The barrel cover being removed by the blade of a small watch screwdriver, the arbor is first taken out and then the broken spring. If, without doubt, the broken spring was the original spring, and the watch is of fair quality, it is well to follow the rule generally adopted by the trade, and replace it with another of "the same width and strength." Frequently, however, it happens that the spring is not the original, but one put in by some careless workman either ignorant of what conditions a spring should fulfil, or contented with the nearest spring to the original that he happened to possess. In such a case, the general rule does not apply.
Suppose, by way of example, that you have a broken spring to replace, which evidently is not of the proper width and strength for the barrel it occupied, and consequently not adapted to the watch. The first consideration is its width, which should be as great as the barrel will fairly admit, reaching from the bottom of the barrel to the groove barely, excepting where the barrel cover is hollowed out, when it may reach it fully. If the spring is not wide enough, its working will be irregular; if too wide, then it will bind in the barrel. The next point is the thickness, and it is most important that this should be correct for the watch to perform satisfactorily. If the spring is too thick, the action of the escapement will be hurried and its rate unsteady, and the chain more liable to break; while, if too thin, the escapement will be sluggish, and the watch apt to stop altogether. The strength of the spring should be such that, when of the proper length, hooked in the barrel and wound up, it may cause the barrel to make about 3/4 of a turn more than is required by the length of a chain that occupies the fusee when fully wound. The length of a spring should be such that when wound in the barrel it should occupy about 1/3 of its diameter.
Having gauged the width and found the corresponding springs, one of the proper strength will be found as a rule to be a little larger in diameter than the barrel, or one that would almost fill the barrel if it were wound in, so that it is necessary to break off a short piece that the barrel may not be too full. This applies to the springs as bought from the makers, coiled within a wire ring, and is merely given as an approximate guide to selection.
Having selected a spring apparently suitable, it must be shortened as much as is necessary, and " hooked in," when it must be finally tested by holding the barrel tight in the left hand and winding up the spring by means of a pair of sliding tongs attached to the squared end of the barrel arbor, and observing how many times it causes the barrel to revolve. If it makes an insufficient number of turns, the spring is too thick; if too many, it is too thin. Although this may be stated as a general rule, it is not without exceptions, as, for example, in verge watches it is occasionally expedient to use a somewhat weaker spring than will only make the proper number of turns, owing to an imperfect and unequal balance wheel not admitting of a close and correct escapement. There are 2 methods of hooking in mainsprings: in one the hook is in the barrel, and the spring only requires a hole in it near the end; in the other the hook is attached to the spring, a hole being formed in the barrel to receive it. In replacing a spring which only requires a hole in the end, it must be carefully tempered by means of a very small flame, so applied that the spring may be gradually and equally tempered from the end where the hole is to be, which should be rather soft, to about 1/2 in. of its length.
The hole should be square, as being the least liable to constrain the spring, and prevent its proper action in the barrel. It is usual, after making the hole, which is punched with a pair of mainspring nippers, to pass a file lightly across the end of the spring and round off the corners, giving it a neat and workmanlike appearance. When the hook is to be attached to the spring, the latter is tempered in the manner already described, and a small round hole is punched in it. A piece of "hooking-in" wire is then fitted to the hole in the barrel, and placed in the jaws of a pair of sliding tongs in such a manner that a pivot may be filed on it to fit the hole in the spring, and cause the piece of hooking-in wire to form a hook standing at the proper angle to suit the hole in the barrel. The hooking-in wire is then put in the vice, and the mainspring is firmly secured to it by riveting, when the length of the wire is cut off, leaving only sufficient to form the hook. The end of the spring is usually finished like the other, but left pointed instead of round.
When this is necessary, it is always a good plan to put in one of steel, and not brass, as they frequently are. The hook should be "tapped" in very tight and nicely shaped, not standing up too high in the barrel.
When a barrel cover is loose, it should be covered over with a piece of thin paper and gently tapped with a round-faced hammer all round the edge, which, if carefully done, will spread the cover a little without marking it.
There are 3 kinds of arbor commonly in use - the plain English, the plain Geneva, and the Geneva with solid ratchet. The fitting of a new one of either kind requires to be done very carefully, it being absolutely necessary that the pivots should be accurately fitted, and the end-shakes very exact, for the barrel to run true and give satisfaction, father of the plain arbors can be made from a piece of ordinary round steel, or an "arbor in the rough" may be obtained from the tool shops. In the former case, it will be necessary to turn the steel somewhat to shape in the lathe; but when bought in the rough, the arbor is quite ready for the more exact turning which is done in "the turns." A screw ferule is attached to one end of the arbor, and the body or centre part is first turned to the proper width and diameter, the measurement being taken from the old arbor by means of the pinion gauge. The arbor is then turned down and polished until it fits the holes in the barrel just tight, when a round broach passed lightly into the holes will give the necessary freedom.