Modifying the ETA Ratchet Wheel to Fit the Asian 7750

[onzenuub 325]

And again another splendid tutorial from Francisco.

 

The “ratchet wheel driving wheelâ€, reference 32.033, is a gear of the automatic winding system of the 7750. In some cases this gear will finish with some teeth damaged, as seen on this picture took by Zigmeister:

 

 

This failure can be produced by frequently manually winding a watch with a 7750 movement. If this failure happens the watch will show a poor power reserve.

 

The ETA 7750 replacement is not compatible with the Asian 7750. The difference is the size of the two pivots of this ratchet wheel. The Swiss spare will not fit in the jewels, as the diameter of the pivots is larger.

 

Here a picture of the Asian ratchet wheel:

 

 

Next picture shows one of the two pivots that are different:

 

 

This is a picture of the another wheel face and of the pivot that has smaller diameter in the Asian wheel.

 

 

The marked pivots have the following dimensions:

 

Asian 7750: 0.19 mm

Swiss 7750: 0.27 mm

 

Therefore, the is a difference of 0.08 mm, 8 cents of mm. This difference is large enough to not to use the Swiss replacement, as the pivot cannot fit the jewel.

 

There are three options to use the Swiss part:

 

1.- To exchange the central part of the ratchet wheel (steel, silver) that it is fitted on the wheel (brass, golden). That means using the central part of the Asian, fitting this part in the Swiss. This task is explained here: http://www.replica-watch.info/phpBB3...p?f=59&t=53355

 

2.- To fit the ETA jewels in the Asian plates. I have not research if these jewels are available. Anyway, this is not an easy task.

 

3.- Modifying the pivots of the ETA ratchet wheel, to reduce the diameter. This task is shown in this tuto.

 

This task requires an extreme precision. It is difficult to show you how small these pivots are. I try ii in the following pictures:

 

First, the size of the gear. The pivot is not visible in the next picture, due to its very small size.

 

 

On this zoomed image it is possible to see the pivot.

 

 

A watchmaker lathe will be employed to reduce the pivot diameter. With a good watchmaker lathe a precision better than 0.02 mm is possible, so we can do this task.

 

First step is to measure the ratchet wheel, as shown, it has a diameter of 6.76 mm.

 

 

Therefore, a collet of 6.8 mm is used:

 

 

Next picture shows the wheel fastened by the collet in the head-stock of the lathe:

 

 

Another image to show the vey small size of the pivot:

 

 

To reduce the diameter a precision milling disk will be employed:

 

 

The disk is connected to the milling attachment of the lathe:

 

 

 

Next step is adjusting the lathe. The milling disk only has to polish the pivot, not other parts of the wheel. We move the disk closer to the pivot until they are touching. In this moment we know we have to polish 0.04 mm, because the total reduction will be 0.08 mm in the pivot diameter.

 

As this task requires so high precision, the scales of the lathe has not enough precision. One of these scales is shown in this picture:

 

 

We need an external indicator, which has a real precision of 0.01 mm. Next picture shows the indicator, located to measure the distance we move the disc to the pivot.

 

 

Once the disk is brushing the pivot when it is rotating, the dial is set to 0.

 

 

 

In a couple of minutes the pivot is ready. I have stopped when the indicator shows 0.05mm, to take into account the tolerances.

 

 

The diameter is checked using the calibre, 0.18 mm…good! Now is time for the another face:

 

 

Same steps here, no more pictures are needed. The same diameter is obtained, 0.18 mm.

 

To be completely sure, a broken movement is used to check that the modified Swiss ratchet wheel fits:

 

 

 

Therefore, now a replacement for this problematic part is ready.

 

Thanks for reading,

 

Francisco.

[jcagara 0]

Sir perhaps replacing an asian 7750 with an ETA Valjoux 7750 on a rep SMP 300m chrono diver or similar watches that use the same movement could be possible? because I'm thinking of this project cause I think it will be worth the longevity?

could you please shed some light sir?

thanks and more power!

[onionbag 1]

Nice tutorial onzenuub, as a toolmaker I appreciate the level of skill and precision demonstrated here.

Edited May 8, 2011 by onionbag

[KBH 7]

Sir perhaps replacing an asian 7750 with an ETA Valjoux 7750 on a rep SMP 300m chrono diver or similar watches that use the same movement could be possible? because I'm thinking of this project cause I think it will be worth the longevity?

could you please shed some light sir?

thanks and more power!

 

I don't think you'll get an answer here, as this tutorial was originally posted on a different forum. And, I doubt Onze has any answer for you.

[MikeMcNair 0]

Sir perhaps replacing an asian 7750 with an ETA Valjoux 7750 on a rep SMP 300m chrono diver or similar watches that use the same movement could be possible? because I'm thinking of this project cause I think it will be worth the longevity?

could you please shed some light sir?

thanks and more power!

 

You will still need to modify it as posted above, but if you do not have a lathe, you can actually use a cordless, variable speed power drill with a collet from a dremel. You will need the caliber to be able to accurately measure the diameters but that's the only "non common household tool" you"ll need.

 

Slow speed and low pressure with a needle file, small grinding stone, or small piece of 2000 grit sand paper.

 

If you really think about what the project is doing I am sure you can get creative. Good luck and post results!

[nofitstate 28]

Thanks for this guide

 

Without it I would of been stuck waiting for an A7750 gear to turn up.