# Why is #5-40 UNC such an orphan thread?



## Brian Rupnow (Dec 10, 2008)

This is one that puzzles me. #5-40 is the perfect size for a 1/8" rod, yet none of the local Nut and Bolt stores in my end of the world stock it in nuts, bolts, nor set screws. When I first started to mess about with model steam engines last year, I bought a #5-40 tap and a #5-40 die, and they were special order and due to their 'Uncommoness" cost me over $100 to buy them. We have #10-24 for 3/16" rod, and we have associated thread sizes from there on up, but 1/8" rod seems to be a Bastard size.---And yet, #6-32 and #6-40 are called out as having a diameter of .131 to .137 which doesn't fit with any standard rod size that I know of.---Weird!!!!


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## joeby (Dec 10, 2008)

Oddball? The common thread sizes would be 00, 0, 2, 4, 6, 8, 10, 12. 5-40 isn't as commonly used, or 3-48 for that matter. I needed a 1-72 die, which wasn't an off the shelf item either.

 Good question though. 

 Possibly because 2 or 4 are close enough that 3 isn't needed? Maybe something to do with clearance drill sizes?

 I've been in the machining trade for a good while and I don't recall hearing a reason for it.

Kevin


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## joe d (Dec 10, 2008)

Brian:

Too late to help you now, but if you break one, check out Micro Fasteners (http://www.microfasteners.com/). I've dealt with them several times for nuts and bolts, and recently a pair of 5-40 plug taps for less than $10. (Bought a pair presuming I'll break one when I really don't want to wait for a replacement). They are perfectly happy shipping to Canada.

Cheers, Joe


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## Philjoe5 (Dec 10, 2008)

Brian,
I had the same thoughts early in my machining days. When I first started cutting threads I was getting poor results even with a fairly good quality die. I would turn rods to the major diameter of the thread I was working with. Over time, in my limited experience, I've gotten much better results by reducing the major diameter of a rod I'm going to thread by 1-5% depending upon size and material. So even though 3/16" drill rod should be good to go for 10-32 threads, I always get better results by turning it down to 0.185" - 0.186". So, if I was going to make a #5 thread, I'd still want to take some material off of a 1/8" rod. The first time I tried threading a 0.250" rod for 1/4-20 threads using a die held in a tailstock dieholder the results were terrible. I thought I had a bad die. But when I reduce the rod diameter to 0.245" I get good results. 

There may be other reasons that relate to scaling threads up or down by common factors. ??? ??? Just my thoughts 

Cheers,
Phil


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## shred (Dec 10, 2008)

I've wondered that too. The 'odd' sizes 1,3,5,7,9 (are there even the last two?) don't get much shrift, even though they do nicely on stock size rods. #12 seems pretty unused as well.

Deep in Machinery's Handbook, along with every other piece of information in the known world, is the thread major size to use for cutting with a die, depending on the class of thread you want, but it's not that easy to extract. I think Marv may have a program for it though.. I have a vague memory of seeing something like that.


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## gbritnell (Dec 10, 2008)

I can't answer your question but here's something more perplexing, why do we have two drills that measure .250, a 1/4 inch and an E drill?
gbritnell


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## ChooChooMike (Dec 11, 2008)

PM Research has a model builder selection of taps/dies including MTP ones :

http://www.pmresearchinc.com/store/search.php?mode=search&page=3

5-40 taps for $4.95 (left hand too !)

http://www.pmresearchinc.com/store/product.php?productid=3430&cat=0&page=3

5-40 die $13.10 (left hand die also):

http://www.pmresearchinc.com/store/product.php?productid=3450&cat=0&page=2

PM Research has been a good source of stuff for me !!


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## greenie (Dec 11, 2008)

If I'm not mistaken, the #5-40 tpi is used mainly in the auto industry, that's one of the reasons you dont find it freely available at your local hardware store. 
Where-as if you go to the auto el-cheapo joints, you'll find plenty of them, there are a couple of other threads used mainly by the auto industry as well.


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## DICKEYBIRD (Dec 11, 2008)

5-40 is the propeller screw size on Cox .049/.051 model aircraft engines. 5-40 is a little out of the ordinary but I've always been able to order allen head screws, taps & dies from several sources at reasonable prices.


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## Mainer (Dec 11, 2008)

It's the fault of the bean counters, trying to reduce the number of thread sizes to cut costs. 

You shouldn't have to pay a hundred bucks for 5-40 taps and dies though. Check out http://www.victornet.com/cgi-bin/victor/subdepartments.html?did=100&id=4bRPzWvz
I've never had occasion to use them, but they list all manner of uncommon taps and dies, for reasonable prices.


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## compound driver 2 (Dec 11, 2008)

why not use 5BA rather? its bang on the money for 1/8th rod. 

come to that if your pushed make your own tap and die or tap and die plate for 5-40
its a nice job just needs some thinking about and a bit of tool steel.


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## mklotz (Dec 11, 2008)

Standard threads have a flat on the crest of the thread. The width of this flat is P/8, where P is the pitch of the thread (P = 1/n, where n is the thread pitch in tpi).

If one works out the height of the small triangular section formed by this flat and then doubles that, the resulting number represents the amount by which the nominal diameter (0.125" in the case of 5-40) should be reduced to yield the requisite flat after cutting the thread.

In the attached diagram this undercut is labeled:

U = diametrical undercut from nominal diameter

and is calculated as:

U = (sqrt(3)/8)/n = 0.2165/n

So, for a 5-40 thread, we have n=40 and U=0.2165/40 = 0.0054". Turning the rod down to 0.120" before threading should work just fine.

Note that the formula is valid for all sixty degree threads.


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## shred (Dec 11, 2008)

Since Marv was so kind as to post that, I popped his 0.2165/n formula into a spreadsheet of the UNF and UNS sizes I'm likely to use and made a little table, attached below. The 'Ext Thread Dia' column is the one to turn stock to for external threading.

I like printing tables for the shop after destroying several calculators in various workshop-related methods.



View attachment ExternalThreadTable.pdf


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## rleete (Dec 11, 2008)

Thank you! I've been meaning to type one up, and this is perfect.


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## rake60 (Dec 11, 2008)

The 5-40 thread is still a common size in the gun smiting trade.
I am lucky enough to have an industrial/hardware supplier near by who 
caters to that market. Going to that store and asking for 5-40 bolts
is answered with: "Hex or socket head? What grade and how many?"

It is strange that the 5-40 has become a bastard size here in the USA 
with it's major diameter being a perfect Imperial 1/8" ???

Rick


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## Lew Hartswick (Dec 11, 2008)

There must no be any other electronic folks here. One of the most common 
screw terminal barrier strips uses 5-40 screws. At least up till I retired 11 years
ago.  They may all be metric now. 
  ...lew...


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## Kludge (Dec 11, 2008)

Try here for not only 5-40 but a few other unusual trheads:

http://www.brownells.com/

Best regards,

Kludge


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## Mike N (Dec 11, 2008)

Try:

http://www.smallparts.com/


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