Fabricating a stem on a lathe, and Tap and Die to cut threads onto it

[GregG]

I want to make a stem for a watch as my first attempt at using the lathe which I restored.

My first question: I have a graver (one of these https://www.cousinsuk.com/product/graver-lozenge).  The majority of watch lathe videos I've seen are of people cutting into brass, but stems are typically stainless steel.

There is another graver I saw from Bergeon, specifically marketed as a stem cutting graver (https://www.cousinsuk.com/product/gravers-bergeon-specialist).

Is there some functional difference between the two types of gravers, or would the Vallorbe one also work?

Second question, how do I cut the threads onto the stem.  I've seen those cheapo tap and die sets (https://www.cousinsuk.com/product/0070mm-to-0200mm-indian?code=T20863), but I don't know if those are specifically for screw threads, or is there a separate style for stem threads?

Thanks

[nickelsilver]

You'll want to cut the stem from a hardenable steel like O1 or what the Brits call "silver steel", in its annealed/soft state. Your Vallorbe graver will do just fine. The Bergeon stem-specific graver is for cutting the slot. It, and the other one "for balance staffs" are both in tungsten carbide. I imagine they are delivered pretty sharp, but you will absolutely have to sharpen them from time to time, which requires diamond grinding wheels*. You can get another Vallorbe and grind it to a form suitable for cutting slots very easily, and the Vallorbes, being tool steel (HSS I believe) are readily dressed and sharpened on India and Arkansas stones.

 

The threads will be metric and adhere to Swiss watch norms, the threads are the same for screws and stems. Unless of course it's a very old (like 100+ years), but even then the threads are usually close enough to modern that you can cut a metric thread and it will work. American stuff is another story!

 

The Indian screw plate you linked ought to work. It honestly can be much worse than the horrible looking German dies they have listed! I would advise cutting the diameter undersize for the thread, like, for a 1mm thread, cut the steel to 0.90mm. Try threading and see how it looks. You have to use oil, preferably cutting oil. Old and/or cheap threading dies/screw plates often cut a bit and also "form" the thread- you might find that your 0.90mm start diameter has grown to a full 1mm after thread cutting. If the thread is only partially formed, increase to 0.95mm to start, etc., until you get a decent thread.

 

When it's all done (don't forget to file the square!), then you can heat treat it.

 

*I know some folks sharpen carbide gravers on cheap diamond plated plates, but they are really quite rough, even the fine ones. Too rough to produce a finish on the graver that will hold up and cut cleanly in my opinion.

Edited October 14, 2022 by nickelsilver

[GregG]

Thank you for the reply.

The widest part of the stem I want to cut is 0.055" (just a hair under 1.4mm)  It's a vintage Timex so it's got no stem tap.  The crown and stem are press fit together and I couldn't separate them, so that's why I plan on making a new one.

I found a few choices for 1/16 inch (1.58mm) rods of medium hardness annealed stainless steel (https://www.mcmaster.com/metals/shape~rod-and-disc/diameter~1-16/tolerance-rating~undersized/).

I'm thinking the Easy-to-Machine 303 version.  Thoughts?

 

[nickelsilver]

I would avoid stainless at all costs, unless a client specifically demanded it, and then I would triple my normal price, haha. 303 isn't hardenable, and even if listed as "easy to machine" will be 10x harder to work than a regular tool steel like O1. McMaster has plenty of O1 in just about any diameter you can imagine, and it's one of the easiest and most forgiving steels to heat treat. It may be that some modern watches use a hardenable stainless for stems these days, but traditionally they are made from regular carbon steel.

[GregG]

1 hour ago, nickelsilver said:

I would avoid stainless at all costs, unless a client specifically demanded it, and then I would triple my normal price, haha. 303 isn't hardenable, and even if listed as "easy to machine" will be 10x harder to work than a regular tool steel like O1. McMaster has plenty of O1 in just about any diameter you can imagine, and it's one of the easiest and most forgiving steels to heat treat. It may be that some modern watches use a hardenable stainless for stems these days, but traditionally they are made from regular carbon steel.

Is there a specific selection for "O1" steel?  Or should I just stick with carbon steel?

EDIT: I see it, but it's $21 for 6 inches 

EDIT 2: I see more inexpensive options, will have to see their full lineup.

Edited October 14, 2022 by GregG

[nickelsilver]

I'm seeing around 3 bucks per 3ft length in sizes around what you're looking for

 

https://www.mcmaster.com/metals/tool-steel-3/tight-tolerance-multipurpose-oil-hardening-o1-tool-steel-rods/

 

It's good to get it a bit oversized from the largest diameter you will end up with; at full stem length you will have a fair bit sticking out of the collet and a general rule is you want about 3x diameter of your material as max stick out. You can go over, but you have to take more care when cutting so it doesn't flex and climb up over your graver. For a typical stem I usually start with 2.5mm or 3mm stock, and end up with a max diameter frequently of 1.2mm or so. You work from the end back, to maintain rigidity.

 

My usual procedure:

-cut pivot to the diameter that the square will be across the flats

-cut the diameter of the square (usually this will be equal to the diameter for the winding pinion)

-file the square. the oversize pivot diameter you cut first acts as a guide for when to stop filing

-finish pivot

-cut diameter for winding pinion

-cut the bearing diameter that rides between the mainplate and barrel bridge

-cut slot

-cut diameter and length for the thread, part off

-flip around and cut thread

 

In some cases it's better to cut the thread first. This depends on the overall stem dimensions and also how freely your die cuts; there can be a lot of torque needed to cut the thread, and you can have a situation where the stem wants to spin in the collet, or if you hold it tightly enough so it doesn't spin you deform the diameter you are holding.

 

Couple of drawings below. For the Valjoux 72 with 0.9 thread I would cut the thread last. On the skinny little AP I'd do it first for sure (also 0.9mm thread).

Edited October 15, 2022 by nickelsilver