As this chapter deals with the sharpening methods only, the reader is referred to the next chap[p. 29]ter, which deals with the manner of setting and holding them to do the most effective work.
When it is understood that a cutting tool in a lathe is simply a form of wedge which peels off a definite thickness of metal, the importance of proper grinding and correct position in the lathe can be appreciated.
Roughing Tools.—The most useful is the roughing tool to take off the first cut. As this type of tool is also important, with some modifications, in finishing work, it is given the place of first consideration here.
| Fig. 24. Tool for Wrought Iron. | Fig. 25. Tool for Cast Iron.[ToList] |
[Fig. 24] shows side and top views of a tool designed to rough off wrought iron, or a tough quality of steel. You will notice, that what is called the top rake (A) is very pronounced, and, as the point projects considerably above the body of the tool itself, it should, in practice, be set with its cutting point above the center.
The Clearance.—Now, in grinding, the important point is the clearance line (B). As shown in this figure, it has an angle of 10 degrees, so that in placing the tool in the holder it is obvious it cannot be placed very high above the center, particularly when used on small work. The top rake is ground at an angle of 60 degrees from the vertical. The arc of the curved end depends on the kind of lathe and the size of the work.
The tool ([Fig. 25]), with a straight cutting edge, is the proper one to rough off cast iron. Note that the top rake (C) is 70 degrees, and the clearance 15 degrees.
The Cutting Angle.—Wrought iron, or mild steel, will form a ribbon when the tool wedges its way into the material. Cast iron, on the other hand, owing to its brittleness, will break off into small particles, hence the wedge surface can be put at a more obtuse angle to the work.
In grinding side-cutters the clearance should be at a less angle than 10 degrees, rather than more, and the top rake should also be less; otherwise the tendency will be to draw the tool into the work and swing the tool post around.