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and more uniformly the change of color appears the more reliable will be the results from the tools. In tools for turning, planing, and shaping, chipping chisels, drills, and many varieties of cutting-out punches, where it is not necessary to have the tool hardened for. the whole of its length, the hardening may be readily done at one heating. The following explanation of hardening and tempering a chipping chisel will serve to illustrate how this is accomplished:
The chisel, Fig. 198, being held by its head H in a pair of tongs, is placed into the fire for about one-third its length A B, and carefully heated to a cherry-red, care being taken that the extreme end E of the chisel
does not become overheated. If the chisel is very thin the end E should be cooled by dipping it into water once or twice during the time that the chisel is being heated. After it has been heated to the required temperature it is dipped into the water for a portion of its length, according to the size of the chisel this may vary from 34 to 112 inches, and held in the water until cold, then remove the chisel, and brighten the hardened portion C by rubbing with a piece of stone or emery cloth. The heat will now travel from the unquenched portion to the quenched portion. The change of color is watched as it travels along from B to A, until the required color appears at the cutting end E, when the chisel is again plunged into the water bath, this time the whole of it will be quenched. This method can be applied to any tools to be hardened at their ends only, but it must be understood that the nature of the work to be operated upon may necessitate the tool being brough down in temperature to a totally different color. For instance, turning tools, dark straw or yellow color, 450 degrees temperature, cutting-out punches for sheet steel, very dark straw or yellow, 490 degrees temperature; cold chisel for chipping cast iron, dark purple color, 550 degrees temperature.
It should be noted that when chisels, drills, or turning tools are being forged, it is advisable to hammer them until the steel has become quite cold, as this hammering gives toughness and fineness of texture, it may then be re-heated for the purpose of hardening. Taps and reamers are sometimes covered with a mixture of Castile soap and lampblack, to preserve their cutting edges, and to prevent them being burnt whilst being heated for hardening. This class of tools may also be heated in a wrought-iron pipe filled with charcoal dust, the ends being plugged with clay. This method generally results in the taps or reamers being heated uniformly, and they are afterwards dipped into water in a vertical position, and held there until cold.
Circular milling cutters may be covered with Castile soap and lampblack with advantage, and the hole of the cutter plugged up with clay, this preserves the center, which is not usually required to be hard. The tools are generally slightly warmed before the mixture of Castile soap and lampblack is applied, and a circular cutter should be plunged into the water bath edgeways. The tempering of a tap or reamer is usually done by introducing it into a cast-iron or wrought-iron collar, which has been made red-hot, the tool is held in a pair of tongs, and passed along the center of the hole. At the same time it assists matters if the tool is rotated whilst being moved along, as the continual change of position prevents one portion becoming hotter than another, and results in a more even temper. Taps, reamers, milling cutters, and similar tools are generally tempered to a light brown color, and quenched in oil.
Small punches from 1/16 to 12 inch diameter, and dies from 14 to 1 inch diameter, are best heated in a wrought-iron pipe about 12 inches long and 2 inches in diameter, with one end closed. This may be done by welding a wrought-iron plug in one end of the pipe. The small punches or dies are placed in the pipe, which is, in turn, thrust into the breeze fire. This method gives much better results than can be obtained when the flame of a fire is allowed to come in contact with such small tools. When the punches or beds, as the case may be, are sufficiently heated the pipe is removed . from the fire, and the tools tipped into a bucket of clean water containing a handful of common salt. Tools hardened in this manner will be found to be quite clean, and ready for tempering. This may be readily done by placing the tools upon a wrought-iron plate, say 12 inches square by 1/8 inch thick, heated over a gas stove, the small round punches should be rolled over the hot plate until the required color appears, while small dies are best placed endways on the plate.
ANNEALING DIES AND TOOLS.
When dealing with cutting-dies, punches, stampingdies, or any piece of steel which has to be worked or shaped by the action of cutting tools when in the cold state, previous to the tool being hardened, it is desirable that the steel be carefully annealed. Particularly is this necessary when stamp-dies of some peculiar and difficult shape have to be worked and finished in a first-class manner. When a die or punch, after having been forged, is thrown down upon the floor of a smith's shop to cool, thereby being exposed to cold air, especially in winter time, it frequently results in the steel being in an unequally hardened condition, which may also be partly caused by the hammering process. Annealing will generally remedy this defect, as the process of annealing reduces the steel to its softest and most uniform condition. The ease with which steel may be worked by the various cutting tools in a lathe, planing, milling, or drilling machines more than repays the little trouble that is necessary for anealing. Small articles, for instance, as delicate tools, cutters, reamers, punches, and dies, may be placed in an iron box, surrounded or buried in powdered charcoal, the charcoal prevents the steel from losing its carbon and assists the uniform heating of these small tools, at the same time preserving their shape and preventing any damage being done to their cutting edges. After being heated the box is placed somewhere to gradually get cool before the small tools are removed. The larger tools can be successfully annealed by moderately and uniformly heating them in a muffle, then allowing them to cool slowly. This is sometimes done by burying them in ashes to retard the cooling.
The anealing is usually done before the forgings leave the blacksmith's shop, a good method being to carefully re-heat them after forging to a dull red and place them into an iron box containing slaked lime, where they should remain until cold, which frequently takes a whole day. They are then taken out of the lime, and will be found to be more easily worked into shape, and are not so likely to leave their shape when undergoing the hardening process. There are instances when dies and punches are annealed, roughed-out, and annealed a second time before being finally shaped to their finished outline with beneficial results. But this is not advisable if the tool can be readily worked fairly easy after one annealing, since too much heating may removed the nature from the steel.