February 13, 1930. The Model Engineer and Practical Electrician.
Setting Jewelled Pivot Holes in
Clocks and other Instruments.
By R. N. U. Pickering, F.B.H.I.
OWING to their hardness and the high polish of which they are capable, jewels are used for the bearings of light machinery such as clocks, watches and for electric instruments, compasses, chemical balances, etc. Being brittle and liable to crack or chip, they are not suitable for heavy machinery. When properly made and fitted they greatly reduce friction and provide a material which resists wear more than metal bearings. They have been used in watches since 1700, and were introduced by Nicholas Facio, of Geneva, at that date, but his invention was not at first favourably received either in Paris or London, and the Clockmakers' Company declined to grant him a patent; nevertheless, he was made a member of the Royal Society in consequence, and jewelling was speedily adopted in England and other countries, but, being expensive, was confined chiefly to the escape- ments of watches. Now in many cases it is overdone, especially in the cheap Swiss watches which are jewelled up to the centre hole with badly made garnet jewels for show, whereas brass holes would be more satisfactory. The stones found to be suitable for jewelling are the diamond, ruby, sapphire, chrysolite, agate and garnet. An arbitrary scale of hardness has been devised in which the diamond is taken as 10, the sapphire 9, ruby 9, chrysolite 7, garnet 6-7, agate 7. A test of hardness is one stone scratching another, the harder scratching the softer. A file will have no effect upon a stone which is suitable for a jewel hole. The diamond, which is pure crystallised carbon, is the hardest substance known (with the exception of the metal tantalum, which is said to be equally hard) is used as endstones to cover jewel holes in watches and chronometers. When powdered, it is used for polishing other
- With some supplementary illustrations and notes
on them by Geo. Gentry. 147 stones and splinters of it are used for drilling or turning them and also for cutting glass, truing carborundum wheels, etc. Its sp. gr. is 3.5. Ruby (red) and sapphire (blue) are both varieties of corundum or oxide of aluminium Al,O, which when pure is colourless, but it usually contains about I per cent. of ferric oxide or of chromic oxide, the latter probably giving the red colour to the ruby. The blue colour of the sapphire is probably due to the presence of titanic oxide. The word corundum is applied to opaque stones, used for abrasive purposes. Emery is corundum mixed with magnetite and other stones of lower hardness. Carborundum, now extensively used in work- shops, is a silicide of carbon C.SI., artificially prepared. There are three varieties of ruby: oriental, sperial, and balar; the first is the hardest and best, and capable of a higher polish; the other two varieties are softer and oil in contact with them is said to rapidly deteriorate. The sp. gr. is about 3.7. Oriental sapphires have about the same hard- ness and density as the ruby but are more brittle; they may be white or milky as well as blue. Chrysolite has a green colour and is an alluminate of beryllium Be,Al,O, containing some ferrous oxide, the density varying from 3 to 3.7. Agate, which is composed mainly of silica, derives its name from the river Achates in Sicily, where it was found at the time of Theophratus. It has a peculiar banded structure, the bands being usually irregularly shaped following the configuration of the cavity in which it was formed. Onyx is an agate in which the bands are well defined and often black and write. Its sp. gr. 2 8" 6 g Fig. 3. Parts of Jewel-hole Settings taken from Models made by Mr. Pickering. 5
