First: The heating temperature should be a dull red, which is less than the annealing heat.

Second: Instead of cooling slowly the article tempered is dipped into a liquid which suddenly chills it.

Third: The materials used vary, but if the article is plunged into an unguent made of mercury and bacon fat, it will impart a high degree of toughness and elasticity.

MATERIALS USED.--Various oils, fats and rosins are also used, and some acids in water are also valuable for this purpose. Care should be taken to have sufficient amount of liquid in the bath so as not to evaporate it or heat it up too much when it receives the heated body.

Different parts of certain articles require varying degrees of hardness, like the tangs of files. The cutting body of the file must be extremely hard, and rather brittle than tough. If the tang should be of the same hardness it would readily break.

_Gradual Tempering._--To prevent this, some substance like soap suds may be used to cool down the tang, so that toughness without hardness is imparted.

The tempering, or hardening, like the annealing process, may be repeated several times in succession, and at each successive heating the article is put at a higher temperature.

If any part of a body, as, for instance, a hammerhead, should require hardening, it may be plunged into the liquid for a short distance only, and this will harden the pole or peon while leaving the other part of the head soft, or annealed.

Glycerine is a good tempering substance, and to this may be added a small amount of sulphate of potash.

FLUXING.--The word _flux_ means to fuse or to melt, or to put into a liquid state. The office of a flux is to facilitate the fusion of metals. But fluxes do two things. They not only aid the conversion of the metal into a fluid state, but also serve as a means for facilitating the unity of several metals which make up the alloy, and aid in uniting the parts of metals to be joined in the welding of parts.

UNITING METALS.--Metals are united in three ways, where heat is used:

First: By heating two or more of them to such a high temperature that they melt and form a compound, or an alloy, as it is called.

Second: By heating up the points to be joined, and then lapping the pieces and hammering the parts. This is called forge work or welding.

Third: By not heating the adjacent parts and using an easily fusible metal, which is heated up and run between the two, by means of a soldering iron.

The foreign material used in the first is called a flux; in the second it is termed a welding compound; and in the third it is known as a soldering acid, or soldering fluid.

The boy is not so much interested in the first process, from the standpoint of actual work, but it is necessary that he should have some understanding of it.

It may be said, as to fluxes, generally, that they are intended to promote the fusion of the liquefying metals, and the elements used are the alkalis, such as borax, tartar, limestone, or fluor spar.

These substances act as reducing or oxidizing agents. The most important are carbonate of soda, potash, and cyanide of pota.s.sium. Limestone is used as the flux in iron-smelting.

WELDING COMPOUNDS.--Elsewhere formulas are given of the compounds most desirable to use. It is obvious that the application of these substances on the heated surfaces, is not only to facilitate the heating, but to prepare the articles in such a manner that they will more readily adhere to each other.

OXIDATION.--Oxidation is the thing to guard against in welding. The moment a piece of metal, heated to whiteness, is exposed, the air coats it with a film which is called an _oxide_. To remove this the welding compound is applied.

The next office of the substance thus applied, is to serve as a medium for keeping the welding parts in a liquid condition as long as possible, and thus facilitate the unity of the joined elements.

When the hammer beats the heated metals an additional increment of heat is imparted to the weld, due to the forcing together of the molecules of the iron, so that these two agencies, namely, the compound and the mechanical friction, act together to unite the particles of the metal.

SOLDERING.--Here another principle is involved, namely, the use of an intermediate material between two parts which are to be united. The surfaces to be brought together must be thoroughly cleaned, using such agents as will prevent the formation of oxides.

The parts to be united may be of the same, or of different materials, and it is in this particular that the workman must be able to make a choice of the solder most available, and whether hard or soft.

SOFT SOLDER.--A soft solder is usually employed where lead, tin, or alloys of lead, tin and bis.m.u.th are to be soldered. These solders are all fusible at a low temperature, and they do not, as a result, have great strength.

Bis.m.u.th is a metal which lowers the fusing point of any alloy of which it forms a part, while lead makes the solder less fusible.

HARD SOLDER.--These are so distinguished because they require a temperature above the low red to fuse them. The metals which are alloyed for this purpose are copper, silver, bra.s.s, zinc and tin. Various alloys are thus made which require a high temperature to flux properly, and these are the ones to use in joining steel to steel, the parts to be united requiring an intense furnace heat.

SPELTER.--The alloy used for this purpose is termed "spelter," and bra.s.s, zinc and tin are its usual components. The hard solders are used for uniting bra.s.s, bronze, copper, and iron.

Whether soft or hard solder is used, it is obvious that it must melt at a lower temperature than the parts which are to be joined together.

There is one peculiarity with respect to alloys: They melt at a lower temperature than either of the metals forming the alloys.

SOLDERING ACID.--Before beginning the work of soldering, the parts must be cleaned by filing or sandpapering, and coated with an acid which neutralizes the oxygen of the air.

This is usually muriatic acid, of which use, say, one quart and into this drop small pieces of zinc. This will effervesce during the time the acid is dissolving the zinc. When the boiling motion ceases, the liquid may be strained, or the dark pieces removed.

The next step is to dissolve two ounces of sal ammoniac in a third of a pint of water, and in another vessel dissolve an ounce of chloride of tin.

Then mix the three solutions, and this can be placed in a bottle, or earthen jar or vessel, and it will keep indefinitely.

THE SOLDERING IRON.--A large iron is always better than a small one, particularly for the reason that it will retain its heat better. This should always be kept tinned, which can be done by heating and plunging it into the soldering solution, and the solder will then adhere to the iron and cover the point, so that when the actual soldering takes place the solder will not creep away from the tool.

By a little care and attention to these details, the work of uniting metals will be a pleasure. It is so often the case, however, that the apparatus for doing this work is neglected in a shop; the acid is allowed to become dirty and full or foreign matter, and the different parts separated.

CHAPTER X

ON GEARING AND HOW ORDERED

The technical name for gears, the manner of measuring them, their pitch and like terms, are most confusing to the novice. As an aid to the understanding on this subject, the wheels are ill.u.s.trated, showing the application of these terms.

SPUR AND PINION.--When a gear is ordered a specification is necessary.

The manufacturer will know what you mean if you use the proper terms, and you should learn the distinctions between spur and pinion, and why a bevel differs from a miter gear.

If the gears on two parallel shafts mesh with each other, they both may be of the same diameter, or one may be larger than the other. In the latter case, the small one is the pinion, and the larger one the spur wheel.

Some manufacturers use the word "gear" for "pinion," so that, in ordering, they call them _gear_ and _pinion_, in speaking of the large and small wheels.

MEASURING A GEAR.--The first thing to specify would be the diameter. Now a spur gear, as well as a pinion, has three diameters; one measure across the outer extremities of the teeth; one measure across the wheel from the base of the teeth; and the distance across the wheel at a point midway between the base and end of the teeth.

These three measurements are called, respectively, "outside diameter,"

"inside diameter," and "pitch diameter." When the word _diameter_ is used, as applied to a gear wheel, it is always understood to mean the "pitch diameter."

[Ill.u.s.tration: _Fig. 121. Spur Gears_]

PITCH.--This term is the most difficult to understand. When two gears of equal size mesh together, the pitch line, or the _pitch circle_, as it is also called, is exactly midway between the centers of the two wheels.

Now the number of teeth in a gear is calculated on the pitch line, and this is called: