Direction of Top Slope for Turning Tools.—Aside from the question of the shape of the cutting edge as viewed from the top, there remains to be determined the amount of clearance that the tool shall have, and also the slope (and its direction) of the top of the tool. By the top is meant that surface against which the chip bears while it is being severed. It may be stated, in a general way, that the direction in which the top of the tool should slope should be away from what is to be the working part of the cutting edge. For example, the working edge of a roughing tool A ([Fig. 11]), which is used for heavy cuts, would be, practically speaking, between points a and b, or, in other words, most of the work would be done by this part of the cutting edge; therefore the top should slope back from this part of the edge. Obviously, a tool ground in this way will have both a back and a side slope.
When most of the work is done on the point or nose of the tool, as, for example, with the lathe finishing tool C which takes light cuts, the slope should be straight back from the point or cutting edge a—b. As the side tool shown in [Fig. 10] does its cutting along the edge a—b, the top is given a slope back from this edge as shown in the end view. This point should be remembered, for when the top slopes in the right direction, less power is required for cutting. Tools for certain classes of work, such as thread tools, or those for turning brass or chilled iron, are ground flat on top, that is, without back or side slope.
Clearance for the Cutting Edge.—In order that the cutting edge may work without interference, it must have clearance; that is, the flank f (Fig. 10) must be ground to a certain angle α so that it will not rub against the work and prevent the cutting edge from entering the metal. This clearance should be just enough to permit the tool to cut freely. A clearance angle of eight or ten degrees is about right for lathe turning tools.
Fig. 12. Illustrations showing how Effective Angles of Slope
and Clearance change as Tool is raised or lowered
The back slope of a tool is measured from a line A—B which is parallel to the shank, and the clearance angle, from a line A—C at right angles to line A—B. These lines do not, however, always occupy this position with relation to the tool shank when the tool is in use. As shown to the left in [Fig. 12], the base line A—B for a turning tool in use intersects with the point of the tool and center of the work, while the line A—C remains at right angles to the first. It will be seen, then, that by raising the tool, as shown to the right, the effective clearance angle α will be diminished, whereas lowering it, as shown by the dotted lines, will have the opposite effect.
A turning tool for brass or other soft metal, particularly where considerable hand manipulation is required, could advantageously have a clearance of twelve or fourteen degrees, as it would then be easier to feed the tool into the metal; but, generally speaking, the clearance for turning tools should be just enough to permit them to cut freely. Excessive clearance weakens the cutting edge and may cause it to crumble under the pressure of the cut.