CHAPTER VII.
Making a Flat Cold Chisel—Spring Tempering—Welding Steel—Case Hardening—Coloring Steel—Annealing—Making a Scratch Awl—Making a Center Punch—Making a Hand Punch—High Speed Steel—Annealing High Speed Steel.
Exercise No. 17.—Flat Cold Chisel.
Fig. 109.
A good cold chisel is an indispensable tool in a shop, and one that is very much abused. Therefore, it should be made with the greatest care. In the forging of a good chisel a piece of ⅝-in. octagonal tool steel, from 75 to 95 point carbon, is used. The piece is cut six inches long. In doing this the bar may be nicked with a chisel. The nicked part is then set over the outer edge of the anvil. A chisel with a handle is set on the nicks and given a good blow with a sledge hammer, shearing the piece from the bar. See [Figure 109]. This method of cutting is quite dangerous, so care must be taken. Perhaps, a less dangerous method, tho not so practical, is to heat the bar red and cut the piece off with a hot chisel and sledge, or on the hardie, if one has no helper. The end is then hammered. See [Figure No. 110].
Fig. 110.
When cut off and hammered round on one end, the piece is caught with a fluted-lip pair of tongs that will hold it firmly and a ring is placed on the ends of the reins to bind them. The end is now heated in a well burned fire, letting the heat soak in slowly, and not forcing it with too much blast. If the fire is lively hardly any blast is used on the start. The piece is brought to a heat somewhat beyond what is commonly called cherry heat. It is then taken to the anvil and drawn out square with hard blows of the hammer, to a long taper, and nearly to a point. This taper should be about 1¾ inches long. See [Figure No. 111].
Fig. 111.
Hammering must cease before the red heat has left the steel. It is again heated and hammered on two sides; in drawing the chisel bends edgewise. Do not strike it on the edge; it will fracture the grain of the steel. To straighten the blade, it should be hammered on the flat side near the concave edge. See [Figure No. 112]. This stretches the metal and straightens the blade. Care must be taken in hammering not to make the chisel wider in one place than in another.
Fig. 112. Fig. 113.
When finishing the chisel, it is hammered lightly until the red is nearly but not quite gone. This hammering packs the grain and makes it fine. The end of the chisel is set on a hardie and cut half thru, so that when it is hardened and tempered it may be broken to note its grain and also require less grinding in sharpening. See [Figure No. 113]. The chisel is now heated very slowly to a dark red and set in a dry place on the forge to anneal. This annealing relieves the strain in the tool due to hammering.
When the chisel is cold it is reheated to harden and temper. Over-heating does not make the tool harder when cooled in water, but increases its brittleness, so care must be taken when heating. The heating must be very slow, and to a dark red, 2½ inches long. The chisel should be cooled as the heat is going up. A common practice of heating the steel more than a cherry red and holding it out of the forge until the heat goes down, before dipping, is wrong. When properly heated the chisel is held in a vertical position and dipped about 1½ inches into 16 gallons of salt and water, heated from 60° to 70° F. See [Figure 114]. The tool is kept in motion when dipped. When cooled it is removed, and the hardened part is rubbed bright with an emery stick or sand paper. This is done so that the temper colors may be seen. Tempering increases the tool’s elasticity and strength, and reduces the brittleness. The temper color will show just a faint yellow against the edge of the remaining heat that was left in the tool after hardening.
Fig. 114.
In hardening the tool, it is heated 2½ inches of its length and 1½ inches is cooled in water to harden. The remaining heat gradually runs thruout the whole chisel and may be noted by the faint yellow color on the bright part of the tool traveling towards the cutting end. This faint yellow temper color, due to the heat and air, is followed with darker colors; if let run too much all of the hardness would be taken out of the tool. Four hundred and thirty degrees Fahrenheit would be about a light straw color, leaving the steel very hard. About 600° F. would be the darkest color, nearly black. This is as hot as steel can be made and still leave a trace of hardness. This temper is too soft for a chisel but about right for springs; therefore when the very dark purple temper color covers the whole bright part of the chisel the point is dipped in water. The chisel is then set in a dry place on the forge to cool slowly. The temper color must run to the end of the chisel very slowly. The reason for this is that if the temper color comes slow, the chisel is tempered farther back from the point. The temper colors on the surface of the bright steel are obtained by different degrees of heat, as it travels from the remaining heat left in the tool when the piece was hardened. The less heat allowed to travel toward the end of chisel, the paler the temper color and the harder the chisel; therefore, the faint yellow color indicates that the steel is very hard. The darker the temper color becomes the softer the tool.
The best chisels are those that are file proof. If, after hardening and tempering a chisel, it cannot be cut with a file, it is too hard and the temper must be run out more. If the grain of steel is very fine when broken the chisel had the proper heat when quenched, but if it looks coarse the tool was too hot when cooled and must be annealed, rehardened and tempered. A little judgment will enable one to determine the proper hardness for all tools of this character by noting these temper colors. The above explanation in a general way applies to the working of all carbon steel tools.
Spring Tempering.
There are many kinds of springs that are hardened and tempered. The methods of handling are about the same with all. As an example, a piece of spring steel 5 by 1 by ¹⁄₁₆ inches is to be tempered. In doing this, the piece is caught at one end with a pair of light tongs. The steel is heated to a dark red and dipped into a can of sperm oil, or equal parts of lard and tallow. When cool it is held over the fire until the surplus oil takes fire and blazes off. It is redipped in the oil, and the oil is burned three times in all. It is then partly cooled in the oil and set on the forge until cool, when it is ready for use. Steel is manufactured especially for springs. It is called spring steel. It is made in a different way from tool steel, by the open hearth process. It differs in quality and cannot be absolutely guaranteed. The steel is never free from all foreign elements which might be detrimental to its quality.
Tempering Thin Pieces of Steel.
In hardening thin pieces of steel such as knives, very thin milling cutters, etc., there is always difficulty in preventing warping after hardening. Two heavy surface plates, planed on one side, are used. On one of these plates equal parts of tallow and lard are spread ¼ inch thick. The knife is heated in a steam pipe with one end plugged and having fire under and over it. When an even red heat is reached, the knife is brought out and set on the oil and at the same time the top plate is set onto the knife until cool. This hardens the blade and keeps it from springing. The knife is brightened and the temper is drawn to a dark straw color by holding it on a hot iron.
Very small pieces of steel are packed into an iron pipe or box surrounded with charcoal. The whole is then heated red and the pieces are dumped out and cooled in water. To draw temper, they are put in an iron ladle filled with lard oil that is heated on the fire.
Welding Steel.
All small pieces of tool and spring steel should be welded with separate heats. A little practice and a clean fire, with some good welding compound, are necessary. In separate heat welding of flat steel, the flat sides of the scarfs are put together instead of the beveled ones. The scarfs are shown in Figure No. 115. The method of riveting and splitting small pieces of flat steel to hold them together while taking the heat is not to be recommended because after they are put together in this manner the lap is double thick, and in raising the heat there is always danger of over-heating each side of the lap. Separate heats and a clean fire is the best method to use to make a good weld, unless the steel is heavy. In this case, it is split and forked as previously explained.
Fig. 115. Welding Thin Steel.
Case Hardening.
The difference between wrought iron and tool steel lies in the absence of carbon in the iron. Tool steel can be hardened because it contains carbon, and when heated and suddenly cooled becomes hard thruout. The surface of wrought iron or mild steel can be carbonized and then made very hard. This is called case hardening because about ¹⁄₁₆ inch or less of the outside of the bar is made hard while the center is soft. There are several methods. One is to place the articles in a tight cast iron box and surrounded with ground bone before placing in a furnace. The box is then brought to a high heat of about 1700 degrees Fahrenheit. It is held at this heat for several hours and then let cool. When cool, the pieces are reheated and dipped in salt water to harden them or they may be cooled with the first heating. By another method the pieces are placed in an iron ladle with cyanide of potassium and heated. Iron may be heated red and rolled in the cyanide, then reheated and plunged into water. Care must be taken in handling cyanide as even the fumes are poisonous.
Coloring Steel.
Very bright pieces of soft steel can be case hardened and colored at the same time. In doing this, cyanide is heated in an iron box, and the steel articles are put into it. When heated they are removed and dipped into a solution of water and salt peter to cool and harden them. This gives them a mottled effect with many colors. A pint of salt peter to about four gallons of water makes a solution strong enough. This bath becomes poisoned from the cyanide. It should be kept clean and labeled “Poison.”
Annealing.
A piece of metal of any kind is said to be “annealed” when made very soft. Steel should be annealed before it is filed, drilled, or machined, as it is a very hard metal to work when cold. The method of annealing is first to heat the piece to a red heat. It is then covered with warm, slacked lime so that the air will not come in contact with it until cool. A simple way to anneal, when in a hurry, is to heat the steel red, setting it in a dry place on the forge until black. It is then plunged into water quickly and brought out. This operation is repeated until the piece is cool. Steel is also annealed by heating the piece red and setting it on the forge until cool. The slower steel is cooled, the softer it becomes. Wrought iron and mild steel forgings should always be annealed when used in work where there is danger of breaking them.
Fig. 116 (above). Fig. 117 (below).
Fig. 118. Scratch Awl.
Exercise No. 18.—Scratch-Awl.
This tool is used to scratch holes on the surface of metal, and also to lay out shapes on metal. [Figure 116] shows the dimensions of stock. The piece should be carbon steel. One and one-half inches from one end, the bar is drawn out until it measures 2¼ inches in length, as shown in [Figure 117]. It is then bent on an angle as shown in [Figure 118]. This part is now heated and hammered over the horn of the anvil to form the eye or ring. It is then twisted by catching one end in the vise and twisting to the right. The point is next drawn out as shown in [Figure 119]. The point is then ground or filed and the awl tempered hard.
Fig. 119. Scratch Awl Complete.
Exercise No. 19—Center-Punch.
[Figure 120] shows the size of stock and [Figure 121] shows the center-punch completed. The top part is first made, then the bottom is drawn out to a taper. In doing this, it is first drawn square, then eight sided and finally rounded. The point is ground and the punch is tempered to a purple color. For heavy centering a larger size steel should be used.
Fig. 120 (above). Center Punch. Fig. 121 (below).
Exercise No. 20—Hand-Punch.
Hand-punches are made of various sizes of stock, ⅝ in., ¾ in. and ⅞ in., and are used for hot punching. [Figure 122] shows the size of stock for a punch that will be useful in the school shop, and [Figure 123] shows the completed punch. It is made in the same manner as described for the center-punch. This punch must not be tempered. For punching square holes the punch is drawn square, and the ends of all hand-punches are made smaller than the hole to be punched.
Fig. 122. Stock for Punch. Fig. 123. Completed Punch.
High speed steels, due to their hardness and durability, retain their edge when cutting at extremely high speeds.
It has only been of recent years that high speed steels came into use. Before this time self-hardening steels were made by Jessop and Mushet which were in general use. They were tempered by heating to a dark red and left to cool in the air. The high speed steels of today are heated to 2,000° or 2,200° Fahr., or a white heat bordering on a welding heat.
The chemical composition of these new steels are only known by their makers. However, it is said that they contain carbon, tungsten, chromium, manganese and other elements.
The great advantage in using high speed steel, is that a machine can be run three times as fast as one using carbon steel, without destroying the edge of the tool. The output is therefore greater. Of course, in order to force this steel to do a great amount of work the machine tools should be constructed to stand heavy strains. All kinds of tools are now being made from high speed steel.
For light lathe work, high speed steel is used in the adjustable tool holder. The most common tool for doing heavy work is the round nose which is made from various size steel.
High speed tool steel is sold under many brands. The method of handling is about the same for all. However each manufacturer will give the method which is best for his particular make of steel. In forging high speed lathe tools, a furnace or clean fire with plenty of coke is used. The steel is heated to a bright red heat, holding the steel at this heat as nearly as possible when hammering. Forging at a low heat is liable to cause the steel to burst. When the tool is forged, it is laid in a dry place on the forge to cool. When hardening, the point of the tool is brought to a white welding heat, about 2,100° Fahr., and this is noticeable by the appearance of melted borax, forming on the nose. The tool is now held in a compressed air blast, or dipped into sperm, linseed or lard oil until cool.
Annealing High Speed Steel.
The process is the same as the one used for carbon steel, heating to a red heat and covering the piece with slacked lime until cold.
In cutting high speed tool steel, the bar may be nicked with the emery wheel, then broken.
In working tool steel or iron of any weight the blows of the hammer must be heavy. Light blows stretch the outer part of the metal and not the center. This is liable to fracture it. The blow must be heavy so as to penetrate thru the bar. A trip hammer of ordinary size run by a belt is a very economical tool for the school shop. It is inexpensive and can be used to advantage in drawing out large pieces of stock, especially tool steel.
Every pupil should have more or less practice in the handling of a trip or steam hammer.
