Fig. 933.
In these considerations we have referred to the angle of the top face only, but if we consider the angle of the two faces one to the other we shall see that they form a wedge, and that all cutting tools are simply wedges which enter the material the more easily in proportion as the angles are more acute, providing always that they are presented to the work in the most desirable position, as was explained with reference to [Fig. 920].
Fig. 934.
We may now consider the degree of a bottom rake or clearance desirable for a tool, and this it can be shown depends entirely upon the conditions of work, diameter, and rate of tool traverse, and cannot, therefore, be made a constant degree of angle. This is shown in [Fig. 934], in which a tool t is represented in three positions, marked respectively 1, 2, and 3. Line a a is at a right angle to the axis of the work w, and the side of the tool is given in each case 5° of angle from this line a a. In position 1 the tool has 3° of clearance from the side of the cut; in position 2 it has 2° clearance, but in position 3 it would require to have 2° more clearance given to it to enable the cutting-edge to meet the side of the cut, without even then having the clearance necessary to enable it to cut. This occurs because the side of the cut is not at a right angle to the work axis, but at an angle the degree of which depends upon the rate of feed.
