The product obtained by multiplying together the center and third wheels = 84 × 78 = 6,552. The two pinions multiplied together = 7 × 7 = 49. Then 6,552 ÷ 49 = 133.7. So that for every turn of the center wheel the escape pinion turns 133.7 times. Or 133.7 ÷ 60 = 2.229, which is the number of turns in a minute of the escape pinion.
The length of the pendulum, and therefore the number of escape wheel teeth, in clocks of this class is generally decided with reference to the room to be had in the clock case, with this restriction, the escape wheel should not have less than 20 nor more than 40 teeth, or the performance will not be satisfactory. The length of the pendulum for all escape wheels within this limit is given in the preceding table. The length there stated is of course the theoretical length, and the ready rule adopted by clockmakers is to measure from the center arbor to the bottom of the inside of the case, in order to ascertain the greatest length of pendulum which can be used. For instance, if from the center arbor to the bottom of the case is 10 inches, they would decide to use a 10-inch pendulum, and cut the escape wheel accordingly with the number of teeth required as shown in the table. But they would make the pendulum rod of such a length as just to clear the bottom of the case when the pendulum was fixed in the clock.
In the clocks just referred to the barrel or first wheel has 96 teeth, and gears with a pinion of eight.
Month clocks have an intermediate wheel and pinion between the great and center wheels. This extra wheel and pinion must have a proportion to each other of 4 to 1 to enable the 8-day clock to go 32 days from winding to winding. The weight will have to be four times as heavy, plus the extra friction, or if the same weight is used there must be a proportionately longer fall.
Six-months clock have two extra wheels and pinions between the great and center wheels, one pair having a proportion of 4½ to 1 and the other of 6 to 1. But there is an enormous amount of extra friction generated in these clocks, and they are not to be recommended.
The pivot holes and all the other holes in the frames, are punched at one operation after the frames have been blanked and flattened. They are placed in the press, and a large die having punches in it of the proper size and in the right position for the holes, comes down on the frame and makes the holes with great rapidity and accuracy. These holes are finished afterwards by a broach. In some kinds of clocks, where some of the pivot holes are very small, the small holes are simply marked with a sharp point in the die, and afterwards drilled by small vertical drills. These machines are very convenient for boring a number of holes rapidly. The drill is rotated with great speed, and a jig or plate on which the work rests is moved upwards towards the drill by a movement of the operator’s foot. All the boring, countersinking, etc., in American clocks, is done through the agency of these drills. Bending the small wires for the locking work, the pendulum ball, etc., is rapidly effected by forming. As no objectionable marks have been made on the surface of either the thick or smaller wires during any process of construction, all that is necessary to finish the iron work is simply to clean it well, which is done in a very effective manner by placing a quantity of work in a revolving tumbling box, which is simply a barrel containing a quantity of sawdust.
Milling the winding squares on barrel arbors is an ingenious operation. The machine for milling squares and similar work is made on the principle of a wheel cutting engine. The work is held in a frame, attached to which is a small index plate, like that of a cutting engine. In the machine two large mills or cutters, with teeth in them like a file, are running, and the part to be squared is moved in between the revolving cutters, which operation immediately forms two sides of the square. The work is then drawn back, and the index turned round, and in a like manner the other two sides of the square are formed. The cutting sides of the mills are a little bevelled, so that they will produce a slight taper on the squares.
Winding keys have shown great improvements. Some manufacturers originally used cast iron ones, but the squares were never good in them, and brass ones were adopted. At first the squares were made by first drilling a hole and driving a square punch in with a hammer; and to make the squares in eighteen hundred keys by this method was considered a good day’s work. Restless Yankee ingenuity, however, has contrived a device by which twenty or twenty-five thousand squares can be made in a day, while at the same time they are better and straighter squares than those by the old method; but we are not at liberty to describe the process at present, but only to state that it is done by what machinists call drilling a square hole.
Pendulum rods are made from soft iron wire, and the springs on the ends rolled out by rollers. Two operations are necessary. The first roughs the spring out on rollers of eccentric shape, and the spring is afterwards finished on plain smooth rollers. The pendulum balls in the best clocks are made of lead, on account of its weight, and cast in an iron mold in the same manner as lead bullets, at the rate of about eighteen hundred a day. A movable mandrel is placed in the mold to produce the hole that is in the center of the ball. The balls are afterwards covered with a shell of brass, polished with a bloodstone burnisher. The various cocks used in these clocks are all struck up from sheet brass, and the pins in the wheels in the striking part are all swedged into their shape from plain wire. The hands are die struck out of sheet steel, and afterwards polished on emery belts, and blued in a furnace.
All the little pieces of these clocks are riveted together by hand, and the different parts of the movement, when complete, are put together by workmen continually employed in that department. Although the greatest vigilance is used in constructing the different parts to see that they are perfect, when they come to be put together they are subjected to another examination, and after the movements are put in the case the clocks are put to the test by actual trial before they are packed ready for the market. As a general rule, all the different operations are done by workmen employed only at one particular branch; and in the largest factories from thirty to fifty thousand clocks of all classes may be seen in the various stages of construction.