Clearance prevents the tool from rubbing on the work, while rake adds to the keenness of the cutting edge, and gives freedom to the removal of the chips. A tool should have sufficient strength at the cutting point to do the work required.
The tool should be set so that the cutting edge will coincide very nearly with a horizontal line passing through the axis of the work. Most machinists set the cutting edge a little above this horizontal line. When so set, the stress tends to force the tool down along the line of its greatest strength. The tool may, however, be set too high. If this is done, as in Fig. 115, the angle of clearance will disappear, and the curve of the work will rub against the bottom of the tool. This will tend to force the tool out; heating the tool and producing a rough surface on the metal being turned. If, on the other hand, the tool is set too low, as in Fig. 116, the cutting edge does not stand in line with the motion of the work at the point of contact. The result will be that the metal will be scraped rather than cut, as there is no rake; and the pressure upon the tool will be in the line of its least resistance, as indicated by the arrow. Such a position might cause the point of the tool to break off. It will also cause the tool to tremble or chatter as it removes the chips, leaving a rough and wavy surface on the metal.
Fig. 115. Tool Set too High.
Fig. 116. Tool Set too Low.
As stated above, most machinists prefer to set the cutting edge a little above the center. The amount the tool is set above the center is slight, and of course depends upon the character of the work, and upon the shape of the cutting tool. The angle ACB, Fig. 117, should be only about 5 or 6 degrees.