Spindle thread question

[Arutha]

I have purchased an old cast iron lathe, and am going to order an insert for my Sorby Patriot chuck (that has an insert to fit my old lathe).

I'm a bit confused, after trying to measure the thread size. It seems to be 1 1/4" x 7 TPI. This seem to be an unusual thread size. 1 1/4" x 8 tpi is available at Sorby, as well as 1 1/8 x 7 tpi. But this size seems odd.

I measured the threads with a Whitworth thread gauge (that usually works for both Whitworth and UNC threads).

Not sure if it's Whitworth or UNC however. I have no means to measure if the thread angle is 55 or 60 degrees. The threads seem to fit the thread gauge quite well, but that does probably not mean much.

 

[CHJ]

Both 1-1/8 & 1-1/4 Whitworth Coarse are 7 TPI

http://www.gewinde-normen.de/en/whitwor ... hread.html

 

[Arutha]

Thanks. I guess that means that it's probable that it's Whitworth threads on my spindle.

 

[YewTube]

Just to confuse the issue. There is a 1 1/4" x 7tpi UNC.

http://www.carbidedepot.com/formulas-tap-standard.htm

Bill

 

[Arutha]

Thanks. I found some bolts on the machine that are marked as UNC, as well as a 1/2" thread that has 13 tpi . which should mean UNC (Whitworth has 12 tpi). This should mean that the spindle thread is also UNC. But to be 100% certain, I'm considering ordering a couple of nuts with UNC/Whitworth thread to check with.

If it's UNC, the Whitworth nut should not fit. If it's Whitworth, both nuts should fit, but the UNC nut would be a bit loose (if I understand it correctly).

 

[jurriaan]

Don't guess, get one of these and measure!

https://en.wikipedia.org/wiki/Thread_pitch_gauge

 

[woodfarmer]

If you have some which are known to be UNC I doubt you will have any Whitworth threads.
Initially I half wondered if you had a 33 x 3.5mm thread but that is more unlikely than Whitworth.

Whitworth threads should be rounded in the bottoms ( and tops, but these wear). UNC and Metric are simpler, cheaper and are straight across.

 

[Arutha]

The tops are flat, and the bottoms are v-shaped. So no round tops or bottoms.

 

[Arutha]

I bought a set of UNC 1 1/4" taps, and did a test today. I drilled a hole in a piece of wood, and threaded it with the taps.

It fit really tightly over the spindle thread. I almost needed a wrench to fit it on the first time. After that it went on and off easily.

I did a second test where I drove the taps through several times, but the same thing happened.

I also tried the taps in the face plates and chuck, but was unable to thread them on by hand (and didn't want to use tools in case I destroy the threads).

When I hold the tap onto the spindle thread, it looks like the spindle thread goes deeper, more steeply. I'm beginning to wonder if it might be 55 degree Whitworth threads on the spindle...

Can that have been an old standard thread on lathe chucks?

 

[beechnut]

Hi Arutha,
I went through a similar process when I got an oldish Wadkin; that has a 1 3/8" 6tpi Whitworth spindle.
Perhaps take a "plaster cast", in putty, off a section of the spindle thread (bake it to harden it) & bring to an engineers shop for confirmation?
good luck

 

[Arutha]

It's now been confirmed. The thread is indeed a 1 1/4" BSW thread.