My understanding of this explanation is not that the thread cuts into the dial foot but that the cut away part of the sharp flange initially leaves the dial foot hole exposed for the foot to be inserted. The screw is then backed out less than a full turn and the flange either engages in a small notch in the foot (or does it cut its own notch ?) thereby holding the dial in place and drawing it tight towards the movement as the screw is effectively loosened from its starting position. Am I correct here ? - even if technical terms like flange are not.

 

I'm a bit confused: first you seem to be saying that the thread does not cut into the dial foot, but then later, you ask if it cuts its own notch. Is there a difference?

 

Just to be clear, I have no specific knowledge of the design of this fastening arrangement. But reading the thread and looking at the photo of the screw it seems that there is some confusion between the thread (the spiral bit) and the sharp shouldered flange in the explanations. Looking at as an Engineer it seems pretty clear to me that the thread engages with a tapped hole in the plate and the sharp shoulder engages with the dial foot. Looks to me like the thread would never contact the dial foot. Of course I may be completely wrong here ...

 

Yep, you are confused. I think I made it clear that the head with the sharp edge engages with the dial foot. My original post said this:

"The screw has very sharp edges on the head, and those slice into the dial foot"

And in my subsequent post:

"You can see that the edge of the screw is very sharp, so it cuts into the post for the dial foot."

"That sharp edge cuts into the foot, and as the screw backs out, it draws the dial tight on the other side..."

You aren't an electrical engineer are you? Okay just kidding...and I'm glad you are looking at it as an engineer, because I wrote it as an engineer, and watchmaker.

So to clarify, the thread comes into play because as you unscrew the screw, there is a vertical displacement of the head - just like any other screw thread has. So after the sharp edge has bitten into the dial foot, it draws the dial in as you unscrew the screw - note this is usually maybe 1/2 a turn or so. And there is no notch in the dial foot originally...it's a straight cylindrical post and the sharp flange on the head cuts into the post - these are soft brass typically. Of course once the dial has been mounted once, there is a cut in the post created, but new dials do not have this. This is a common method of holding dials in place, although there are slight variations. Some dial feet clamps swing in and out, and have a sharp edge that cuts into the foot, like so on this ETA 7750:



This style is also used on other watches, like the ETA 2824-2, 2836-2, etc.

There is a variation on the style used in the Cal. 321 where it isn't a screw, just a clamp that rotates in place. like this:



In this style there is no vertical displacement of the clamp, and you just rotate the clamp with a screwdriver, and it bites into the dial foot. The one shows is for an ETA 6497, but the same style is used on all other movements like the 2892 (so Omega 1120, 2500, etc.), and even the 8500.

Hopes this helps you understand how these work.

Cheers, Al

 

Thanks, Al. From an engineering perspective, do you prefer the cal 321 method, or the later 861, where the dial foot is grabbed by the screw tip?

 

They both have advantages and drawbacks. The 321 are sort of self-tightening of the dial to the movement, but the screws can come loose and fall out during service (in the cleaning machine), and you have to handle the movement with the dial on to flip it over and tighten those screws, of course wearing finger cots while doing so...

The 861/1861 the screws also fall out, sometimes while the watch is being worn as I've received them for service with the screw missing, and I find it in the movement as I'm disassembling it. However the movement holder that Omega makes for these allows you to have the movement sitting in the holder dial up, drop the dial in place, and then access the screws without ever having the touch the movement directly or handle it at all.

I suppose if I was forced to pick, I'd pick the 861/1861 just because of the ease of handling while mounting the dial as it's in the movement holder.

Cheers, Al

 

I work in the Oil Industry in the North Sea but my work over the years has touched on almost all branches of engineering. Thanks for the further clarification and photos. I did fully understand your first explanation but it seemed to me that others participating in the thread had not fully understood and I thought that I could clarify. It could be another case of two countries divided by a common language. Always enjoy reading your posts - very informative.