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Domed Acrylic Sizing


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I replaced the acrylic crystal on a simple little ~1960s 3-hander (leBernco w/ ST1686).

Crystal opening measured 27.5mm. Sternkreuz Hw 276 fits in well but will can rotate in the case with not a lot of tangential thumb pressure. Can't get Hw 278 to deform enough to get into the case with claw or press. There doesn't seem to be a Hw277 available. 

Should I be OK with the rotating crystal? Try adding some glue to keep it from rotating? Keep looking for a 277 sized crystal? Or crank down on the Hw278 until it goes in?

 

 

 

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1 hour ago, KOwatch said:

Try adding some glue to keep it from rotating?

Plain acrylic crystals  are supposed to be kept  with a bit of cement. The ones that should be cement free are tension ring crystals.

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Thanks @watchweasol. Cousinsuk domed acrylics go up in .2mm sizes, hence the issue. Going through the Sternkreuz catalog again, I'm realizing I can try a St topglass that does come in a 277 OD.

For deformation I can expect in these acrylics, I had always thought .1mm was appropriate in this case, it's not enough, and .3mm is too much, so .2mm is the number. I just now re-read a tutorial where they explicitly indicate .2mm.

@jdm, these domed crystals are intended to be pressure-fit based on my understanding and experiences.

 

 

 

Edited by KOwatch
correction
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What a concept!

Actually using G-S cement for its intended purpose!

(im joking)

It seems lately I use it for everything but. 

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9 hours ago, KOwatch said:

 

@jdm, these domed crystals are intended to be pressure-fit based on my understanding and experiences.

Slight pressure and glued. Unless you like the idea of the crystal leaving at some time. The evolution from that was the tension ring and then nylon gaskets with glass crystals. 

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I have only ever used glue with flat acrylic crystals. Never needed it with domed and never had any issues with water ingress or popping out. A correctly sized crystal will fit securely without rotating, but it does need to be a tight fit which is why I asked about the type of press you used.

I have always struggled to get sufficient compression using a claw type tool, and if you simply push the crystal in with force from above you will have difficulty unless it is a tension ring crystal which can only be installed that way. 

The best way to install a standard acrylic crystal (and I believe the way they were designed to be installed) is to apply a compressive force between the centre of the underside and the perimeter of the top of the crystal. This effectively increases the doming of the crystal and reduces the circumference of the skirt. Apply enough pressure and crystal will fit straight into the rebate without any force. Then when you release the pressure the crystal resumes its shape and firmly grips the case.

There are both plier presses and screw operated presses designed to do this. I use a Robur screw press which is excellent. 

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If the crystal is slightly larger you can try and use the claw tool to install it.

Alternatively, I've used fine grit sandpaper, in the past, to slightly decrease the diameter of the crystal but afterwards you're going to have to polish the edge.

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I have a claw tool and a lever bench press (the blue body ones). Both are low-end versions and (usually) work enough for my level. I've read about the doming presses before, where the diameter is reduced by pressing on the concave side in conjunction with pressure on the outer convex circumference. I once saw an excellent illustration of the technique from a old manual online, but now I can't even remember what that kind of press is called. I suppose I can try rigging up my current press to perform that function by using different dies. I'd like to see a link or photo for this type of press or illustration of the setup. @Marc I enjoyed your blog, nice to see work on uncelebrated watches like the ones I encounter.

The claw has worked for me the times I've used it, however, it often seems to require more compression force than I can twist into it by hand alone. I use a leather work glove to hold the body and a plumbers wrench (with rubber over the jaws) to twist the top, with some success, although it does seem a little unelegant. In this case, I couldn't get the .3mm diameter reduction even with my heavy-handing. Not sure how much a better-quality claw would work; definitely a larger diameter on the body, and a better handle design would help. I've heard of gently adding some heat with a hair dryer to soften the acrylic while using the claw, might be worth a try.

I hadn't thought of the sanding technique @Chopin, that's a good option.

 

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Although this info above is from the manufacturer, I have been told to never touch the inside of an acrylic crystal. It can never be cleaned as well as it arrives from the factory. 
 

I have taken that to heart (particularly with the cost of vintage Rolex crystals) and I never touch the inside. 

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5 hours ago, Tudor said:

never touch the inside of an acrylic crystal

It would probably be a bad idea to use a plug with a hard surface on the inside of the crystal as it could result in marking unless it was mirror polished and scrupulously clean.

However, the tops of the Robur plugs (which are bakelite) have a thick felt pad attached, and the brass plugs on my plier type press are rubber topped, and neither leaves any marks or residue on the crystal.

At a push if you were really concerned you could put a piece of fresh cling film over the top of the plug. Any dust that might settle on the inside (which is likely to start the minute you take the crystal out of the packet irrespective of how you intend to fit it) is simply blown out with a puffer before closing the watch.

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10 hours ago, KOwatch said:

@Marc I enjoyed your blog, nice to see work on uncelebrated watches like the ones I encounter.

Thank you 🙂 I really should try and get around to updating it more often.

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