thread for standard light fixture nipple?

Was helping cousin hang a pendant type light fixture over the weekend. Fixture came with solid downrod that screwed together with those nipples that are used all over the place in lighting. I thought to myself, "self, if I had a tap for that thread I could make the rod any length she wanted, and not be limited to the foot-long increments that come with the fixture." Well, she was happy with no downrod at all in the location it was going (hung from bulkhead over kitchen sink) but it got me thinking, what the heck *is* that thread? Google suggests it is a "straight pipe thread" but what size? Does anyone make sets of those taps for a reasonable price? Even McMaster-Carr only sells them individually (for over $20 apiece!)
nate
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Nate... I do a lot of desk lamp building and repair. The size is 1/8 X 27 thread per inch. Ace hardware will have both taps and dies. 1/8 x 27 NPT. (national pipe thread. WW
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Bingo! NPS = National Pipe Straight
My local Ace did not have the 1/8"-27 NPS (straight pipe thread) die nor tap. I balked at the McMaster prices & bought them from MSC online.
Great lamp parts source http://www.grandbrass.com /
Lamp part sizes IPS 1/8, 1/4 & 3/8 IPS meaning iron pipe size (but some say it's internal pipe size)
Lamp parts are typically NPS (National Pipe - Straight) as opposed to the much more common plumbing thread NPT (National Pipe - Tapered)
THREAD SIZE Actual Unthreaded OD 1/8 -27 NPS .405" 1/4 -18 NPS .540" 3/8 -18 NPS .675"
To add to the confusion NPS is an abbreviation that is also used to denote "nominal pipe size" :(
When I first started to repair some ancient lamps & wall sconces, I had a very hard time figuring out what the parts were made from.
cheers Bob
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wrote:

If they were truly ancient they may have had NPT threads because they were converted gas lamps. That is where the pipe thread standard got established in the first place but they switched to straight threads for flexibility and convenience. The fact that so many lamp parts are available in so many shapes and colors did create the whole hash pipe market.
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On 5/3/2011 7:19 PM, Nate Nagel wrote: ...

They're 1/8" NPT...
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On 05/03/2011 09:01 PM, dpb wrote:

NPT is tapered innit?
nate
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YES ww
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Being able to thread the outside surface of a rod or hollow tube using only a tap would be a neat trick man...
What you are looking for is called a die...
Sets of piping dies cost megabucks because normally only a tradesmen or machine shop would even want to look for such a thing...
~~ Evan
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wrote:

Being able to thread the outside surface of a rod or hollow tube using only a tap would be a neat trick man...
What you are looking for is called a die... =============================================This is a job for Einstein's curved spatial taps and dies. Only $24 (does not include black hole required for powering device).
I wonder who the first guy was that held a nail up and decided - this would would better with a spiral groove wrapped all around it.
That made me curious enough to search and guess whose name popped up. It's once familiar to any of the seniors here who ever had an English car:
http://en.wikipedia.org/wiki/Joseph_Whitworth
" . . .moved to London where he found employment working for Henry Maudslay, the inventor of the screw-cutting lathe . . .In 1841 Whitworth devised a standard for screw threads with a fixed thread angle of 55 and having a standard pitch for a given diameter. This soon became the first nationally standardized system; its adoption by the railway companies, who until then had all used different screw threads, leading to its widespread acceptance. It later became a British Standard, "British Standard Whitworth", abbreviated to BSW and governed by BS 84:1956."
Well, since Archimedes knew about screws in the time before Christ, I doubt Whitworth was the first man to decide a twisted nail holds better than an untwisted one.
http://en.wikipedia.org/wiki/Screw#Historical_issue says:
"While a recent hypothesis attributes the Archimedes' screw to Sennacherib, King of Assyria, archaeological finds and pictorial evidence only appear in the Hellenistic period and the standard view holds the device to be a Greek invention, most probably by the 3rd century BC polymath Archimedes himself.[49][dubious - discuss]"
-- Bobby G.
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No, I'm looking for a tap, the downrod was made of tubing with a little standard lighting nipple at each end. So if I wanted a custom length, I would have cut the tubing, squared off/dressed the end, then tapped it so the nipple could be threaded back in.
nate
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wrote:

Ace hardware has pipe taps here pretty cheap. Less than 10 bucks for 1/8"
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