These seldom go satisfactorily for any length of time with the treatment they ordinarily receive. In addition to the usual careful examination of depths, end-shakes, sizes of holes, etc, it is necessary to bear in mind the following principal causes of their bad performance - defective calibre, roughness of finish, and faulty escapements. Defective calibre is unalterable, for you cannot prudently make any useful alteration in the proportions of the various parts, as the expense would probably be more than the timepiece would be worth. There is, however, one very important part which demands attention, and that is the mainspring. This usually has to make such a large number of turns for the timepiece to go the prescribed 8 days that considerable skill is required to make an escapement which will give a fairly uniform rate. Therefore it is always desirable to have a thin mainspring, in order to obtain as many turns as the size of the barrel will admit.
Rough finish must be remedied, especially in the parts farthest from the motive force. To this end, thin down the third, fourth, and escape wheels, when found unnecessarily thick, by filing with a fine-cut file, and finish smooth with a piece of water-of-Ayr stone. Take care not to raise a "burr" by using too coarse a file, and look out for imperfections in the teeth. If the pivots of the escape pinion and pallet arbor are left any too large, reduce their size by "running" in the turns, and burnish them well.
In these timepieces, faulty escapements are almost invariably found, and may be considered their greatest defect. With the object of rendering the pendulum insensitive to the varying power of the mainspring, the pallets are made as close to the arbor as possible, embracing only 1 or 2 teeth of the escape wheel. The inside pallet communicates impulse to the pendulum, but the outside one, forming part of a circle struck from the centre of motion, gives no appreciable impulse, as the escape-wheel teeth merely rest "dead" on it. Unfortunately, this principle is carried too far, and the result is that at times there is insufficient force at the escape wheel with such a small amount of leverage to maintain the vibrations of the pendulum, and the timepiece stops. As no beneficial alteration of the original pallets can be made in a proper workmanlike manner, it is best at once to condemn them, and make a new pair. By very carefully following the instructions here given, no great difficulty will be experienced in making them give favourable results.
The object of making new pallets is to obtain a longer leverage, so that the occasional diminished force may prove sufficient to keep the pendulum vibrating; and the difficulty which arises is to make them of such a shape that this varying power of the escape wheel does not influence the time of the pendulum's vibrations, however much it may the extent. The object is attained by making the pallets embrace a larger number of teeth, which brings them a greater distance from the centre of movement, and thus increases the leverage. The difficulty is overcome by making the pallets of such a shape that the escape-wheel teeth rest as "dead" as possible during the excursion of the pendulum beyond the distance necessary for the escape to take place. From a consideration of the shape of the escape-wheel teeth, and the distance the pallet arbor is pitched from the escape wheel, it will be readily seen that, though the outside pallet can be easily made to give the desired effect, it is impossible to make the inside one of any shape that will not produce more recoil than is desirable.
To render this recoil as insignificant as circumstances admit, great care must be bestowed in suiting the pallet to the wheel, and for the same purpose it is advisable to make it nearer than the outside one to the pallet arbor. Before making the new pallets,file off the old ones, guarding the pivot so that the file cannot slip and break it off, and leaving the arbor round, smooth, and slightly taper. Procure a small piece of card, and make a straight line down the centre; then, with a pair of compasses, take the distance from the escape-wheel pivot-hole to the pallet-arbor pivot-hole, and make 2 small holes through the card upon the straight line that distance apart. In one of these holes fit the escape-wheel arbor so that the wheel rests flat upon the card, and in the other fit the pallet arbor. The number of teeth most suitable for the new pallets to embrace must be decided by the character of the train; if it is fairly good, 4 will be found sufficient; if very rough, 5 had better be the number. Select a piece of good steel, of suitable thickness; having softened it, drill a hole through it, and fit the pallet arbor in to the proper distance.
Put the escape-wheel arbor through one of the holes in the card, and the pallet arbor with the piece of steel on it in the other, and see how much requires filing off, so as to leave only sufficient to make the pallets of the proper length. Now mark off the position of the opening between the pallets, the distance of the inside pallet from the line of centres being equal to the space between 2 of the escape-wheel teeth, leaving the space between the points of 3 teeth on the opposite side of the line of centres. Fig. 221 shows the escapement. It is advisable not to file out the full width until the pallets are roughly shaped out and ready for escaping. They should be made of the shape shown, keeping them flat across the surface; and they may be roughly "scaped" for trial upon the card, which, by bending, can be made to move the pallets nearer or farther off as desired. When nearly right, finish the escaping in the frame, taking great care not to get too much drop on to the inside pallet, as there is no way of altering it should there be an excess.
The drop on to the outside pallet is easily adjusted, as the hole in the front plate is in a movable piece, which can be turned with a screwdriver.
Respecting the shape of the inside pallet, it will be seen that its point resembles a half tooth of an ordinary wheel; this is to cause the friction and recoil, which are unavoidable, to take place, with the least impediment to the pendulum, as this shaped-point rolls upon the faces of the escape-wheel teeth, whilst the ordinary form scrapes them. When the pallets are properly "scaped," it only remains to finish their appearance in a workmanlike manner, and harden and temper them. The sides should be nicely "greyed" by rubbing them on a flat piece of steel with oilstone-dust and oil, and the acting faces polished with diamantine or redstuff. It will be generally found sufficient to secure them by driving the pallet arbor in tight; but if thought necessary, they may be pinned on. The timepiece may then be cleaned and put together, observing that it is nicely "in beat," according to the conditions already stated.
When these drum timepiece movements are fitted into large gilt or bronze cases, where there is plenty of room for any motion the pendulum may take, it is a great improvement to suspend the pendulum with a spring, for the pallet-arbor pivots, being relieved of the dead weight of the pendulum, do not wear the holes so quickly, and, as the friction is considerably reduced, the pendulum is kept in motion with less power. The best way to put a spring suspension is as follows: - If there is sufficient substance in the cock above the pivot hole, drill a hole through the cock and tap in a piece of 3/16-in. brass wire, with a slight shoulder, and rivet it in secure. Cut off so as to leave it about 1/2 in. long, and make a saw-cut to receive the brass mount of the pendulum spring. The underneath part of this stud should be left nearly in a line with the centre of the pivot hole. When the pivot hole is too near the top edge of the cock to allow this to be done, a piece of brass must be fitted on to the cock to receive the stud; a very convenient shape is shown at a in Fig. 222. Procure one of the thinnest and most suitable French clock pendulum springs, fit one of the brass mounts into the saw-cut in the stud, and arrange it so that the spring, when in action, may bend as near as possible in a line with the centre of the pivot hole; then drill a hole through the stud and brass mount, and secure it with a pin.
Fit a steel pin on which to hang the pendulum, in the hole through the other brass mount. The pendulum rod should be a piece of straight, small-size steel wire tapped with a thread at both ends. Make the hook exactly like the ordinary French clock pendulum hooks, only very much smaller and lighter, and fit it on one end of the pendulum rod; screw the pendulum bob upon the other. Cut the old pendulum-rod in two, so that the piece remaining attached to the pallet arbor reaches to opposite the centre-wheel hole; file a short pivot on the end, and fit on it a crutch. All the parts must be as small and light as possible, and the pendulum bob must be round and turn tolerably tight. Silk suspensions are sometimes used, but rarely give satisfactory results, as they are so sensitive to atmospheric changes.