The simplest form of metal cutting tool is the chisel, called a cold chisel. The mechanical principle of the cutting edge of the chisel is that of the wedge. Chisels for machine work differ from wood chisels in several ways, the principle difference being that cold chisels have no handles. There are many kinds of chisels in common use in the metal trades, some small and some large, but all are generally made of 3/4 in, octagonal (8-sided)
(a) Ball- Peen Hammer.
(6) Straight- Peen Hammer.
(c) Cross-Peen Hammer.
Fig. 123. - Machinists' Hammers.
tool steel 8 in. long. After the chisel is forged (hammered in a hot condition) to the required shape, the end is hardened by drawing the temper (heating) to a purple color. There are three elements to be considered in making a good chisel, namely, shape, temper, and cutting edge. Chisels must be forged and tempered at a low heat, as a high heat will burn the steel (burn the carbon out of the steel).
(a) Flat Chisel (Side View).
(b) Flat Chisel (Front View).
(c) Flat Chisel with Curved Edge.
(d) Cape Chisel (Side View).
(e) Cape Chisel (Front View).
(f) Gouge Chisel.
(g) Diamond-Point Chisel.
(h) Round-Nose Chisel.
(i) Side Chisel (Front View).
(j) Side Chisel (Side View).
Fig. 124. - Chisels.
Chisels made for use in the pneumatic hammer are longer than hand-driven chisels. The shanks are fitted to the holder or socket in the hammer and the chisel head should be tempered to keep it from upsetting. Ordinary chisels should never be used in the pneumatic hammer. The flat chisel (Fig. 124, a and b) is the form most commonly used. The cutting edge is generally drawn out about 1/8 in. wider than the stock from which it is made and then ground to an angle of 60°. For cutting soft metal the angles should be less; 30° for lead or Babbitt metal (a soft mixture of metals), and 45° for brass and soft cast iron, may be used. For very fine chipping, the cutting edge may be curved slightly, as shown by Fig. 124c. A small cutting angle used for cutting steel would soon break, while a blunt or large angle would not cut Babbitt metal but would simply tear it off. The flat chisel is used for all-around chipping, snagging castings, etc.
Figure 124, d and e, shows another common form of chisel called a cape chisel. It is made of the same steel and tempered in the same way as the flat chisel, but the point is drawn down to a width of about 3/8 in. The cape chisel is made wider on the cutting edge at A than it is at B to provide a clearance, and keep the sides of the chisel from breaking out the edges of the groove or channel which is being cut. The cutting edge is ground to the same angle as on the flat chisel.
There are four other forms of chisels used, but they are not so common as the flat and cape chisels. These are the gouge (Fig. 124f), the diamond-point (Fig. 124g), the round-nose (Fig. 124h), and the side chisel (Fig. 124, i and j). They are made of the same stock as the other chisels and tempered in the same way. The diamond-point and round-nose, like the cape chisel, should be made wider at the cutting edge than farther back, for clearance. The round-nose is very much like the cape chisel except that the cutting end is rounded and the bevel is on one side only. The side chisel is ground with only one bevel, like a wood chisel, but with angles just the same as if it had two bevels. This chisel should also be ground thinner or "backed off" near the point for clearance.
The gouge is used for work on round corners and on all concave surfaces. The diamond-point is used for cutting V-shape grooves and finishing out square corners; it is also used for drawing drilled holes and for cutting round coiners and oil grooves.
There are several other forms of chisels used especially by boiler-makers. Figure 125 shows four handle chisels, so called because they are held by the wooden handle when used. A hot chisel (Fig. 125a) is used for trimming or cutting hot plates, etc. The cold handle chisel, used for general chipping on boiler work as well as erecting wort, is very similar, except that the cutting edge is not drawn so thin as that of the hot chisel. Figure 125c shows a round punch used for knocking off rivet heads and driving out stay-bolts, rivets, etc. Figure 125b shows a square punch used for driving keys and knocking off rivet heads. Figure 125d shows a side set which is also used for cutting off rivet heads. All these tools are made of tool steel. The hot chisel is tempered to a dark straw color and the cold chisel to a blue color; the set is also tempered slightly. It is customary not to temper or harden punches since they would be apt to break off. The handles in all these tools should fit loosely and should be made of soft yielding wood so that the shock or jar of a glancing blow will not hurt the hands.