1919] 



on The Hardening of Steel 



483 



dependiDg on the work to which it is to be put ; this temperature 

 varie.^ usually from 200'-300'' C. This tempering process, while it 

 withdraws some of the hardness produced by quenching, confers a 

 most valuable property on the tool, namely, toughness, by means of 

 which it stands up to its work, at any rate for a time, without 

 cracking. Then comes the last stage, namely, the grinding of the 

 tool on the grindstone, whereby a clean cutting edge of the required 

 shape is produced ; the tool is then ready for use. 



Let us suppose that it is to be used in taking a cut from a 

 cyhndrical bar of an unhardened steel. The latter is fixed in a lathe 

 and rotated at an appropriate speed (Fig. 1). The tool, held in a tool- 



Fx^.l 



<^7S5.A.} 



holder, actuated by suitable mechanism, is gradually brought up to the 

 end of the rotating bar and a given rate of feed maintained. For an 

 instant there is actual contact between the cutting edge of the tool 

 and the bar ; the moment, however, that this happens a chip is 

 formed and a shearing stress is set up, as a result of which the work 

 falls not on the actual edge of the tool, but on an area inside. If 

 the metal which is being machined is ductile, it is cut away in long 

 shavings w^hose thickness depends on the feed ; if, on the other 

 hand, it is brittle, it breaks off into short chips. The effect of this 

 friction between the tw^o metals, and the pressure of the shaving, is 

 to generate heat, and to raise the temperature of the tool to a much 

 greater extent than that of the bar which is being machined, and 



