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Tap Terminology: For Light Fixture Threaded Tube

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  • Tap Terminology: For Light Fixture Threaded Tube

    I need to order a tap for a light fixture threaded tube. I am talking about the common threaded tube that is used for holding the parts of lights together. In my case it is for making a nut to fit the short piece of this threaded tube that holds the glass cover on a ceiling fixture. This size of threaded tube is very common on light fixtures and lamps in the US. For clarity, here is a photo of the present, plastic nut and of a short length (1.5") of the tube.

    Click image for larger version

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    The OD of the tube measures 0.380" with a +/- of about 0.005" or so. The TPI is 27. I am fairly sure it is a pipe thread, probably 1/8" nominal size and it is not tapered. But there are several and likely different 1/8 - 27, non tapered pipe threads. So I want to know which tap I should be looking for. NPS, NPSC, NPSM, NPSF, NPSL and probably more. Or will any of them work?

    So far I have not been able to find any OD listed for the available taps, just the nominal pipe/tube sizes which are not even accurate IDs.
    Paul A.
    SE Texas

    And if you look REAL close at an analog signal,
    You will find that it has discrete steps.

  • #2
    Here in Europe they are M10x1. Sounds like it would be within decent tolerance with your measurements.

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    • #3
      Click image for larger version

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      • #4
        Buy the nut, cheaper than the tap. It is a very common fixture part. When I was a kid I worked weekends with my dad wiring houses. My job was assembling all the lighting fixtures, chandeliers and the like.

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        • #5
          You can use a tapered tap if you can run it all the way through. Still need a lock nut.

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          • #6
            Sounds a lot like 1/8 NPT to me. If the nut was short, a tapered tap could be used and stopped when the thread matched. The NPSM taps with parallel threads might be hard to get hold of.

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            • #7
              https://www.mscdirect.com/product/details/01549112

              I'm sure there are cheaper options out there.
              George
              Traverse City, MI

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              • #8
                Well, there is a thread series I've never hear of. Thanks everyone! It's been educational -- previously I had thought that things like that were British Standard Brass at 26TPI.
                25 miles north of Buffalo NY, USA

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                • #9
                  the "brass threads" series is different from most others. A lot are 27 tpi, which just happens NOT to be available with a lot of QCGB lathes unless you add gears to the QCGB drive.
                  2730

                  Keep eye on ball.
                  Hashim Khan

                  Everything not impossible is compulsory

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                  • #10
                    Originally posted by J Tiers View Post
                    the "brass threads" series is different from most others. A lot are 27 tpi, which just happens NOT to be available with a lot of QCGB lathes unless you add gears to the QCGB drive.
                    I ran across the following on PM, from a poster who is doing the exact same task: 27 TPI for lamp fixtures.

                    "I have a sheet from SBL (Form 968, Section 3, date: 4-5-65) that covers cutting 27 tpi. It states:

                    It is possible to cut 27 threads per inch on any South Bend Lathe by procuring an additional gear. Our present 9" and Light Ten Models B & C Lathes and 10"-1" Collet Lathes with wide range gear box are already equipped to cut 27 threads. For other South Bend Lathes see list below:
                    And in the chart, it lists for 9" and 10K Model A, Double Tumbler.....Part No: PT32K42NK1. In addition, it has the proper set-up for this gear in the gear train. For the 9",10K and Heavy Ten (without the wide range gear box) the stud gear is the 20 tooth gear and the gearbox input gear on the gearbox would be the above mentioned 42 tooth gear. The gear box tumblers are set for C-2 (left tumbler set on "C" and right tumbler set in the second position).

                    Personally, I would have thought of using a 54 tooth gear instead of the standard 56 tooth gear on the input to the gear box and then set the tumblers to 28 tpi."
                    25 miles north of Buffalo NY, USA

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                    • #11
                      Several ways to get there, but you do need gears not on the standard setup.
                      2730

                      Keep eye on ball.
                      Hashim Khan

                      Everything not impossible is compulsory

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                      • #12
                        Probably not the best quality, but: https://www.grandbrass.com/category/...g_tools/tools/

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                        • #13
                          I have taps in either NPS F or NPS C in 1/8 inch....l can mic the OD of tap and tell you the size..
                          Last edited by 754; 07-23-2021, 01:39 PM.

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                          • #14
                            It is 1/8"-27 RMC or Rigid Metal Conduit.
                            It is similar to 1/8"-27 IPS or Iron Pipe Size
                            but only straight thread, not tapered 3/4"
                            per foot like IPS.

                            -Doozer
                            DZER

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                            • #15
                              Amazon.com : lamp shade nut Take your pick, no tap needed.
                              “I know lots of people who are educated far beyond their intelligence”

                              Lewis Grizzard

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