Mallets. The difference in effect between a blow given by a hammer and one given by a mallet is so great that, although similar in many respects, the two tools are adapted to widely different uses. A blow from a hard, elastic hammer is sharp and decisive, and its force is absorbed almost as soon as it is received. Comparatively speaking, therefore, its effect must be local. If such a blow is received on a chisel handle, for example, a large part of its force is wasted in affecting the handle, a part only being transmitted through the handle to the cutting edge, the only place where it can be of use. A blow from a soft, less elastic mallet, on the contrary, is more general in its effect. Much of the force remains for an instant stored in the mallet, by which it is given out somewhat gradually, allowing time for the impulse to pass beyond the point where it is received. The effect of two different explosive agents will serve as an illustration. As compared with nitroglycerine, powder burns slowly, and, when put into a rifle barrel, gradually develops its force upon the bullet until, when the latter reaches the end of the barrel, it has gained velocity enough to carry it a mile or more. But if a charge of nitro-glycerine, having a total explosive force no greater than that of the powder, be substituted, the result will be very different. The rapidity with which nitro-glycerine burns - the suddenness of the impulse - is such that, before the bullet can respond to its influence, the breach of the barrel is destroyed.

Fig. 129

The blow of a mallet on a chisel resembles the action of powder on a bullet. It is a pushing action, and, in this respect, is unlike that of the hammer. A chisel, therefore, will be driven deeper into the work by a blow from a mallet than by one of the same force from a hammer, while a chisel handle which has withstood blows from a mallet for years, may be shattered in a single hour by use under a hammer.

An excellent form of mallet is shown by Fig. 129.

103. Sand-Paper is neither a tool nor an appliance, strictly speaking, but, on account of its tool-like action, it should be mentioned with them. The "sand" used in making sand-paper is crushed quartz, and is very hard, angular, and sharp. It is graded as to degree of coarseness, by precipitation, and then glued to paper. The finest sand-paper is marked 00, from which the gradations run o, 1/2, 1, 1 1/2, 2, 2 1/2, and 3, which is the coarsest.

104. Miter-Boxes are useful in cutting the ends of light strips of wood at an angle of 45 degrees; they are frequently adapted to cutting at other angles. When of wood, like the one represented by Fig. 219, they are usually made by the workman himself.

A wooden miter-box is composed of three pieces - a bottom and two sides. It is necessary that the bottom piece be uniform in width and thickness, and have jointed edges, and it is well to prepare the other pieces in the same way. After the box is nailed, the sides should be square with the outside face of the bottom piece; this surface may now be used as a working-face. Lay off across the working-face two lines at a distance apart equal to the width of the face, thus forming with the outside edges of the box, a square. The diagonals of this square will represent the two oblique cuts, one. marked c, and the one taken by the saw, Fig. 219. Project up the sides such lines from the points thus fixed, as will be useful in making the cuts; the sawing is then done with the back-saw. No special directions are required for laying off the cut d.

105. Iron Miter-Boxes are now in general use. The accuracy with which work may be done by the use of one will more than compensate any bench-worker for the money invested in it. Fig. 130 may be taken as a type; the work A is supported by the frame as shown, while the proper position of the saw is maintained by the uprights B, which, in the sawing process, slide down into the standards C. The saw may be set at any angle with the back of the box D, by swinging the frame E, which supports the standards C; E is held in position by a suitable fastening operated by F.

Fig. 130

106. Bench Clamps are useful in holding two or more pieces of material together temporarily. They are particularly valuable for keeping pieces that have been glued, in place until they are dry.

Wooden clamps, or hand-screws, are of the form shown by Fig. 131. The whole length of the jaws, AB and A'B', may be made to bear evenly upon the work, or to bear harder at certain points, as AA' or BB'.

Iron clamps are illustrated by Fig. 132, but the mechanical arrangement differs in different makes. Such clamps are very useful in many kinds of work, but, all things considered, it is doubtful whether they are as serviceable to the bench-worker as the wooden ones just described.

Fig. 131

Fig. 132

107. Grindstones are selected with reference to their "grit." A coarse, soft-grit stone will remove material much more rapidly than one of finer grit, but the surface produced will be very rough compared with that produced by the other. Thus, when it is necessary to remove material for the purpose of giving shape to a casting or forging, the coarse, soft-grit stone is better; but if a smooth cutting edge is required, one of fine grit should be used. For wood-working tools, a stone rather fine and soft is found best. The speed of a power grindstone must vary from 500 to 1000 circumferential feet a minute, depending upon its diameter, and the accuracy and steadiness with which it runs. It may not be well to run a 20" stone beyond the minimum limit, while one of 4' or 5' may give good results if run beyond the maximum. As a rule, a stone for tool grinding is at its maximum speed when, if run faster, it would throw water from its face.

By circumferential speed is meant the speed of the circumference of the stone. This is found by multiplying the diameter of the stone, in feet, by 3.1416 (ratio of diameter to circumference), which will give the circumference of the stone, in feet, and this product by the number of revolutions per minute.1

Example I

A 4' stone is run at 30 revolutions a minute; what is its circumferential speed?

The circumference of a 4' stone is 4' X 3.1416= 12.56'.

This would be the speed of the stone if it were to make but I revolution per minute; but, since it makes 30 revolutions, its speed is 12.56' X 30= 376.80' or 377' (nearly).

Example II

It is desired that a 30" stone should have a circumferential speed of 280' per minute. How many revolutions should it make?

30" = 2.5'. The circumference of a stone 2.5' in diameter is 2.5' x 3.1416 = 7.85'.

This would be the speed of the stone if it were to make 1 revolution per minute. But the circumferential speed is 280' per minute, and therefore the number of revolutions made must be 280' % 7.85 = 36 (nearly).

108. Water is used on a stone as a means of carrying off the heat resulting from friction between stone and tool; it also washes away the particles of stone and steel that come from the grinding, and which, without the water, would fill the interstices between the cutting points of the stone, and make the surface so smooth as to be useless.

A grindstone, when not in use, should not stand in or over water. 'Water softens a stone, and one unequally exposed to moisture will be found softest in such places as are most exposed. When brought into use, the softer parts wear away more rapidly than the others, causing the stone to become "out of round." Water is best supplied from a tank, or from service pipes, so arranged that it may be shut off when the stone is not running, the drip-pan under the stone being at all times perfectly drained. After every precaution has been taken, the stone will in time become untrue and need attention.