A Practical Course of Instruction in The Science of Horology Or The Construction and Repair of Time-Measuring Instruments
by Orville R. Hagans and D. L. Thompson
Properties of Steel-Making a Setting Spring - Hardening Steel Tempering Steel - Stoning Flat Steel Parts - Grinding Flat Steel Parts - Polishing Flat Steel Parts.
10. PROPERTIES OF STEEL
Many of the parts replaced by the horologist are made of a special steel which can be sawed, filed, or turned to shape and then hardened and tempered to prevent wear and to give a certain elasticity to the metal.
This steel, sometimes called crucible steel, is produced by refining molten pig iron. In the process the iron is purified of most of the alloyed metals naturally found in it and a certain percentage of carbon is added. For commercial purposes steel is often classified in three grades of hardness: low-carbon, which contains less than .25 of 1% carbon and is called mild or soft steel; medium-carbon, which contains from .25 to .6 of 1% carbon and is called medium steel; and high-carbon, which contains from .6 to 1.7% carbon and is called hard steel. The latter, sometimes called tool-steel, is the kind that the horologist uses for watch parts which are to be hardened and tempered. It can be obtained from a watch material house or machinists' supply house in rods and sheets of convenient sizes.
There are other hard steel alloys which are used in the manufacture of high-speed drills and other cutters for use on tempered steel and to stand the heat of fast cutting, some of which will continue to cut well after reaching a temperature that would soften ordinary tool-steel. The horologist, however, has little use for these.
11. MAKING A SETTING SPRING
Flat steel watch springs, such as setting springs, click springs, etc., which are made in various shapes and sizes, break from frequent flexing. These springs are usually obtainable for watches that are in production and for most others; however, there are watches still in use for which material is scarce or not obtainable. It is for the latter that the horologist must use his skill and experience to produce a part which will equal the one to be replaced in utility and appearance.
A part is usually made by using the old part for a sample and filing out one of the same size and shape from a piece of sheet steel, after which it is hardened and tempered and then given a finish matching that of the original, all of which will be explained in this lesson.
One of the most frequent parts to be replaced is a setting spring, Fig. 1, or a click spring of similar shape. To make such a spring, secure a piece of sheet steel a little wider and longer than the part and of the required thickness; remove the scale, if any, with emery-paper; and then color the metal to a dark blue. Lay the broken pieces of the old spring on the metal in their proper relation, and mark the outlines and the screw hole with a sharp pointed instrument.
A center is now to be made, with a center-punch, to start the drill centrally and a hole of the size of the screw tap drilled. If there is a countersink for the screw head it can now be made with a twist drill of the size of the screw head, or it may be started with a flat drill, after which the drill is to be sharpened to a rather flat angle and the sink then drilled to the proper depth.
Having drilled the hole for the screw, and one for the steady-pin if required, the metal is placed in a bench-vise and one edge filed to shape, as shown in Fig. 1 a. Tool-steel, even when soft enough to file easily, is difficult to saw and filing to shape will be found more practical. After filing one edge, the metal is to be turned in the vise and the other one filed. If the metal is much wider than the part, a V groove, as shown in Fig. 1 b, is filed close to the marking for the edge and about half-way through the metal which will weaken it enough so that it can be easily broken off and thus save considerable filing.
The spring portion of the-part should be left somewhat wider than necessary, as shown in Fig. 1 c, and then ground on an emery-wheel to final size, as shown in Fig. l d, after hardening and tempering. Otherwise, the thin spring portion would be overheated in the hardening process which would run it. The spring shall be left as it appears in Fig. lc, until the hardening and tempering processes have been mastered, after which it can be given its final shape as shown in Fig. Id and than a surface finish.
12. HARDENING STEEL
High-carbon steel can be hardened by heating it to a dull red heat, or about 1200· F., and then dropping it into any liquid substance which will quickly cool it. Water, at medium temperature, is the most common cooling agent, but cooling in oil, of any kind, will add some toughness to the metal. Molted tallow and beeswax can be used with similar results.
In testing these rods for resistance to breaking it will be found that the first mentioned will require but little pressure to break it; the second will require somewhat more pressure; and the third will require considerably more pressure. This shows that the temperature to which steel is heated before cooling governs both hardness and resistance to breakings. If steel is heated to dull red heat and then allowed to gradually cool it will be found to be softer than before.
These tests can easily be made and the fractures observed with an ordinary loupe. Heat a 1/8 inch steel rod at the end to white heat and then quickly cool it in water. Fasten it in a benchvise and put enough pressure on it to break it. It will break with little pressure and the fracture will show an irregular surface. Heat a similar rod to bright red heat and cool it. It will require somewhat more pressure to break it and the fracture will show a somewhat cleaner break. Now heat a rod to dull red heat and cool it. It will require considerably more pressure to break it and the fracture will show a quite clean, smooth surface. Heat one of these rods to dull red heat and allow it to gradually cool. It can now be bent considerably before breaking.
Hard-steel should be so treated before making bends and for easy drilling and tapping. Short bends should be made while the steel is red-hot. Steel can be made softer yet by heating it in ashes or sand and then allowing it to cool in these substances, which will take considerable time, this is called annealing.
If steel is heated to white heat small sparks will be seen to fly off as it reaches incandescence which shows that the metal is disintegrating and this is called burning. If heated to a bright red heat there is also a slight burning of the metal which renders it unfit for springs, pivots, or cutting tools. If overheated, or heated hotter than a dull red, and then suddenly cooled, tempering will reduce its hardness, but it will not give it safe elasticity and it will easily break under pressure. Theoretically, the reason for this is that the structure of the metal is then quite coarse and the molecules have weak cohesion.
The method most commonly used by the horologist to harden a small steel part is to lay it on a heating block, as shown in Figure 2, and to direct the flame of an alcohol lamp on it with a blow-pipe until it is heated to a dull red, after which it is dropped into water or oil to quickly cool it. The addition of oxygen from the breath to the flame increases its heat considerably and the thinner the flame the hotter it is. A thin flame should be used to heat a very small part, which is produced by holding the pipe within the flame, while a flaring one will be required for larger parts, which is produced by holding the pipe outside of the flame. The part is heated only slightly at first and then dipped into liquid soap, or rolled on a bar of common laundry soap, until it is fully covered. This coating of soap prevents the oxidizing effect of the heat and air which would otherwise cause a hard scale to form on the part. It will flake off when the part is reheated to a dull red heat and dropped into water or oil and there will be silvery white spots on the metal which shows that it is quite hard. To test the hardness of the metal, it can be tried with a file and if hard the file will slip over it without leaving a mark. If found to be soft, the process can be repeated, being careful not to overheat the metal.
Another method commonly used is to wrap a short length of fine iron wire around the part with which it is held in a flame, as shown in Figure 3, until it is heated to a dull red and then quickly cooled. This method is especially good for parts which are thin or are smaller in diameter in the center than at the ends as it permits cooling them by placing them endways into the cooling liquid which can be held in a test tube or a wide-mouthed bottle close up to the flame. Such parts when cooled broadside are apt to warp due to the unequal contraction of the metal on opposite sides of the part which is caused by the cooling of one side quicker than the other.
The student should harden several three-fourth inch lengths of one-eighth inch steel rod and several pieces of sheet steel, of about one-fourth inch by three-fourth inch by one-thirty-second inch in dimensions, using both of the methods above described.
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