Fig. 938.
Now, in proportion as the length of the cutting edge is disposed parallel to the work axis, the tool has a tendency to spring (under an increase of cut) into the work, and also to dip into soft places or seams in the work, and the amount of its front rake must be decreased, because such rake causes a pressure pulling the tool deeper into its cut, as was explained with reference to [Fig. 921]. Round-nosed front tools, therefore, such as in [Fig. 938], cannot be given so much front rake as ordinary ones, such as in the preceding figures.
Fig. 939.
Round-nosed tools are used to cut out round corners, and the roughing tools are given a less curvature than that to be formed on the work, thus in [Fig. 939] is an ordinary form of small round nose shown operating in what is termed a hollow corner, the directions of tool feed being marked by arrows. The tool may be fed by the feed traverse, and the tool gradually withdrawn, thus forming the work to the required curve.
The amount of cut a lathe will drive, the degree of hardness which the tool may be given, the length of time the tool will last without grinding, the speed at which the work may run, and the cleanness and truth of the cut, depend almost entirely upon the perfect adaptability of the tool to the conditions under which it is to be used. Upon the same kind of work, and using the same kind of tools, some workmen will give a tool from 20° to 30° more angle than others.
It is a difficult matter to determine at just what point the utmost duty is being obtained from cutting tools, because the conditions of use are so variable; but one good general guide is the speed at which the tool cuts, and another is the appearance of the cuttings or chips.