Making a stem for a watch, I don't understand the tap sizes

[GregG]

I recently purchased a brand-spanking new Sherline lathe, which has the ability to cut threads.  However, I am still unsure of how the stem tap measurements work.  Some websites say that the tap is proportional to the thread diameter (larger diameter = larger tap).  Some websites say that the tap is inversely proportional to the thread diameter (larger diameter = smaller tap).

Then there is the issue of what the actual thread pitch of the stem is.  Assuming the simple approach of tap being proportional to diameter, what is the thread pitch or TPI of these tap numbers?  I've read that it closely resembles the Bourgeaux thread series.  But I can't get a straightforward answer of what exactly the Bourgeaux thread series is.  Different websites report different numbers.  I can't find an authoritative source to get the actual numbers.

[Neverenoughwatches]

1 hour ago, GregG said:

I recently purchased a brand-spanking new Sherline lathe

That defo warrants a photo Greg, what on earth were you thinking ? That you could get away without one

[GregG]

1 hour ago, Neverenoughwatches said:

That defo warrants a photo Greg, what on earth were you thinking ? That you could get away without one

Because I purchased it so recently that it hasn't even shipped from Sherline yet.

[Neverenoughwatches]

15 minutes ago, GregG said:

Because I purchased it so recently that it hasn't even shipped from Sherline yet.

I'm excited for you

[JohnR725]

3 hours ago, GregG said:

TPI

That might be a minor issue I thought threads on stems were metrics

there is a website that has a little information

https://blog.esslinger.com/stem-tap-mm-size-cross-reference-chart/

3 hours ago, GregG said:

Sherline lathe,

Then did you get inch or metric?

 

 

[GregG]

Hi John, that was one of the websites I found before making this post.  I bought the inch version of the lathe, but you can still cut metrics threads.  The instruction manual for the thread cutting attachment includes the gear setup for cutting metric threads on an inch lathe and vice versa.

[oldhippy]

Here is the table for  Bourgeaux thread series.

https://www.sizes.com/tools/thread_bourgeaux.htm

 

[GregG]

9 minutes ago, oldhippy said:

Here is the table for  Bourgeaux thread series.

https://www.sizes.com/tools/thread_bourgeaux.htm

 

Thanks hippy.  That was one of the websites I was referring to when I mentioned that different websites report different numbers. 

Mainly

https://www.sizes.com/tools/thread_bourgeaux.htm

and 

https://www.chiark.greenend.org.uk/~alanb/martin.html

The sizes.com also has a page on the Latard series, and gives a reference to a nearly 100 year old German book.  So at least it's something to go by.  But there are no such references for the Bourgeaux thread.

Then there's also this book (p69 and p70) which reports that Bourgeaux had a pitch of 0.94^n or 0.9^n, neither of which conform to those two websites.

https://www.researchgate.net/profile/Randall-Brooks/publication/233455494_Standard_Screw_Threads_For_Scientific_Instruments_Part_I_Production_Techniques_And_The_Filiere_Suisse/links/568c0ec708ae153299b63b04/Standard-Screw-Threads-For-Scientific-Instruments-Part-I-Production-Techniques-And-The-Filiere-Suisse.pdf

However, this whole point is moot if there is a place that can give the exact dimensions of current stem threads.

[JohnR725]

13 hours ago, GregG said:

this whole point is moot if there is a place that can give the exact dimensions of current stem threads

The problem with looking for certain specifications is the specifications only exist if someone is trying to make that item typically. The problem becomes finding the specifications become problematic if nobody's actually making stems and him.

Then at one time they used to make a set for re-tapping the crown. The that a holder for the crown a drill and a tap and it was made by star and that's all the information you get the sizes are intent's of a millimeter. But it doesn't say give the guy information you're looking for as I didn't assume you be using it to try to make a stem.

There is a couple links you might have missed perhaps

https://mb.nawcc.org/threads/watch-stem-threads-made-on-sherline.157058/

I think this one covers the same stuff you have but to put it here anyway

https://mb.nawcc.org/threads/watch-stem-tap-sizes.124651/

I think probably what you're going to have to do is get stems of the size you like to make and imitate the threads that you find on them.

 

[nickelsilver]

For modern (last 100 years) non-U.S. stuff it's metric. You can find the thread pitches here for each diameter. Don't worry about how metric (sort of) corresponds to Martin threads, especially if you will be cutting them on a lathe.

[GregG]

On 7/28/2023 at 12:15 PM, Neverenoughwatches said:

That defo warrants a photo Greg, what on earth were you thinking ? That you could get away without one

As promised.

[JohnR725]

are those stepping motors I see? did you go with their CNC package or you going with somebody else's?

[GregG]

I went with the CNC-ready package, so I had to supply my own motors. I've written a rudimentary "CNC" program in Python that takes an SVG outline of a balance staff that I create and moves the motors to cut accordingly.  But I haven't stitched all the pieces together yet.

They are also closed loop stepper motors, meaning they have an internal feedback mechanism to accurately control the position.

Edited August 14, 2023 by GregG

[Neverenoughwatches]

4 hours ago, GregG said:

As promised.

Nice , I'd love to know what its capable of, it looks far simpler than i imagined.

[oldhippy]

Are you sure its big enough to make a watch stem. 

[GregG]

5 hours ago, oldhippy said:

Are you sure its big enough to make a watch stem. 

On a somewhat related note, I found a video about the American Precision Museum, and they had one of the machines used to make watch screws (at 48 seconds if it doesn't automatically jump to that point).

 

[JohnR725]

1 hour ago, GregG said:

and they had one of the machines used to make watch screws

here's a book that talks about things like this unfortunately not the kind of detail I would like but still interesting.

The Evolution of Automatic Machinery: As Applied to the Manufacture of Watches at Waltham, Mass

https://archive.org/details/evolutionautoma00marsgoog

 

[nickelsilver]

1 hour ago, GregG said:

On a somewhat related note, I found a video about the American Precision Museum, and they had one of the machines used to make watch screws (at 48 seconds if it doesn't automatically jump to that point).

 

 

I believe this is an identical machine in action, amazing for 1870s!

 

 

 

[GregG]

1 hour ago, nickelsilver said:

 

I believe this is an identical machine in action, amazing for 1870s!

 

 

 

I think the video you linked to is the video that is playing on the iPad in the museum video. XD

Edited August 14, 2023 by GregG