The Modern Clock / A Study of Time Keeping Mechanism; Its Construction, Regulation and Repair

by Ward L. Goodrich

Assuming the pinion teeth to be satisfactorily cut, the next operation will be hardening and tempering. A good way of doing this is to enclose one at a time in a piece of gas pipe, filling up the space around the pinion with something to keep the air off the work and prevent any of the products of combustion attacking the steel and so injuring the surface. Common soap alone answers the purpose very well, or it may have powdered charcoal mixed with it; also the addition of common salt helps to keep the steel clean and white. The heating should be slow, giving time for the pinion and the outside of the tube to both acquire the same heat. Over-heating should be carefully avoided, or there will be scaling of the surfaces, injurious to the steel, and requiring time and labor to polish off. There is no better way of hardening than by dipping the pipe with the pinion enclosed in plain cold water, or if the pinion should drop out of the tube into the water it will do all the same. To be sure the hardening is satisfactory it will be as well not to trust to the clean white color likely to result from this treatment, but try both ends and the center with a file. After all this has been successfully accomplished the pinions will require tempering, the long arbors straightening, and the teeth polishing.

The drilled pinion heads, if hardened at all by the method last mentioned, will, on account of their short lengths, be equally hardened all over, but if the pinion and arbor should be all in one piece care will be needed to ensure equal heating all over, or one part may be burnt and another soft. Also, to guard against bending the long arbors, the packing in the tube will need to be carefully done, so as to produce equal pressure all over; otherwise, while the steel is red hot, and consequently soft enough to bend, even by its own weight, it may get distorted before dropping in the water. A long thin rod like this almost invariably bends if heated on an open fire unless equally supported all along; if hardened so, a little tin tray may be bent up, filled with powdered charcoal, and the pinion bedded evenly in it. Either this way or with a tube the long arbor may get bent before being quenched; but if the arbor, though kept straight up to this point, should happen to be dropped sideways into the water the side cooled first would contract most. To avoid this, the arbor should be dropped endways, as vertically as possible.

Tempering the Pinions.—For common cheap work the usual and quickest way is what is called “blazing off.” That is done either by dipping each piece singly in thick oil and setting the oil on fire, allowing it to burn away, or placing a number of pieces in a suitably sized pan, covering with oil, and burning it. The result is the same either way, the method being simply a matter of convenience regulated by the number of pieces to be tempered at one time. As the result of blazing off is to some extent uncertain, and the pinions apt to be too soft, it will be advisable to adopt the process of bluing, by which the temper desired may be produced with more accuracy. The first thing to do will be to clean the surface of the arbor all along on one side; the pinion head may be left alone. As the pinion head would get overheated before the arbor had reached the blue color, if the piece were simply placed on a bluing pan or a lump of hot iron, it will be necessary to provide a layer of some soft substance to bed the pinion on; iron, steel or brass filings answer well because the heat is soon uniformly distributed through the mass, and by judiciously moving the lamp an equable temper may be got all along, as determined by the color. There is another and very sure way of getting a uniform temper, in using which there is no need to polish the arbors. The heat of lead at the point of fusion happens to be just about the same as that required for the tempering of this work; so if a ladle full of lead is available each pinion may be buried in it for a few seconds, holding it down beneath the molten surface with hot pliers. The temper suitable is indicated by a pale blue, a little softer than for springs, and a piece of polished steel set floating on the lead will indicate whether the heat is suitable; if found too great some tin may be added, which will cause the metal to melt at a lower temperature. Over-heating the metal must be avoided: it should go no higher than the bare melting point.

Straightening Bent Arbors.—When all care has been taken in the hardening, the long pieces of wire are still apt to become bent more or less, and this is especially the case with solid pinions; so before proceeding further the pieces must be got true, or as nearly so as possible, and it will be found impracticable to do this by simple bending when the steel is tempered. If the piece is placed between centers in the lathe and rotated slowly, the hollow side will be found; this side must be kept uppermost while the steel is held on a smooth anvil, and the pene, or chisel-shaped, end of a small hammer applied crossways with gentle blows, stepping evenly along so that each portion of the steel is struck all along the part which is hollow; this will stretch the hollow side, and, by careful working, trying the truth from time to time, the piece can be got as true as may be wished, and probably keep so during the subsequent turning and finishing, though it is advisable to keep watch on it, and if it shows any tendency to spring out of truth again, repeat the striking process, which should always be done gently and in such a way as to show no hammer marks. Having got the pieces sufficiently true in this way, each arbor may have a collet of suitable size driven on to it for permanency, and as the collets will probably be a little out of truth they may have a finishing cut taken all over them and receive a final polish.

Polishing.—To polish the steel arbors after turning, a flat metal polisher, iron or steel, is used; this with emery or oilstone dust and oil produces a true surface, with a sharp corner at the shoulder; the polisher will require frequent filing on the flat and the edge to keep it in shape with a sharp corner, and a grain crossing like the cuts on a file to hold the grinding material. The polishing of arbors is not done with the object of making them shine, but to get them smooth and true, so there is no need of using any finer stuff than emery or oilstone dust.

An old way to polish the leaves was to use a simple metal polisher of a suitable thickness, placing the pinion on a cork or piece of wood, or even holding it in the fingers; working away at a tooth at a time until a good enough polish was obtained; but this method, while being satisfactory as to results, was also tedious and very slow. It was in some cases assisted by having guide pinions fitted tight on one or both ends of the arbors to prevent rounding of the teeth, the polisher resting in the guide and the tooth to be polished. On the American lathes an accessory is provided called a “wig wag.” This is a rod fastened at one end to a pulley by a crank pin near its circumference; the pulley being rotated by a belt from the counter shaft pulleys causes the rod to move rapidly backwards and forwards. On the other end of the rod a long narrow piece of lead or tin is fixed, the pinion being fitted by its centres into a simple frame held in the slide rest so that it can be rotated tooth by tooth; the lead soon gets cut to the form of the teeth, and the polishing is quickly effected. Another way is to take soft pine or basswood, shape it roughly to about the form of space between two teeth and use it as a file, with emery and oil or oilstone dust. The wood is soon cut to the exact shape of the teeth, and then makes a quick and perfect job. The pinion is held in the jaws of the vise and the wooden polisher used as a file with both hands.

Where there is much polishing to do a simple tool, which a workman can form for himself, produces a result which is all that can be desired. It consists of an arbor to work between the lathe centres, or a screw chuck for wood, with a round block of soft wood, of a good diameter, fixed on it, and turned true and square across; this will get a spiral groove cut in it by the corners of the pinion leaves. The pinion is set between centres in a holder in the slide rest, with the holder set at a slight angle, so that, instead of circular grooves being cut in the wood a screw will be formed, the angle being found by trial. On the wood block being rotated and supplied with fine emery the pinion will be found to rotate, and, being drawn backwards and forwards by the slide rest, can be polished straight, while the circular action of the polisher will cause the sides of the pinion leaves to be made quite smooth and entirely free from ridges.

If it should be desired to face the pinions, like watch pinions, it may be done in the same way, by cutting hollows so as to leave only a fine ring round the bottoms of the teeth, and using a hollow polisher with a flat end held in the fingers while the pinion is rotating. A common cartridge shell with a hole larger than the arbor drilled in the center of the head makes a fine polisher for square facing on the ends of pinions, while a stick of soft wood will readily adapt itself to moulded ends.

The pinion heads being finished and got quite true, the arbors may be turned true and polished. It is not advisable to turn the arbors small; they will be better left thick so as to be stiff and solid, as the weight so near the center is of no importance, the velocity on the small circumference in starting and stopping being also inappreciable. The thickness of the arbors when the pinion heads are drilled is determined by the necessity of having sufficient body inside the bottoms of the teeth; but when solid they may with advantage be left thicker; however, there is no absolute size. The ends on which the collets for holding the wheels are to be fixed may be turned to the same taper as the broach which will be used for opening the collet holes, while the other ends may be straight.

None of the wheels in a fine clock should be riveted to the pinion heads; even the center wheel, which goes quite up to the pinion head, is generally fixed on a collet. The collets are made from brass cut off a round rod, the outside diameters being just inside the edges of the wheel hubs, and a shoulder turned to fit accurately into the center hole of each wheel. These collets should first have their holes broached to fit their arbors, allowing a little for driving on, as they may be made tight enough in this way without soldering. Be careful to keep the broach oiled to prevent sticking if you want a smooth round hole.