The simplest form of metal-cutting tool is the chisel. The several types in common use are shown in Fig. 46.
The flat chisel is used for snagging castings, for chipping surfaces having less width than the edge of the chisel, and for all general chipping operations. It is the form most commonly used, and is often called the cold chisel. Generally it has a cutting edge about an eighth of an inch wider than the stock from which it is forged.
The cape chisel is used for cutting keyways, channels, etc., and also for breaking up surfaces too wide to chip with the flat chisel alone. Channels are driven across such a surface, leaving raised portions or "lands" to be removed by the flat chisel. The cutting edge of this chisel is usually an eighth of an inch narrower than the shank, and the part just in the rear of the cutting edge is made thin enough to avoid binding in the slot. As this weakens the chisel, it is made comparatively thick in the plane at right angles to the cutting edge.
The diamond point chisel is made by drawing out the end of the stock to about 5/16 inch square, and grinding the end at an angle with the axis of the chisel, leaving a diamond-shaped point. It is used for drawing holes, making oil grooves, and cutting holes in flat plates.
The small round-nosed chisel is cylindrical in section near the cutting end, the edge being ground at an angle of 60 degrees with the axis of the chisel. When used to "draw" the starting of drilled holes to bring them concentric with the drilling circles, they are called center chisels. The round-nosed chisel is also used for cutting channels, such as oil grooves and similar work. The larger sizes of round-nosed chisels are of the general shape of the cape chisel with one edge rounded, making a convex cutting edge. Large round bottomed channels and all concave surfaces are the proper work of the round-nosed chisel.
Fig. 46. Hand Chisels.
All the accompanying forms should be made from a good grade of tool steel, carefully forged, hardened, and tempered to a purple color. The stock generally used is octagonal, and the chisels for heavy work are about 8 inches long and f inch in diameter.
The two bevels forming the cutting edge of a chisel should make with each other as small an angle as is possible without leaving the cutting edge weak. If the angle is too small, the chisel will soon become dull, while if large, more force will be required to drive it. The best angle for cutting cast iron, all things considered, is about 70 degrees, while for wrought iron and mild steel a slightly smaller angle, say 60 degrees, will be better.
When there are two bevels, they should be alike in width and form equal angles with the center line of the chisel. Small round-nosed chisels and some slotting chisels are ground one-sided, that is, with but one bevel like a wood chisel. The angle between the surfaces which form the cutting edge should be the same, whether these surfaces are both bevels, or one a bevel and the other the straight side of the chisel. In a one-sided chisel, therefore, the angle that the bevel forms with the center line of the chisel should be twice as large as in one having two bevels.
To cut well, chisels should be sharp and, therefore, should be ground at once when they become dull. This may be done on an emery or carborundum wheel, not finer than No. 60, care being taken to avoid heating, which draws the temper and spoils the tool.