or of sheet steel. A proper way to make the box of sheet steel is with the aid of the oxy-acetylene welding process. The box may be of almost any size convenient to handle and heat. It must be large enough, since the packing material should be used generously around the articles. For a box 12 × 14 × 24 inches in size, a thickness of sheeting of 0.4 to 0.6 inch is proper. A box such as described should last through perhaps 12 or 15 occasions of its use. What disintegrates the boxes is not so much wear and tear as the action of the atmosphere on the highly heated metal. It may be well to state that it is advisable to make the boxes small rather than large. We must not forget the considerable amount of packing material. A small box will naturally get heated up more quickly than a big one and will be more likely to have the same temperature all the way through. The box should be made with a lid and the two should be so designed that the edges overlap.

In charging a box with the work, it is proper first to coat the inside bottom with a paste made by mixing clay and water. This coat should be allowed to dry thoroughly before going on. After the drying is complete, we put in a layer of the packing material. This layer should be, say, 11⁄2 inches thick. This packing material should be in the condition of a fine powder and should be very dry. The first layer of the articles is put in place. We are careful not to put one against another, but to allow at least 114 inches between them. We then pack in the carbonaceous material. Care should be taken to fill in crevices and cavities and other irregularities of the work. We then put on a layer over the work, making this layer, say, 1 inch thick, if more work is to be put in, and 11⁄2 inches thick, if no more work is to go in. In case the box is not full when we have put in 11⁄2 inches of packing material over the top layer of work, then we fill in packing material clear on up to the top. The cover

is now put on, fire-clay made pasty with water being used to seal the joints between lid and box.

We are now ready to put the box into the furnace. We put it near the door in order to give the moisture in the fire-clay paste, and any other moisture that may be in the packing material, time to evaporate. After the moisture has evaporated, the box is placed in the hottest part of the furnace.

Almost any kind of a furnace will answer, if it can be made hot enough and provided the temperature in the region where the box is can be managed so that there will be only very small differences at different points. This latter requirement is a very important one. A good gas furnace is, however, easily able to meet it. It is considered undesirable that in the working part of the furnace the temperature at one point should differ by more than 50°F. or 60°F. from that at any other point. If improperly heated, the work may, accordingly, come out of the box in various conditions. Or, if the box is used to hold one large article, then one part of the article may have received more carbon than another.

A proper temperature to use in case-hardening is 1740°F. This is a light orange color verging to a yellow. Case-hardening may be done at a lower temperature—as low, it seems, as a light red, under favorable circumstances. It should be remembered that it is not simply the box that has to acquire the temperature but the work itself; and that a sufficient time must be allowed to get the depth of impregnation desired.

One of the simplest materials is that made according to the following formula:

Wood-charcoal

Common salt

9 parts

1 part

In using this mixture, it may be necessary to go to high temperatures to get results in a reasonable time. Tem

peratures up to light yellow may be used. Another mixture, claimed in authoritative quarters as better, is the following:

With this mixture, a mild tool steel coating, very thin, is obtainable at an orange heat or higher; and a highcarbon tool steel coating, 32 inch or thicker, at high temperatures near light yellow. An advantage of this mixture is that it may have its activity restored after it has suffered from use, the simple means of restoration being exposure of the material in a thin layer to the influence of the air.

There are many substances used for packing material -for example, wood-charcoal, leather, bone, common salt, sodium carbonate, saltpetre, resin, sawdust, soot, etc. These are used in various combinations.

Case hardening to a very small depth may be accomplished by putting the cold, or moderately heated, article into a bath of potassium cyanide. The bath should be heated, say, to a bright cherry red prior to the immersion of the work. The article may be hung by a fine iron wire and allowed to remain until it acquires the temperature of the bath. This will, in some ordinary cases, require about a quarter of an hour or a little longer. It is necessary to point out that cyanide of potassium is a very deadly poison and that even the fumes from it are poisonous. The cast iron pot or other crucible holding it should be enclosed with a hood connecting with a chimney or ventilating shaft.

CHAPTER VI

THE USE OF ABRASIVES

Abrasive equipment for the model engineer's workshop—Grinding and polishing Grinding attachments for small grinding head—Bonds used in making abrasive wheels—How to choose a wheel for certain work—Precautions to be taken in mounting wheels.

There are many instances, in certain work, where the short-cut lies in grinding, and owing to the scarcity of published data on this very important subject many remain ignorant of the great utility of simple abrasive materials and equipment in their application to mechanics.

Every workshop should contain a small grinding and polishing head. The one shown in Fig. 55 is a very good machine for the work generally required in the small shop, as it can be used for grinding, polishing and buffing. The machine should be belted to a %-H.P. motor of sufficient speed. In the event the mechanic is unable to procure such a motor and has a small bench grinder, the arrangement shown in Fig. 60 may prove to be of interest to him. The clamping disc of the bench grinder is replaced by a wooden pulley with a groove large enough to accommodate a sewing machine belt. The polishing head is placed close enough to the bench grinder so that one may work conveniently at the former while driving it by means of the bench grinder. If a motor is used, a rheostat would make a very valuable addition to the outfit, as a variation of speed is desirable for different classes of work.

The grinding head should be provided with an assortment of wheels of various shapes, sizes and grits, as every wheel should be adapted to the particular kind of work

it is to be used for. A wheel 3 inches in diameter by 1/2 inch thick is a very good size for general work when used with the small polishing head shown in the picture. As such wheels can be purchased for about 40 cents each, it is advisable to have four or five on hand of various

grits; from very fine grit to coarse grit. Several round edge wheels of varying thickness should also be on hand, as there are many different jobs and operations where such wheels can be employed with great convenience, as in the cutting of grooves, etc. For very fine and accurate work, small wheels of fine grit should be employed. Such wheels are commonly known as jewelers' wheels, and owing to the difficulty of procuring them with an arbor large enough for use on a half-inch spindle, the little "kink" shown in Fig. 56 may be used. The wheel is clamped between two washers by means of a 10-24 machine screw and nut. The protruding end of the machine screw is then placed in the chuck of the polishing head. As these