Chapter X: Jewelling
98. The use of jewels is a recent improvement in watchmaking. It is clearly a significant advance to use a material for pivot holes that cannot be destroyed by friction, is unaltered by chemicals and will take the highest polish; and at the same time obtains the stability of the gear depths (14), the fluidity and purity of the oil, and ensures the reduction of friction to the smallest amount.
99. All jewel holes must be carefully examined before they are used. If a jewel hole is not carefully polished or the corners are cracked, it is worse than a metal bush and very rapidly wears out the pivot.
100. In my view, movements should be fully jewelled. The price of a pair of jewel holes is not so high that it should be an obstacle to their use, and the lever holes should not be left without jewels. It is true that the angular movement of the lever is very small, but experience teaches us that a reciprocating movement is most effective when sharpening something, and the wear of a pivot in its hole is just the same as a very gentle sharpening process. Besides we can expect a reduction of friction by using jewel holes for the lever pivots and this is very important in the lever escapement, with which inertia and frictional resistance have to be overcome anew with each tick of the watch.
101. For similar reasons the 4th and 3rd wheels should also have jewel holes, if the quality and the intended value of the watch permit.
102. The use of end stones for the escape wheel and lever is more a matter of taste than of practical usefulness. However, the balance has a fast movement over about 450° and it is extremely important to avoid the increased friction which would come from contact of the arbor shoulders with the surfaces of the jewels; and so we cannot omit end stones from the balance. On the other hand, it is obvious that the lever and escape wheel work under very different circumstances. In watches of normal design the lever makes an angular movement of 10-15° with each oscillation of the balance; and the escape wheel, if it has 15 teeth, covers 12° of one revolution in the same time. Besides, we cannot assume their weight exercises as much pressure in the upright position, because they are actually light and work under continual and significant side pressures. But the greatest difference is that the escape wheel and lever are made as light as possible while the balance is and must be much heavier.
103. The difference between the friction of a simply set pivot and one with end stones is extraordinarily small. According to the generally valid law of mechanics, that the magnitude of friction at equal pressures is independent of the surface area of contact, this difference should be zero. However, in our case adhesion must be considered, particularly because oil must be applied to the pivot. But the friction of a pivot provided with caps will only be smaller in relation to the difference in the contact surfaces, and the difference between the surface of the end of the pivot and that of a shoulder reduced in size is small. However, with an angular movement 30 times larger than those of the escape wheel and the lever it assumes a greater importance with the balance. And therefore the balance must have end stones. I agree that a small saving of energy can be obtained by putting end stones on the escape wheel and lever, but I believe this very slight advantage is usually overrated. The fact that a number of best English watches are without end stones on the escape wheel suggests that English watchmakers understand the matter roughly in the way described above.
104. The use of diamond for the end stone of the upper balance pivot is probably recommended because, if the watch operates in the usual horizontal position, the balance rests with its whole weight on the end of the upper balance pivot and the friction and wear on it are reduced to the smallest amount by the extraordinary hardness and fine polish of the diamond surface. But we must use great care when selecting diamonds, because we will occasionally find badly polished pieces amongst those which we buy from shops, and they will contribute to the destruction of the pivot instead of protecting it.
I want to report one case which occurred in my own business and caused me to avoid using diamond caps if they were not expressly required. I supplied a pocket chronometer, which went to the great satisfaction of its owner; but after 6 or 7 months I learnt that its rate had become irregular and that in certain positions it was noisy. When it was sent to me for examination I found the upper balance hole was oval. With the help of a strong microscope I discovered a tiny fault in the surface of the diamond cap and could now understand it. I made a new jewel hole, put on another diamond cap and repolished the pivot a little, although this had not noticeably suffered. The watch went as before, but I got it back 6 months later with the same complaint. My investigation showed that the jewel hole was again oval. I believe this could only be explained by a microscopically small fragment of diamond sticking to the pivot and which may have been pressed into the surface when it was repolished. Now another balance hole and a new balance staff were made. Since then the watch has gone for many years without problems. This shows how carefully diamond caps must be selected and that a strong loupe is not sufficient for this purpose. The difference of friction between a good ruby or sapphire cap and one of diamond is very small and I do not recommend exposing a watch to such dangers if the profit which can be obtained from it is not more important.
105. The exact and careful execution of the balance holes is the most important part of the jewel work of a watch. They must not only have, as all other jewel holes, an error free polish, but also be rounded in a suitable way, in order to make the friction in the upright and horizontal positions completely equal or as equal as possible.
106. It is a good plan to make the balance holes with a conical sink in order to give them more strength and to help the pivot enter the hole when putting on the cock. But its form is very important or adhesion will increase. Besides, if the cock has its steady pins made in the way described above (Art. 83) there will no difficulty in putting on the cock without damaging the jewel hole.
107. Setting jewels is done in different ways. In some, including the better class of English watches, the jewel holes are set in brass or gold bushes which fit into sinks and are fastened by screws; part of their heads are sunk into the setting and part into the plate so that they lie flush with the surface, with the thread cut in the plate.
Jewel hole setting.
108. The advantage of setting jewels in this way is that it is easier to replace a damaged or broken jewel without having to re-gild the bridge or the plate. However this is not very important, because if we have a good supply of jewels in stock then it will be simple to find one which fits into the old setting; and even if this is not possible we can set the new jewel into a piece of brass wire of a suitable diameter. This wire, after it is turned slightly tapered and exactly concentric with the jewel hole, is turned until it fits into the hole in the plate, and then cut off so that it remains a little longer than is necessary. The setting is driven carefully into the hole in the plate until the right amount of end shake is obtained. Then the plate or bridge is put on a face plate, centred to the jewel hole and the chamfer is turned. When the brass setting has been turned to a suitable size, then is it easy to ensure that the chamfer is cut a little larger into the plate; if this is carried out well in a simply set watch, then replacing a jewel in the way just described will be barely visible afterwards.
109. A work with simply set jewels is in no way inferior to one with screwed settings even in the rare cases where a jewel has to be replaced, provided it is done as explained above. Work with screwed settings has a higher reputation, but if it is not implemented with understanding and great care it results in a large increase in work during production and still more during repair. In order to clean a watch carefully we have to take out and then replace the screws and jewels, and the small depth in which the screws are held is a large source of annoyance for the repairer; particularly in English watches with their thin upper plates of brass made completely soft by gilding and their screws with rather rough threads (Art. 22). Each loose screw must be replaced by another of a larger diameter, and this will have less chance of holding securely because of its greater thickness, so that it is often necessary to drill new holes in fresh places. If screwed settings offer the advantage of easier replacement of broken jewels without leaving any noticeable traces, then we can say that this small advantage is offset by the disadvantages mentioned above.
110. However, we can change the screwed setting in such a way that these problems are eliminated. There is not the slightest necessity to sink the screw heads into the upper plate. They could, without any disadvantage, have a flat head which only serves to hold the jewel in its place, and then the whole thickness of the plate is available for the thread. A screwed setting can also be provided with marks so that it is always inserted the same way into the sink, which is not unimportant. If it is necessary to ensure the position of the jewel when careless repairers ignore the marks, then this can be easily done by drilling a small hole in the bottom of the sink and putting in a pin, which enters a small cut made for it in the setting.
14 "…die Stabilität der Wirkungen …" is literally "the stability of the action". I think Grossmann means the stability of the gear depths. [Trans]