How to make acrylic crystals?

[szbalogh]

My Roskopf pocket watch has its backs acrylic crystal missing and the one over the dial is dark yellow. How are such crystals usually made?

[StuartBaker104]

Not sure what's behind your question, but if it's gone yellow, it will be polymethylmethacrylate, better known as PMMA or acrylic. More here about how it's made here: https://en.m.wikipedia.org/wiki/Poly(methyl_methacrylate)

There are many ways to achieve the shape - die moulding, die forming, machining, etc. At one time, I understand it was common to use a flat crystal and spring it into place with a domed press. I have a box full of flat round crystals, but don't have a press!

The yellowing is caused by cross-linking of the long polymer strands which make up the material, which in turn is caused by exposure to UV light. The cross linking will also make the material more brittle.

S

[szbalogh]

18 hours ago, StuartBaker104 said:

Thank You StuartBaker 104!

Since this watch meant to be cost efficient, the crystal over the dial seems to fit perfect in the back. So i have a sample. 

Will try to make a plaster die mould with the original piece. 

[StuartBaker104]

Crystals are pretty widely available. If you post a picture and some dimensions then someone on here may be able to help you out.

[szbalogh]

Made the mold for the crystals from plaster. Hope it will work.

Now i need 1mm thick PMMA or polycarbonate. You are right, i need PMMA :

https://www.fasttech.com/forums/vapers/t/1413346/pmma-acrylic-vs-pc-polycarbonate/1

I have access to a heating oven. 160 Celsius is needed to make the PMMA soft.

These high diameter crystals were fitted while heating up the bezel ring. The book says it is just dropped in, no press needed, just a heating plate.

[szbalogh]

Heatmolded some dvd-cover pieces which are 1mm thick polystyrene.

One of four seems acceptable.

At this stage I turned my attention to glass since railroad watches were usually equipped with glass crystal.

Heatglued a piece of 2mm glass on my special "lathe". The circe was cut with a tungsten carbide bit and the edge shaped with diamond powdered dremel disk and co**BLEEP** honing stone.

After fine shaping with 2000 grit sandpaper the glass was fitted on a special crystal fitting tool (heating the bezel over a candle and copper plate).

Perfect

And suddenly .... I couldt close the lid 2mm glass is to thick.

 

[StuartBaker104]

I'm very impressed... but have you tried searching on ebay for "pocket watch glass"?  They only cost a few pounds to buy...

 

Stuart

[szbalogh]

27 minutes ago, StuartBaker104 said:

Stuart

I must admit i didnt look for it. But making it was fun, and now i can make it in any size for any watch

[szbalogh]

Now i cant find thin glass (<2mm). One solution might be slide-projector glass or microscope slides and cover plates. Someone has any idea for more different sources? 

[szbalogh]

Ordered some laboratory hourglasses. They are 1.7-1.8mm thick.

Just finished the domed glass for the Roskopf from one of them. The final diameter is 40.05mm. First, 45-46 mm middle was cut. 

Here is the finished crystal, the old plastic one, the front bezel and the tools i used. Desired diameter was reached with the diamond disc. 1000 Grit sandpaper, 3000 grit and finally 10000 grit diamond paste was used on the edge. 

Perfect heat-fitting. No fancy cement or glue   However, i added some diluted UV glue on the edges after fitting to make sure it wont fall off. It will cure tomorrow on the sun  Now i am waiting for the thin flat glass for the back bezel.

Edited May 27, 2016 by szbalogh

[Marc]

@Szbalogh I genuinely admire your ingenuity and your "can do" approach to your projects. You put me to shame for my reticence to tackle some of the more challenging aspects of watch repair just because I haven't got the "right" tools. I just love the solutions you put together to get the job done, and by the look of it the end results show that ecxellent results can be achieved without spending a small fortune on tools, just so long as you have a bit of inventiveness and a positive and determined approach.

I salute you

[ramrod]

i believe that borel's has flat glass less than 2 mm.

[szbalogh]

Thank You Marc! The hardest part was to determine if the diameter is perfect. If i grind it to small it is falling off. If its to big when i get it from the chuck the it is a pain to get it smaller evenly without turning. Luckily it is easy to get the screw nut off the turn and measure if it just over snapping in. 

2 hours ago, ramrod said:

i believe that borel's has flat glass less than 2 mm.

Yes, i wanted thinner but couldt find. Even thin flat is hard to get. I was reading somewhere that they were between 1 and 2 mm, it is just unusual today

[szbalogh]

Fianlly i colud acqure six pieces of 1mm thick 50mm round glass. Already broke one. Wanted to try shellack as a glue instead of hot glue for mounting on the chuck screw, but shellack has a much higher softening temperature. More intense heating was needed, i tried it evenly but while letting it cooling down the glass was suddenly broken. 

Ah yes, that is needed  for the back cover.

Edited September 8, 2016 by szbalogh

[parmenion31]

Great job , i am impressed , mostly by the "primitive" tools i see in the pictures producing an excellent outcome, maybe you should make us a video how you cut-polished that glass .Great effort

[szbalogh]

3 minutes ago, parmenion31 said:

Great job , i am impressed , mostly by the "primitive" tools i see in the pictures producing an excellent outcome, maybe you should make us a video how you cut-polished that glass .Great effort

Ah yes, maybe. I will grind one in the near future

[szbalogh]

Here is an intro how i grinding the glass.

 

[szbalogh]

And the second part.

 

[parmenion31]

Thats what i was talking , nice videos sz ,lot of hard work but the outcome was impressive