There are many methods of cutting metals and necessarily many cutting tools, and a general knowledge of the main principles of the methods is of importance to any one who has to manipulate metals.

Only the elementary cutting tools which can be sharpened without expensive equipment will be dealt with here.

The most efficient cutting angle depends not only on the material of the tool itself but also on the material and condition of the metal to be cut.

To minimize the heat generated by the action of cutting, oil or some other lubricant must be used.

Cutting operations may be classified as follows: Chiselling, sawing, filing, shearing, punching, drilling, turning (including cutting with a diamond), screwing, milling, fusing.

Some of these processes are performed when the metal is cold, some must be done when the metal is hot, whilst for others the metal may be either hot or cold.

The action of a metal cutting tool is generally considered a shearing action. Shearing takes place when sufficient force is applied to tear away the metal from the main body so that the action may be taken as follows:-

Cutting with a metal cutting tool consists of pressing, tearing, and shearing the metal away with the one side or face only of the tool under pressure, as the opposite side is generally clear of the work, as in the action of a turning tool. This is illustrated in Fig. 1, No. 1. With a pair of shears both sides or faces of the tool are under pressure; this is shown in Fig. 1, No. 2.

In many instances a moving force is applied to the cutting edge of the tool and the substance is severed particle by particle. The tenacity of the material opposes the entry of the tool, but the first motion given to the tool, which severs a portion or a shaving which, directly it is away from the main body, is curled up, at the same time still presses down on the slope of the tool so keeping it up to the work. When the tool is too keen, as may be the case with a badly ground turning tool or twist drill, the cutting edge gets drawn into the metal by the same action and the point or edge gets sheared off.

In the use of a "hot sett"orchiselthe action is rather that of wedging than that of shearing.

Fig. i.-Examples of cutting tools, their actions and cutting angles.

Chisels

Chipping, the removal of metal by means of a chisel and hammer, is becoming obsolete, for machinery does the work better and quicker, and in large works pneumatic tools often take the place of the chisel and hammer. In small shops, however, the chisel is still a very necessary tool, and the one illustrated in Ch. VI, f. 1, is very useful for general purposes; but when used for chipping only, the cutting edge instead of being straight across (as drawn) should be slightly curved, so that in actual use the corners will not break off or be drawn into the metal. For cutting soft metals such as brass, copper, etc., a more acute cutting angle could be used with advantage, say 50° instead of 6o°, and it could be left a little harder by cooling out when temper colour is a light brown.