Tap Terminology: For Light Fixture Threaded Tube

[Paul Alciatore]

Gary,

In case you did not notice in my photo, I HAVE a standard, inexpensive nut. It came with the fixture which came with the house.

I WISH to make a different type of nut that, according to several hours or searching, CAN NOT BE PURCHASED. I looked! I asked at more then one home supply! They tried, but all I got was head shaking.

The point of this post is to know what TAP to order, not to find a nut that is exceedingly common and usually fairly inexpensive. I am not stupid, just not completely informed on the fine points of pipe/tubing threads.

Jihe,

I have a metric 10x1 tap and it may make a loose fitting thread that would work. But for my idea, the difference (27 TPI - 25.4 TPI = 2.6 TPI) is probably too much. It would weaken the grip and I will only have a few threads engaged.

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.

[J Tiers]

Yes indeed, and I thank goodness for the SB documentation and support. I know Paul has that lathe, same as me -- so the specific info might be useful to him. In a similar vein, I have wondered for years about how to calculate the gears for oddballs, when you already have the QCGB.All the available pitches on the SB step by increments of two, so I wonder how to calculate the necessary gears to get all the "in betweens".

In many cases you can do as you did in your first post, find a pitch that is very close, then figure the "offset" from that to get the new number of teeth needed.

Or start from the fact that you are wanting a certain number of turns of the leadscrew per turn of the spindle. (or more usually, turns of the spindle per leadscrew turn)

[nickel-city-fab]

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

Yes indeed, and I thank goodness for the SB documentation and support. I know Paul has that lathe, same as me -- so the specific info might be useful to him. In a similar vein, I have wondered for years about how to calculate the gears for oddballs, when you already have the QCGB.All the available pitches on the SB step by increments of two, so I wonder how to calculate the necessary gears to get all the "in betweens".

[Dave C]

Amazon.com : lamp shade nut Take your pick, no tap needed.

[Doozer]

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

[754]

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..

[reggie_obe]

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

[J Tiers]

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

[nickel-city-fab]

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."

[J Tiers]

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.

[nickel-city-fab]

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.

[George Bulliss]

https://www.mscdirect.com/product/details/01549112

I'm sure there are cheaper options out there.

[old mart]

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.

[deltap]

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

[garyhlucas]

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.