July 22, 1922] 



NA TURE 



119 



depends on the cutting angle of the tool, and also 

 upon its rake and clearance, but investigation has not 

 proceeded far enough as yet to define accurately the 

 influence which each element has upon its position. 



A somewhat remarkable change from these conditions 

 is produced when the cutting edge is not so perfectly 

 sharp as it is possible to make it. It then becomes 

 apparent that the shaving is no longer cut from the 

 main body, but is broken or torn away by the continual 

 forcing of a wedge between the shaving and the main 

 body of the material, after the latter has once been 

 penetrated. The shaving breaks away at a point A 

 above the cutting edge (Fig. 3), mainly owing to the 



bending action exerted by the raking face of the tool, and 

 isso much overstressed at this place that all coloureffects 

 are obliterated. An intense and permanent black 

 patch is therefore produced separated by a less stressed 

 part B, in which plastic stress colour effects are observ- 

 able from a similarly much more overstressed part C 

 immediately preceding. These effects are repeated 

 at regular intervals as is indicated in Fig. 3 in a, 

 somewhat diagrammatic form, and are accompanied 

 by a rhythmic pulsation of the colour bands in 

 phase with this phenomenon. The tearing away of 

 the shaving in this manner produces a rough uneven 

 surface on the material, which in planed work is 

 therefore not truly flat, and in turned work is not 

 perfectly cylindrical. It is probable that this in- 

 fluences the character and kind of chip produced in 

 brittle materials, as it undoubtedly influences the 

 shaving from an elastic material which is capable of 

 assuming a plastic condition. Moreover, it is some- 

 times found that when this latter condition occurs the 

 tool is acting in a two-fold capacity, for not only does 

 it break off a shaving, but it may also pare off the 

 irregularities as the point of the tool comes in contact 

 with them, so that occasionally a second and much 

 thinner shaving, D, is produced, and peeled off as 

 indicated in Fig. 3, by a true secondary cutting action. 

 Double shavings are sometimes produced in this 

 manner when steel is turned in a lathe, and it is probable 

 that the fine powder, which can often be observed 

 falling away from a tool working on cast iron, is due 

 to this secondary cutting action. 



In tools with multiple cutting edges the same general 

 features of stress distribution are observed in the 



NO. 275 1 , VOL. I io] 



material. They are sometimes accompanied by addi- 

 tional phenomena, as, for example, with the milling 



Fig. 4.- 



nilling cutter operating 



plate of nitro-cellulose 



cutter, shown in Fig. 4, where the depth of cut is 

 variable owing to the uniform movement of the 

 material up to the cutting edges, which are also turning 

 at a definite and uniform rate. The shaving cut from 

 the trochoidal contour is continuously increasing in 

 thickness therefore, as the cut advances, and in the 



Fig. 5. — Isoclinic lines and colour bands observed and calculated for a 

 wedge of angle 6o° when a load of 50 pounds is applied at an angle 

 of 20 with the axis. Colour bands observed are shown in full lines ; 

 equivalent stress lines given by theory are shown dotted. From the 

 Proceedings of the Institution of Mechanical Engineers, by permission 

 of the Council. 



present instance is being torn off rather than cut, as 

 the characteristic markings for this kind of action 

 occur in a very pronounced manner. If sufficient 

 travel is given to the work, the separate lobed colour 

 bands springing from each cutting edge, and denoting 

 approximate radial stress, ultimately coalesce lied ire 



