Fig. 930.
Fig. 931.
[Fig. 930] represents a section of a tool t, whose feed traverse is in the direction of a. Now all the force that is expended in bending the cutting c out of the straight line, or in other words the pressure on the top face of the tool, acts to a great extent to force the tool to the left, and therefore traverse it to its feed. The more side rake a tool has the nearer the thickness of its cutting will accord to the thickness of the feed traverse. For example, if a tool having a side rake of say 35 degrees of angle feeds forward 1⁄32 inch per work revolution, the thickness of the cutting will but slightly exceed 1⁄32 inch, but if no top rake at all be given, as shown in [Fig. 931], then the cutting will come off nearly straight, will be considerably thicker than 1⁄32 inch, and will be ragged and broken up, and it follows that the thickening and the bending of the cutting has required an expenditure of the driving power of the lathe, diminishing the depth of cut the lathe will be capable of driving. With such a tool the pressure of the cut will fall downwards as denoted by the arrow b.
Fig. 932.
In the practice of many tool makers in the Eastern States the tool is ground to a point a, [Fig. 932], that is, ground sharp and merely rounded off with an oil-stone. This may serve when the lathe has an exceedingly fine feed, and the strain being in that case very slight the tool point may be made to stand well above the level of the body of the steel, as in the figure, and thus save forging; but this is a slow method of procedure, and produces no better work than a tool which is rounded at the point, and therefore capable of producing smoother work with a much coarser feed.
The diameter of the curls of the cutting, shaving, or chip produced by a turning and also the direction in which it moves after leaving the tool, depends upon the amount of the top rake and the direction in which it is provided. The greater the amount of rake, whether it be front or side rake, the larger the coils of the cutting, and, therefore, the less the amount of power expended in bending it. Furthermore, it may be remarked that the thickness of the cutting is always greater than is due to the amount of feed traverse, and it requires power to produce this thickening of the cutting. The larger the coils of the cutting the nearer the thickness accords with the rate of feed.