148 The Model Engineer and Practical Electrician. February 13, 1930.
is about 2.62. The hardness is 7, about the same as quartz. It is porous. It is used as a knife-edge for chemical balances and for jewelling ordinary clock pallets and holes, but is scarcely good enough for observatory regulators. The garnet, so-called from the Latin Granatus, seed-like, from the appearance of the spherical crystals embedded in a matrix in which it is found, is composed of lime and magnesia, com- bined with oxide of iron and manganese and alumina and chromic oxides. Its sp. gr. varies from 3.55 to 4.20. Its hardness is 7. It is used for jewelling common watches, but soon shows wear. Attempts have been made for many years to produce precious stones in the laboratory, and in 1877 Fremy and Teil produced some ruby flakes, which were exhibited in the Natural History Museum. In 1805 red stones having the characters of genuine rubies came from Geneva, and were found to be reconstructed stones, i.e., frag- ments of real stones melted by the oxy-hydrogen blowpipe and coloured by the addition of potassium bichromate. In 1904 Verneuil invented the inverted blowpipe and was able to produce synthetic corundum from alumina and obtain the colour of the ruby by the addition of chromic oxide and that of the sapphire by the addition of titanium oxide and magnetic oxide of iron. They are now made in large quantities and have the same density, hardness and optical characters as the natural stones, and, being much cheaper, are excellent for holes. Synthetic stones can be dis- tinguished by circular or curved striation and by the presence of minute spherical bubbles in their substance, whereas in the natural stone the striations are straight and any bubbles are elongated in 3. 32 5 If, however, it is desired to make some holes and an unlimited amount of time and patience is available, there is no reason why they should not be produced in an ordinary workshop. A copper lap about 2 ins. diameter and in. thick must be rigged up in a lathe which can be run at a high speed, and charged with coarse diamond powder and paraffin, and the stone ground flat on both sides by holding it against the surface of the lap and moving it about. Carborundum powder can be used to cut agate and soft stones in place of the diamond powder. The stones is then cemented to a small brass. faceplate with shellac by heating over a spirit lamp and while still hot run true in the lathe by holding a piece of pegwood against it by means of the hand-rest, a centre is then turned with a diamond graver, and the stone drilled with a diamond drill. Diamond gravers and 2 Shellac 6 Outside 3 (4) Outside 32 (10) (8 Pirot hole Joint Fig. 2.-Details, mostly in Section, showing the procedure in mounting Jewel-holes and Finished Settings in Sections. the direction of the striation. The main bulk of holes are made in Switzerland, though some of the factories, like the Waltham in America, make their own. The Swiss cement the stones on a block and grind them flat by presenting them to a disc charged with diamond powder, driven at a great speed; when sufficiently reduced, they are reversed on the block and the other side ground till of the required thickness. They are then placed in a holder six at a time and held in a slide-rest and drilled from a lathe by a steel wire charged with diamond powder. The objec- tion to this method is that the holes may not be central nor vertical. The sides and oil sinks are turned with a splinter of diamond and the holes opened and polished by means of diamond powder carried on a copper wire. It requires a good deal of practice to make satisfactory jewel holes, and it is far better to obtain them ready made. drills are made by drilling a hole in the end of a brass wire and embedding the fragment of diamond in a bed of shellac by heating and allowing it to cool. The hole when made through the stone is enlarged to size with a brass broach charged with diamond powder and polished with finer grades of diamond powder. The oil sink is turned with the diamond graver and polished with a copper wire and powder and the periphery turned true. The stone may then be detached by warming the brass faceplate cleaned from the shellac in spirits of wine, and its surface polished by rubbing it on a ground-glass plate with very fine diamond powder. Stones can be slit by means of a thin steel circular disc about in. diameter run in the lathe and charged with diamond powder. Where- possible, as with small mechanism, jewels. should be set with the plate cemented to a chuck and the hole run true in the lathe. A hole is then turned out to a depth corresponding to the
