selecting the right glass

Discussion in 'Watch Repair' started by Britannicus, Aug 17, 2019.

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  1. Britannicus

    Britannicus Registered User

    Apr 26, 2016
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    Hi - I have a particular issue that I need a bit of advice on ...

    I have a 1903 Waltham Traveller 7J 12212093- the database says it's a 14S. It's running nicely in a star Hunter case, and I've replaced a missing second hand. I now need to fit it for a new crystal - I measured the bezel and got 40.4 mm - I ordered a low dome Sternkreutz, and got set a 404 N labelled "normalgewolbt" which fits the bezel BUT - the case won't close - clearly the dome is too high. is there a lower dome spec ? - a flat glass will interfere with the cannon pinion - so I need the flattest 404 I can get I think.

    How do I pick the right one ?? - I've attached a few images.

    IMG_20190817_093637.jpg IMG_20190817_093748.jpg IMG_20190817_093943.jpg IMG_20190817_102109.jpg
     
  2. gmorse

    gmorse Registered User
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    #2 gmorse, Aug 17, 2019
    Last edited: Aug 17, 2019
    Hi Britannicus,

    Hunter crystals are a particular shape, they need to be very thin and quite flat, as you've found. This is from an old reply on this MB, but I'm afraid I can't credit the author:

    The height that is measured is depth of concavity from flat to the deepest point on the savonnette glass not accounting the thickness of the glass when considering clearance for hands.

    The thickness of glass should be measured as well and taken into account when considering the total height of glass for clearance to the case cover.

    The height increment from one size to another is 0.4 mm and the sizes range from #4 to #8 are as follows:

    #4 height of 2.4 mm
    #5 height of 2.0 mm
    #6 height of 1.6 mm
    #7 height of 1.2 mm
    #8 height of 0.8 mm

    I can only say from experience that the expected size can vary up to 0.1mm, e.g. if one takes savonnette glass #6 that is nominally 1.6 mm in practice it is somewhere in between 1.5mm and 1.7mm which is fairly accurate for normal use.


    If you want a mineral glass one (as opposed to acrylic) your best bet is probably to contact @Dave Coatsworth who can almost certainly supply you with the correct item. The NOS mineral glass crystals aren't necessarily perfectly circular and can vary from the marked diameter as well.

    Sternkreuz acrylic savonettes are available from Cousins if you're happy with them, but they only come in one height.

    Regards,

    Graham
     
  3. Skutt50

    Skutt50 Registered User

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    I have replaced a number of these very thin and fragile crystals......

    After having broken a few I nowdays replace with one that seems a tiny bit too loose. Then by turning the crystal I usually find one position where it fits well. Then comes closing the lid, and sometimes I am back to where I started.....

    My recommendation is to get more than one flatness and test which fits the best..... Too flat and it hits the canon pinion (pops out or cracks!) and too doomed the lid does not close......
     
  4. gmorse

    gmorse Registered User
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    Hi Britannicus,

    You should be able to measure the height of the cannon pinion above the dial, the height of the crystal seat in the bezel above the dial, and the inside height of the hunter lid above the dial, which will give you the two critical clearances for determining the crystal height. The acrylic savonette crystals are about 0.3mm thick and the old mineral ones can vary quite a bit, but seem mostly to be in the range 0.3mm to 0.7mm. Given these thicknesses you can see why they're so fragile. The very lowest ones, the #8, are not common, but I've just bought one for a Waltham and #4, (2.4mm high), seems to be about right.

    Remember to allow for the undercut in the bezel when measuring the diameter, and take it across several different diameters because the bezels aren't necessarily quite circular; best to measure with the bezel closed, which will prevent any undue flexing from giving misleading readings.

    Regards,

    Graham
     
  5. Britannicus

    Britannicus Registered User

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    thanks for the sage advice - being in the UK - where's my best option to get a crystal ?
     
  6. gmorse

    gmorse Registered User
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    Hi Britannicus,

    I don't know of anyone here in the UK who can provide a similar service to Dave's. I can order the precise size and height from him and it's usually here in about a week or 10 days, at a sensible price. I guess you could try your luck on eBay, but prices there can be quite silly.

    Regards,

    Graham
     
  7. Britannicus

    Britannicus Registered User

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    Thanks - sage advice as ever - found that HS Walsh & Sons seem to do a good line in Sternkreuz savonnette acrylics at a sensible price - ordered them and go tthem the next day - seem to work fine - useful to know of Dave's service though for anything more special. One thing I do find difficult is measuring the bezel precisely - I use a digital vernier that is supposed to be accurate to 0.1 mm - but frankly I find it gives different figures from 40.4 to 41mm in reality- I averaged and went for a 40.8 which seems to fit the job, but any more accurate tooling available ??
     
  8. gmorse

    gmorse Registered User
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    Hi Britannicus,

    You've done the right thing by taking a range of measurements and using the average, because the bezels aren't always perfectly round, and often neither are the old glass crystals. It isn't the measuring tool, it's the bezel itself which is out. Measuring with the bezel installed on the case band is advisable, because that's how it will spend most of its life.

    Regards,

    Graham
     

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