a fine 24 TPI thread had exactly the same geometry (thread depth, flat width) as a coarse 24 TPI thread. I wonder if all the special NS threads I'm seeing are just labelled as NS because the diameter is a non-standard size (like 1 15/16") but the actual thread depth and flat width can be pulled straight from the standard table (Fine or Coarse) for the specified TPI?

 

[Sutts]

So in terms of this special thread:
1 15/16-16 NS-3

Looking in the tables you provided, both fine thread 3/4-16 and coarse thread 3/8-16 have the same spec for 16 TPI:

Pitch: 0.06250
Thread Depth: 0.04059
Width of Flat: 0.00781

For thread class -3, the tolerance on the major diameter is 0.0090".
From the maximum minor diameters I can work out the minimum thread depth for 16 TPI is 0.03385"

So for this special thread:
Major diameter 1.9375 - 1.9285"
Minor diameter 1.85632 - 1.8608"

 

[ThomasP]

By Jove, I do believe he's got it.

The only aspect/dimension you did not cover was the pitch diameter. If you look farther down in the class of fit threads per inch column, below major/minor diameter you see maximum, minimum, and tolerance numbers for the pitch diameter. For a class -3 you will see that the tolerance is .0036". This is an important number due to the variation of error that can arise when combining thread angle (i.e. 60 deg.), major diameter, and minor diameter. If the thread angle is perfect, the major diameter and minor diameter are exactly in the middle of their tolerance, and the flats are exactly in the middle of their tolerance, then the pitch diameter will be exactly in middle of its tolerance, and the thread form will be perfect. If, however, any of these are off in the right number combination then the pitch diameter could be out of tolerance and you would not know it. The measurement equipment used to verify the pitch diameter is usually either a pitch micrometer (aka thread micrometer), or the 3-wire method. Another method is by the use of an optical comparator (aka ~shadow graph).

 

[Sutts]

By Jove, I do believe he's got it.

The only aspect/dimension you did not cover was the pitch diameter. If you look farther down in the class of fit threads per inch column, below major/minor diameter you see maximum, minimum, and tolerance numbers for the pitch diameter. For a class -3 you will see that the tolerance is .0036". This is an important number due to the variation of error that can arise when combining thread angle (i.e. 60 deg.), major diameter, and minor diameter. If the thread angle is perfect, the major diameter and minor diameter are exactly in the middle of their tolerance, and the flats are exactly in the middle of their tolerance, then the pitch diameter will be exactly in middle of its tolerance, and the thread form will be perfect. If, however, any of these are off in the right number combination then the pitch diameter could be out of tolerance and you would not know it. The measurement equipment used to verify the pitch diameter is usually either a pitch micrometer (aka thread micrometer), or the 3-wire method. Another method is by the use of an optical comparator (aka ~shadow graph).

I'd ignored pitch diameter as I couldn't understand it's relevance...I do now. Thanks for sharing your experience with me Thomas, I've learned a lot.

Dave

 

[Elvis]

That last one denotes "national fine" thread, which I recall as being 24 threads to the inch.

 

[ThomasP]

For Elvis,

The American National classifications of Course or Fine is based on the proportion of the diameter of the screw to the thread pitch, said screw diameter/thread geometry providing a specified mechanical strength and fit. 24 TPI is considered Fine on 5/16" and 3/8" diameter screws, while the same 24 TPI is considered Course on #10(.190 dia.) and #12(.216 dia.) screws. If the 24 TPI geometry is applied to a 1" diameter screw it is classed as a Special.

For the 1-14 NF-3 screw that Sutts listed, the proportion of 1" diameter and 14 TPI is considered Fine, as opposed to the 1-8 NC-3 screw where the proportion of 1" diameter and 8 TPI is considered Course. The 14 TPI geometry would be considered Course on a 7/16" diameter screw.

The American National series of screw threads was derived partly from science and partly from everyday trial&error/use. The series was designed to provide a standard set of screw sizes (diameter and lengths) and thread geometries that could fulfill the vast majority of needs of the engineers and mechanics involved in the design, assembly, and maintenance of various parts and assemblies. This standardization allowed a reduced number of tools required for manufacturing threads, and allowed the gradual retirement of most other geometries of thread (Whitworth, stub, ACME, etc.) except in specialized uses - at least in US designs and usage. As such it did a pretty good job - the series met the needs more than 99% of the time.

The American National standard series, with a few tweaks, later evolved into the American National (Unified) series (usually just called the Unified series). The changes added what is called the Extra Fine class (EF) and also incorporated most Metric ISO standard threads of a similar geometry. This was due to a need to interact more efficiently with other countries when engaged in the same development and manufacturing projects. The Unified series is the one in current use in the US and the threads are designated as UNC, UNE, UNEF, UNS, or their Metric equivalents. Again, the new system meets more than 99% of the day-to-day needs for screw type fasteners.

 

[Elvis]

If..."..24 TPI is considered Course on #10(.190 dia.) and #12(.216 dia.) screws...", then why are those machine screws always listed as "10-24" or "12-24" and not "10NC" or "12NC" ?

 

[ThomasP]

Hey Elvis,

The reason is simply that it is easier for the day-to-day user (say a machinist, mechanic, or assembler) to say #10-24, and be understood immediately by another day-to-day user, than for them both to try remember all the different combinations of diameters and TPI that define Course and Fine. In effect they adopt a shorthand type of communication. If they need more detail they look it up.

If you go to the link I provided above, read it over, then try to imagine yourself remembering all of the information well enough that you would not make mistakes, or have to constantly be looking the specifications up, I think you will understand what I mean. I have worked in the engineering and manufacturing industry for most of my adult life, and have had to deal with thread specifications a lot, and I do not even try to remember the combinations of diameter and TPI that define Course and Fine. I do remember a fair number of them, but that is only because of constant use.

 

[Elvis]

I don't see a link, but its common knowledge that 24 tpi is national fine and 18 is national coarse.
Anyone who makes their living spinning wrenches will tell you the same....sorry if you don't agree. It is what I learned many years ago.

 

[ThomasP]

Hey Elvis,

Here is the link again:

https://www.govinfo.gov/content/pkg...VPUB-C13-e37fe604c6638ab929cfb4fe8af50ac2.pdf

Page 3 lists the Course series and page 8 lists the Fine series.

If you go to page 3 (Course series) and look in column 1 (size) and column 2 (threads per inch) you will see 24 TPI for the #10 and #12, and 18 TPI for the 5/16".

If you go to page 8 (Fine series) and look in column 1 (size) and column 2 (threads per inch) you will see 24 TPI for the 5/16" and 3/8", and 18 TPI for the 9/16" and 5/8".

The same 24 TPI is considered Course on sizes #10 and#12, and Fine for 5/16" and 3/8".
The same 18 TPI is considered Course for size 5/16", and Fine for 9/16" and 5/8".

 

[Shortround6]

When I worked at P & W many, many years ago as an apprentice in the tool room somebody "designed" a table to mount a new EDM machine on.
The "designer" goofed and specified (or just made an error) a non-standard thread pitch for some holes in the table. This thing was being worked on by both 1st and 2nd shift. Somebody went to the tool crib and found a tap for the threads specified on the print and drilled and tapped the holes, the table was heat treated (hardened) and surface ground before they figured out they had no bolts in stock to fit the holes. And no way to get any in a short period of time. result was they had a tool maker spending time making bolts one at a time on a lathe.
There were a few other screw ups on this project and what was supposed to save the company money (only buy the working bits of the EDM machine and make everything else in house) wound up costing them scores of hours in highly skilled labor.