1881.] 



The Action of Cutting Tools. 



131 



It will be seen that there is little difference in any of these, though 

 the materials are of all degrees of hardness, and vary in thickness 

 from three- eighths of an inch, the thickest iron shaving examined, to 

 •003. 



Indeed the action at the edge of the tool seems identical in all 

 cases, such differences as there are being due to the action of the face 

 of the tool on the shaving, while the latter is being pushed out of the 

 way ; and this action depends on some of the physical constants of the 

 substance operated on, chiefly its coefficients of friction on the metal 

 of the tool and on itself, but in part also on its ductility, and in some 

 cases, as in lead, on the property which freshly formed surfaces have 

 of reuniting under pressure. 



The tools do not act, properly speaking, by cutting but by shearing, 

 and the shaving removed by them may be accurately described as a 

 metallic slate. 



This remark does not apply to acute-edged tools, such as razors and 

 penknives. 



The difference between cutting and shearing may be defined thus : 

 Conceive the substance to be cut to be divided into an infinite number 

 of cubic elements by parallel planes at right angles to one another ; if 

 in a portion of this removed by a tool the elements remain cubes, the 

 removal has been effected by pure cutting. If, however, they are 

 only distorted but are all unaltered in volume, the removal has been 

 effected by pure shearing ; if they are both deformed and altered in 

 volume, both cutting and shearing have been called into play. 



Fig. 9. 



Let ABCD be a section of the substance under the action of the 

 tool, GEF the tool, H the shaving, CD the undisturbed surface of the 

 substance, and AB the direction of the cut. 



