DON'T start on cast iron with a razor edge on the tool. Take an oil stone and wipe three or four times over the razor edge.

DON'T use tool holder steel from bars without hardening the nose of each individual tool bit.

AIR-HARDENING STEELS.--These steels are recommended for boring, turning and planing where the cost of high-speed seems excessive.

They are also recommended for hard wood knives, for roughing and finishing bronze and bra.s.s, and for hot bolt forging dies. This steel cannot be cut or punched cold but can be shaped and ground on abrasive wheels of various kinds.

It should be heated slowly and evenly for forging and kept as evenly heated at a bright red as possible. It should not be forged after it cools to a dark red.

After the tool is made, heat it again to a bright red and lay it down to cool in a dry place or it can be cooled in a cold, dry air blast. Water must be kept away from it while it is hot.

CHAPTER XI

FURNACES

There are so many standard furnaces now on the market that it is not necessary to go into details of their design and construction and only a few will be ill.u.s.trated. Oil, gas and coal or c.o.ke are most common but there is a steady growth of the use of electric furnaces.

[Ill.u.s.tration: FIG. 92.--Standard lead pot furnace.]

TYPICAL OIL-FIRED FURNACES.--Several types of standard oil-fired furnaces are shown herewith. Figure 92 is a lead pot furnace, Fig.

93 is a vertical furnace with a center column. This column reduces the cubical contents to be heated and also supports the cover.

[Ill.u.s.tration: FIG. 93.--Furnace with center column.]

A small tool furnace is shown in Fig. 94, which gives the construction and heat circulation. A larger furnace for high-speed steel is given in Fig. 95. The steel is supported above the heat, the lower flame pa.s.sing beneath the support.

For hardening broaches and long reamers and taps, the furnace shown in Fig. 96 is used. Twelve jets are used, these coming in radially to produce a whirling motion.

[Ill.u.s.tration: FIG. 94.--Furnace for cutting tools.]

[Ill.u.s.tration: FIG. 95.--High-speed steel furnace.]

Oil and gas furnaces may be divided into three types: the open heating chamber in which combustion takes place in the chamber and directly over the stock; the semim.u.f.fle heating chamber in which combustion takes place beneath the floor of the chamber from which the hot gases pa.s.s into the chamber through suitable openings; and the m.u.f.fle heating chamber in which the heat entirely surrounds the chamber but does not enter it. The open furnace is used for forging, tool dressing and welding. The m.u.f.fle furnace is used for hardening dies, taps, cutters and similar tools of either carbon or high-speed steel. The m.u.f.fle furnace is for spring hardening, enameling, a.s.saying and work where the gases of combustion may have an injurious effect on the material.

[Ill.u.s.tration: FIG. 96.--Furnace for hardening broaches.]

[Ill.u.s.tration: FIG. 97.--Forging and welding furnace.]

[Ill.u.s.tration: FIG. 98.--Semi-m.u.f.fle furnace.]

[Ill.u.s.tration: FIG. 99.--m.u.f.fle furnace.]

Furnaces of these types of oil-burning furnaces are shown in Figs.

97, 98, and 99; these being made by the Gilbert & Barker Manufacturing Company. The first has an air curtain formed by jets from the large pipe just below the opening, to protect the operator from heat.

[Ill.u.s.tration: FIG. 100.--Gas fired furnace.]

[Ill.u.s.tration: FIG. 101.--Car door type of annealing furnace.]

Oil furnaces are also made for both high- and low-pressure air, each having its advocates. The same people also make gas-fired furnaces.

Several types of furnaces for various purposes are ill.u.s.trated in Fig. 100 and 101. The first is a gas-fired hardening furnace of the surface-combustion type.

A large gas-fired annealing furnace of the Maxon system is shown in Fig. 101. This is large enough for a flat car to be run into as can be seen. It shows the arrangement of the burners, the track for the car and the way in which it fits into the furnace. These are from the designs of the Industrial Furnace Corporation.

Before deciding upon the use of gas or oil, all sides of the problem should be considered. Gas is perhaps the nearest ideal but is as a rule more expensive. The tables compiled by the Gilbert & Barker Manufacturing Company and shown herewith, may help in deciding the question.

TABLE 27.--SHOWING COMPARISON OF OIL FUEL WITH VARIOUS GASEOUS FUELS Heat units per thousand cubic feet Natural gas 1,000,000 Air gas (gas machine) 20 cp 815,500 Public illuminating gas, average 650,000 Water gas (from bituminous coal) 377,000 Water and producer gas, mixed 175,000 Producer gas 150,000

Since a gallon of fuel oil (7 lb.) contains 133,000 heat units, the following comparisons may evidently be made. At 5 cts. a gallon, the equivalent heat units in oil would equal:

Per thousand cubic feet Natural gas at $0.375 Air gas, 20 cp at 0.307 Public illuminating gas, average at 0.244 Water gas (from bituminous coal) at 0.142 Water and producer gas, mixed at 0.065 Producer gas at 0.057

Comparing oil and coal is not always simple as it depends on the work to be done and the construction of the furnaces. The variation rises from 75 to 200 gal. of oil to a ton of coal. For forging and similar work it is probably safe to consider 100 gal. of oil as equivalent to a ton of coal.

Then there is the saving of labor in handling both coal and ashes, the waiting for fires to come up, the banking of fires and the dirt and nuisance generally. The continuous operation possible with oil adds to the output.

When comparing oil and gas it is generally considered that 4-1/2 gal. of fuel oil will give heat equivalent to 1,000 cu. ft. of coal gas.

The pressure of oil and air used varies with the system installed.

The low-pressure system maintains a pressure of about 8 oz. on the oil and draws in free air for combustion. Others use a pressure of several pounds, while gas burners use an average of perhaps 1-1/2 lb. of air to give best results.

The weights and volumes of solid fuels are: Anthracite coal, 55 to 65 lb. per cubic foot or 34 to 41 cubic feet per ton; bituminous coal, 50 to 55 lb. per cubic foot or 41 to 45 cubic feet per ton; c.o.ke, 28 lb. per cubic foot or 80 cubic feet per ton--the ton being calculated as 2,240 lb. in each case.

A novel carburizing furnace that is being used by a number of people, is built after the plan of a fireless cooker. The walls of the furnace are extra heavy, and the ports and flues are so arranged that when the load in the furnace and the furnace is thoroughly heated, the burners are shut off and all openings are tightly sealed.

The carburization then goes on for several hours before the furnace is cooled below the effective carburizing range, securing an ideal diffusion of carbon between the case and the core of the steel being carburized. This is particularly adaptable where simple steel is used.

PROTECTIVE SCREENS FOR FURNACES

Workmen needlessly exposed to the flames, heat and glare from furnaces where high temperatures are maintained suffer in health as well as in bodily discomfort. This shows several types of shields designed for the maximum protection of the furnace worker.

Bad conditions are not necessary; in almost every case means of relief can be found by one earnestly seeking them. The larger forge shops have adopted flame shields for the majority of their furnaces. Years ago the industrial furnaces (particularly of the oil-burning variety) were without shields, but the later models are all shield-equipped.

These shields are adapted to all of the more modern, heat-treating furnaces, as well as to those furnaces in use for working forges; and attention should be paid to their use on the former type since the heat-treating furnaces are constantly becoming more numerous as manufacturers find need of them in the many phases of munitions making or similar work.

The heat that the worker about these furnaces must face may be divided in general into two cla.s.ses: there is first that heat due to the flame and hot gases that the blast in the furnaces forces out onto a man's body and face. In the majority of furnaces this is by far the most discomforting, and care must be taken to fend it and turn it behind a suitable shield. The second cla.s.s is the radiant heat, discharged as light from the glowing interior of the furnace. This is the lesser of the two evils so far as general forging furnaces are concerned, but it becomes the predominating feature in furnaces of large door area such as in the usual case-hardening furnaces. Here the amount of heat discharged is often almost unbearable even for a moment. This heat can be taken care of by interposing suitable, opaque shields that will temporarily absorb it without being destroyed by it, or becoming incandescent.

Should such shields be so constructed as to close off all of the heat, it might be impossible to work around the furnace for the removal of its contents, but they can be made movable, and in such a manner as to shield the major portion of the worker's body.

First taking up the question of flame shields, the ill.u.s.tration, Fig. 102, is a typical installation that shows the main features for application to a forging machine or drop-hammer, oil-burning furnace, or for an arched-over, coal furnace where the flame blows out the front. This shield consists of a frame covered with sheet metal and held by brackets about 6 in. in front of the furnace.

It will be noted that slotted holes make this frame adjustable for height, and it should be lowered as far as possible when in use, so that the work may just pa.s.s under it and into the furnace openings.

Immediately below the furnace openings, and close to the furnace frame will be noted a blast pipe carrying air from the forge-shop fan. This has a row of small holes drilled in its upper side for the entire length, and these direct a curtain of cold air vertically across the furnace openings, forcing all of the flame, or a greater portion of it, to rise behind the shield. Since the shield extends above the furnace top there is no escape for this flame until it has pa.s.sed high enough to be of no further discomfort to the workman.

In this case fan-blast air is used for cooling, and this is cheaper and more satisfactory because a great volume may be used. However, where high-pressure air is used for atomizing the oil at the burner, and nothing else is available, this may be employed--though naturally a comparatively small pipe will be needed, in which minute holes are drilled, else the volume of air used will be too great for the compressor economically to supply. Steam may also be employed for like service.

[Ill.u.s.tration: FIGS. 102 to 108.--Protective devices for furnace fronts.]