Fig. 1299.
In this tool the thickness of the end a is of importance, since if it be too thin it will jar or chatter. This is especially liable to occur when a broad scraper is used, having a great length of cutting edge in operation. This may be obviated to some extent by inclining the scraper as in [Fig. 1296], which has the same effect as giving the top face negative rake, causing the tool to scrape rather than cut. The dividing line between the cutting and scraping action of a tool is found in the depth of the cut, and the presentation of the tool to the work, as well as in the shape of the tool. Suppose, for example, that we have in [Fig. 1297], a piece of work w and a tool s, and the cut being light will be a scraping one. Now suppose that the relative positions of the size of the work and of the tool remain the same, but that the cut be deepened as in [Fig. 1298], and the scraping action is converted into that class of severing known as shearing, or we may reduce the depth of cut as in [Fig. 1299], and the action will become a cutting one.
Fig. 1300.
But let the depth of cut be what it may, the tool will cut and not scrape whenever the angle of its front face is more than 90° to the line of tool motion if the tool moves, or of work motion if the work moves to the cut. In [Fig. 1300], for example, the tool is in position to cut the angle of the front face, being 110° to the direction of tool motion.
Fig. 1301.