We were now given the chance to start making complete winding stems from start to finish in preparation for our first micro mechanics exam. For the exam would we would be given 8 hours to make a winding stem from scratch to match the technical drawing provided by WOSTEP in Switzerland.
We had a choice of number of technical drawings, most of them were practice stems designed to prepare us for the exam but there was also a drawing for the ETA 6497 stem. This caliber will feature prominently throughout the next part of the course when we move on to basic servicing. We were also told that all stems from this point onwards would be marked by the tutors to help us prepare for the exam and find areas to improve upon.
Having the option of choosing to make the stems in any order we liked, I decided to tackle the 6497 winding stem first as its design and size made it a slightly easier proposition than the practice exam stems. This time though I would have to complete some new processes that I hadn’t attempted before and combine everything I had been practicing over the past few weeks.
The technical drawings provide tolerances for each length and diameter of the winding stem, most areas have to be within +/-0.03mm and some within +/-0.01mm. Any slight mistake during the exam would loose marks as it could stop the stem from functioning. A mistake on the 6497 stem could stop it from functioning and if it was tested in the movement and proved not too work, it would mean starting all over again.
The new processes I would have to complete were lapping the stem, tempering the stem and burnishing the pivot. I’ll start with the lapping, using lapping paste and a piece of brass this process is first served to remove scale left from hardening the steel. The stem is rotated between centres while a brass lap, applied with paste, is pressed against the surface to remove the scale. The same process is also used to polish the completed stem, although the grade of paste used gets finer and finer to achieve a better finish.
In my last post I explained that the hardening process left the stem very hard, but also very brittle, so it needed to be toughened up again. Tempering is the process that we use to do this. The stem is placed in a pot of brass filings and covered over leaving a small area still visible. The underside of the pot is then heated using a blow lamp, the colour of the stem begins to change as the metal heats up. It must be removed to cool when it reaches a deep blue colour. The colour change happens extremely quickly so once the stem reaches the correct colour, it must be removed immediately. Steel tempered to a blue colour gives us the perfect properties for a winding stem.
The final process I would have to learn was burnishing, this process hardens the out surface of the pivot by rubbing the very minute grooves (A result of turning) flat and work hardening the surface. To carry out this process we use a Jacot tool and a burnisher. The winding stem is rotated in the Jacot tool using a bow and the burnisher is pressed flat against the bed where the pivot sits. The burnisher slowly work hardens the pivot leaving the surface with a highly polished finish. The pivot is now a lot more resistant to wear and tear.
As you can see, making a winding stem is very lengthy process and any small mistake along the way can lead to the whole process being started all over again. I’m pleased to say that I managed to complete this 6497 stem first time and I plan to put it to use in a project later in the year. I still had a lot more practice to do though as the winding stem exam had to be completed in 8hrs and this first completed one took me 2/3 days! I was confident that I could produce a good quality stem but could I do it in the time limit?
Here is the process of making a winding stem from beginning to end:
Turn a point on each end of a piece of mild steel bar.
Turn down the steps for the pivot, square and thread.
File the square.
Create the thread.
Mark the location of the slot.
Harden the stem followed by lapping it.
Temper the stem.
Cut the slot.
Burnish the pivot.
Finally lap all diameters to ensure they are in tolerance and have a good consistent finish.