Hardening, tempering, annealing and forging of steel; a treatise on the practical treatment and working of high and low grade steel .. . PIG. 91.—PORMED BUTT MILL. teeth, or cutting edges, which are, of course, the parts that arewanted hard, are hard; while the central part or core of the toolis comparatively soft, which is in all cases a desirable condition. When milling cutters are hardened in water they often crack,cracking taking place not, as might be imagined, when the hotsteel is first plunged into the water, but about that time the centralpart is becoming real cold, as the outside cutt
Hardening, tempering, annealing and forging of steel; a treatise on the practical treatment and working of high and low grade steel .. . PIG. 91.—PORMED BUTT MILL. teeth, or cutting edges, which are, of course, the parts that arewanted hard, are hard; while the central part or core of the toolis comparatively soft, which is in all cases a desirable condition. When milling cutters are hardened in water they often crack,cracking taking place not, as might be imagined, when the hotsteel is first plunged into the water, but about that time the centralpart is becoming real cold, as the outside cutting edges are properly I50 HARDENING, TEMPERING AND ANNEALING. hardened while the inside is yet comparatively hot; and when theinside cools and contracts while the outside remains rigid, thecracking takes place. The slower the tool is cooled, the longer thetime in which the central metal has a chance to adjust itself tomolecular changes, and consequently there is less liability of crack-. I^IG. 92.—FORMED BUTT MILI*.
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