Both these guides, however, can only be applied to metal not unusually hard, and to tools rigidly held, and having their cutting edges sufficiently close to the tool point or clamp that the tool itself will not bend and spring from the pressure of the cut. The cutting speed for chilled cast-iron rolls, such, for example, as calender rolls, is but about 7 feet per minute, and the angles one to the other of the tool faces is about 75 degrees, the top face being horizontally level, and standing level with the axis of the roll.
Fig. 940.
Fig. 941.
When a tool has front rake only, the form of its cutting will depend upon the depth of its cut. With a very fine cut the cutting will come off after the manner shown in [Fig. 940], while as the depth of the cut is increased, the cutting becomes a coil such as shown in [Fig. 941]. These coils lie closer together in proportion as the top face of the tool is given less rake, as is necessary for steel and other hard metal. Thus [Fig. 940] represents a cutting from steel, the tool having front rake only, while [Fig. 941] represents a cutting from a steel crank pin, the tool having side rake. The following observations apply generally to the cuttings.
The cleaner the surface of a cutting, and the less ragged its edges are, the keener the tool has cut; thus, in [Fig. 941], the raggedness shows that the tool was slightly dulled, although not sufficiently so to warrant the regrinding of the tool. Such a cutting, however, taken off wrought iron would show a tool too much dulled, or else possessing too little top rake to cut to the best advantage. In wrought iron, the tool having a keener top face, the cuttings will coil larger, and the direction in which they coil and move as they leave the tool will depend upon the shape of the tool and its height to the work.