Honing down a centre wheel jewel

[Jon]

Hi guys, 

Some of you who have been stuck down the rabbit hole of watchmaking know that a fair part of it is finding a solution to a particular problem, especially when it come to vintage watches and all their idiosyncrasies. And, to find the solution, some unorthodox approaches and methods are employed. I find a good watchmaker will find a way to overcome.

So, I'm servicing this Tissot 784.2 and find the centre wheel jewel is toast. The flat side of the jewel which the centre wheel sits on is intact, but the oil sink/dial side is crumbling and half of it is gone. It might last this way for some time, but maybe not and I'm guessing more on the latter. When I pressed it out, it fell apart.

I teach my students there are two golden rules of watchmaking. Rule one: You are like the S.A.S. with servicing. You are in and out with no one knowing you were ever there. Not leaving gnarled screw heads and scratched bridges, etc.

Rule two: Always think and consider the next watchmaker who is going to work on the watch. Don't leave obvious problems, or bodge something together, knowing it will create problems for the next watchmaker who works on the movement.

If rule one is adhered to, rule two covers itself. The point I'm making here is, I could have left the centre wheel jewel and it might have lasted, but that's leaving the problem for the next guy and not very SAS.

For some reason Seitz have discontinued the jewel size I needed (Sods law!), so I found one in a scrap movement that was the correct inside diameter. 0.8 mm to take a 0.79 centre wheel arbor. The outside diameter of the replacement jewel was 1.5 mm, so I had to ream the centre hole a little bigger from 1.19 mm to 1.49 mm. The problem was the jewel was 0.4 mm deep, so sat proud on the dial side and stopped the cannon pinion from seating properly, as you can see from this photo. I needed the jewel to be 0.27 mm deep like the cracked old one.

 

Now, synthetic jewels are made from aluminium oxide with some colouring thrown in. Aluminium oxide is what is coated on wet and dry paper and as I don't have a diamond honing wheel, I decided to glue a piece of 400 grit onto a blank 8 mm collet and glue the jewel to a faced off wax chuck. Make sure the jewel is off-centre.

 

 

Then I used my 8 mm lathe to hone the jewel from 0.4 to 0.27 mm. I can only use a GIF, so here it is...

Then I lay another jewel that is 0.4 mm deep next to the one I'm honing and compare how much has been honed.

I use a feeler gauge between the wax chuck and the wet and dry to gauge how deep the jewel is, so when I remove the jewel from the chuck using acetone, I know it is pretty close to what I want and found on my bench micrometre that I had honed it to 0.275 mm, so 0.005 mm bigger, which I'll force myself to live with. I was pretty impressed by the accuracy of doing this by eye with no CNC or even micrometre gauges whilst honing, just a rudimentary feeler gauge I made.

Then I pressed in the jewel with a jewelling tool and set the correct end-shake for the centre wheel and ensured the cannon pinion sat correctly without touching the centre wheel jewel.

There was no real need to polish the face of the jewel, as it isn't an acting surface, but could have done using the same method with different diamond micron pastes to get a mirror finish.

The whole process probably took me close to an hour to figure out and work. Obviously the next time, it would be half as long. The diamond wheels would have made the job a lot quicker, as the wet and dry was wearing out making honing more difficult. I think with a diamond wheel, this could be done in a matter of minutes. A lot of the time was taken soaking the wax chuck in acetone and gently teasing the jewel away from the chuck. Because the jewel is so slim it is also very brittle and will easily break if this stage is rushed, so it's a matter of being patient and letting the acetone do its work and wipe away the melted superglue and gently prying it off with your fingers, not tweezers. You can't gauge how much pressure you are putting on the side of the jewel if you use tweezers.

I could have saved myself time and effort by making a brass bush to fit, but I didn't like that idea I had robbed a jewel and broken one of my cardinal rules, which was more 'Dad's Army' rather than S.A.S.!

It's a very similar way I hone down brass bushes, as parting them off on a lathe can't get total accuracy on the depth, so I finish them off like this. Now I've been successful using this method, I'm going to invest in some different grade diamond wheels for my lathe. 

These kind of skills and problem solving, I teach here in South London in evening first and second year courses.

I hope this gives you ideas of overcoming obstacles and another reason to buy a lathe!

Edited June 1, 2024 by Jon

[RichardHarris123]

Excellent as usual. 

[mikepilk]

Great work. How long did the actual grinding take, and did you use water?

[Jon]

The honing on the lathe took about 15 to 20 mins with plenty of oil. Not too much pressure, as the grit would wear really quickly and I wiped away the slurry often and used fresh oil.

I could have used shellac to set the jewel on the wax chuck, but I find using superglue easier and it doesn't require heat. Parts this small and mini-blowtorches don't go together!

Edited June 1, 2024 by Jon

[Neverenoughwatches]

Might not be quite as accurate Jon, but do i get points for initiative?  

Edited June 1, 2024 by Neverenoughwatches

[RichardHarris123]

I see no reason why you can't have points, if it works, it works.

[Neverenoughwatches]

6 minutes ago, RichardHarris123 said:

I see no reason why you can't have points, if it works, it works.

Can i have more than 2, not "nil points"  please it will be Black Lace 1979 all over again

[mikepilk]

I assume you have all seen how jewels were made. It's a long video, and poor quality, but I found it facinating.

 

[Jon]

50 minutes ago, Neverenoughwatches said:

Might not be quite as accurate Jon, but do i get points for initiative?  

Nice! Now that's what I'm talking about. Definitely points for that.

[Neverenoughwatches]

5 minutes ago, Jon said:

Nice! Now that's what I'm talking about. Definitely points for that.

thanks Jon, thats because there are good teachers here that inspire to do better. I'm  sure i can apply it to grinding out a staff as well. The Proxon 240v micro drill, its a decent bit of kit with few accessories added to it. 

[nevenbekriev]

I have often to reshape stones, for example when I need to replace 'rolled over' or burnished type stone with a modern one. The other case is when a wider pallet stone has to be sized to tight fit the slot in the pallet fork. The tool that does this jobs is this kind of diamond disk.

Grit 1000 or 800 is perfect for the honing.

If the stone height is to be resized in the way Jon did, this will be enough to put the disk (the big one) on the table top diamond side up, put the stone on the disk, press it with finger and move it forward-reverse for a while on the disk surface, then try the height. If one is carefull and take measures  not to grind the surface bent, this will take no more than minute to resize the jewel. The same way I use to resize the pallet stones.  If a stone edge have to be rounded, then I turn on the top of a rod a 'nest' to receive the stone and glue there the stone with shellac, then while the whole thing is rotated, use the disk as a file to shape the stone.

Edited June 9, 2024 by nevenbekriev

[Knebo]

Oh man, @Jon (and also @Neverenoughwatches)... so cool

I've been dreaming of having a lathe, but it's a bit out of budget and I don't even have space for it..