Fig. 926.
In [Fig. 926] is a left-hand tool, its direction of feed traverse being from left to right; hence edge g is the cutting one, edge f being dulled by the side angle b.
It is obvious that various combinations of side rake and front rake may be given to produce the same degree of keenness to the tool. For example, a tool may have its keenness from side rake alone, or it may have the same degree of keenness by using less side rake and some front rake. The principles governing the selections of these combinations are as follows:—
Fig. 927.
Suppose that in addition to say 20 degrees of side rake a tool is given a certain amount of front rake as denoted in [Fig. 927] by e e, and suppose that the tool is moved in to its cut by the cross feed screw. During this motion and until the tool point meets the work surface the contact between the cross feed screw and feed nut will be on the sides of the threads facing the line of lathe centres, and all the play between those threads will be on their other sides, but so soon as the tool meets the cut it will jump forward and into the work to the amount that the play between the threads will allow it, and this is very apt to cause the tool to break. Furthermore the point of the tool is apt from its extreme keenness to become dulled quickly.