Fig. 922.
If the tool be given no traverse, but be merely moved in towards the centre of the work, the cut will move outward and in a line with the body of the tool, the cutting coming off as shown in [Fig. 922].
So soon, however, as the tool is fed to its feed traverse the form of the cutting alters to the special form shown in [Fig. 917], and moves to one side of the tool, as well as outwards from the work.
Fig. 923.
[Fig. 923] is a top view of a tool and piece of work, and the arrow a denotes the direction of the resistance of the work to the cut, being at a right angle to plane of the cutting edge.
Now the duty of the side edge is simply to remove metal, while that of the point is to finish the surface, and it is obvious that for finishing purposes the most important part of the tool edge is the point, and this it is that requires to be kept sharp, hence the angle or rake should be in the direction of the point. But when the object is to remove metal and prepare the work for the finishing cut the duty falls heavily on the side edge of the tool, and the angle of the top face and the direction of its rake may be varied with a view to increase the efficiency of the side edge, and at the same time to diminish the amount of power necessary to pull the tool along to its feed traverse. This may be accomplished by altering the top rake from front to side rake, which is done in varying degrees according to the nature of the work.