In cases, however, where the tool point requires to cut up to a sharp corner, the form of tool shown in [Fig. 1115] (which represents a top and end view) may be used. Its end face c is at an obtuse angle to the length of the tool, so that on passing up a bore and meeting a radial face the point only will meet that face. This angle, however, gives to the tool a keenness that will cause chattering on brass work unless the top face be bevelled to the tool body, as is a to b in the figure.
Fig. 1116.
It frequently happens in boring cast iron that the skin or the surface of the metal is very hard, rapidly dulling the tool and forcing it away from its cut, unless the cut is deep enough to allow the point of the tool to cut beneath it, as shown in [Fig. 1116], in which the hardness is supposed to extend from the bore to the dotted line.
In this case a tool formed as at c is employed, the point cutting in advance of the rest of the tool, and entering the soft metal beneath the hard metal; the hard metal will then break away in lumps or pieces, without requiring to be absolutely cut into chips or turnings, because of being undercut, as shown at b.
Fig. 1117.
The cross slider or tool rest of a lathe should be adjusted to closely fit the cross slide of the lathe if true and parallel work is to be bored, because any lost motion that may exist in the slide is multiplied by the length the tool stands out from the tool post. Thus the centre of motion of the rest if it has play, as at b, [Fig. 1117], and the direction of motion at the tool point, will be an arc of a circle of which b is the centre, the bend of the tool from the pressure of the cut will have its point of least motion or fulcrum at a; hence, both tend to cause the tool point to dip and spring unequally under the varying cut pressure that may arise from hard or soft places in the metal, and from inequalities in the cut depth.