Robert Gibson Griswold
Small tools, dies, etc., can hardly be satisfactorily handled in the flame of a blast lamp or in an open fire, as great risks are run which may result in the burning of the piece or, if hardening be the process in hand, insufficient or irregular heating may be the result. Furthermore the direct heat of a blast lamp is so intense that the thin portions of a tool such as a die may become overheated long before the heat has penetrated to the interior of the heavier portions. Such uneven heating often leads to very unsatisfactory hardening and soft spots may result in the very portions of the work that should be hard.
Such conditions are more easily controlled in a furnace and a small combination type is described and illustrated herewith. As is plainly shown, it is divided into two compartments - one directly over the burner and the other separated from this by a vertical bridge-wall, over the top of which the heated gases pass into the longer chamber for the uniform heating of die-blocks, cutters, etc. The smaller chamber, situated directly over the burner is used for melting metals in a crucible, which may be introduced through a trap in the top. Being directly over the flame and surrounded by it, the crucible becomes intensely heated and such metals as lead, tin, zinc, copper and brass may be readily melted.
A small door in the side permits the introduction of small rods and soldering irons for heating. In the large chamber is a false floor upon which larger pieces, such as die-blocks, may be laid for heating. As the gases play over this floor, heating it to a red heat, any piece laid upon it will soon become evenly heated throughout, as the heat is allowed to "soak " into the work. This "soaking" is seldom given the time it requires, but much of the resulting success in the proper hardening of a piece depends on the length of time given for the heat to penetrate into the center of the work. In this type of furnace the heating of the work is done very gradually as the heated gases only play over the piece and the fierce blast cannot reach it.
The walls are made of firebrick, lined on the outside with 1/4 in. asbestos board. This firebrick is made by moulding a paste of 3 parts fire-clay and 2 parte Portland cement into wooden moulds of the shape and size required. Strips £ in. in height are tacked to a smooth plank so that the inside dimensions correspond with those scaled from the drawing. In order that the pieces may be firmly bound together, 1/4 in. rods, threaded at each end, are let into the cement while moist, the ends protruding on either side sufficiently to pass through the adjoining walls. Holes for these ends to pass through are also made at the time of moulding the sides, as are also the door openings. In fact, all work must be done while the material is plastic, as it is almost impossible to drill or cut it when hard.
Washers are placed beneath the nuts on the asbestos board, and the top cover may be wired on for greater security. The supports for the furnace are of 1 x 1 x1/8 in. angle iron with a toe bent outwards to receive a screw at the lower end. Two shelves of 1-16 or 1/8 in. iron are attached as shown. These form supports for the hot doors when the latter are removed and also as rests for soldering irons, or bars.
Surrounding the hole over the burner are four supporting lugs upon which a crucible may rest so that the flame will reach all sides. These lugs are moulded on as a part of the bottom. The other hole in the bottom of the larger chamber permits the escape of the hot gases.
The burner is of the blast lamp type and produces an intense heat. It is of cast iron and supported between the legs of the furnace. The details are sufficiently clear to render a lengthy description unnecessary. The gasoline is led around the upper part of the burner through a small hole drilled through the legs until they intersect at the corners. These holes are tapped and provided with screws where shown, so as to form a tight passage around the burner. A needle valve controls the admission of gas to the burner, and in the passing of the gas into the burner carries with it sufficient air for combustion. A cast iron plate, liberally perforated with 1-16 in. holes, is placed across the inside of the burner at the joint, and the upper one securely holds it in place. The ignition of the gas occurs at this plate and a solid blue flame passes up through the hole in the bottom of the furnace.
The gasoline must be fed to this burner under pressure, which may be secured by feeding the gasoline from an elevated tank or, better still, from an airtight reservoir fitted with a pump such as is used for inflating bicycle tires.
The burner must be thoroughly heated before it will operate and this is done by burning a small quantity of gasoline or alcohol in a cup beneath the burner. When heated, and while in operation, the gasoline in passing around the burner is vaporized and passes through the needle valve as a gas. The flame is very easily regulated by turning the needle valve handle one way or the other.