November 4, 1922J 



NA TURE 



603 



Letters to the Editor. 



\The Editor does not hold himself responsible for 

 opinions expressed by his correspondents. Neither 

 can he undertake to return, or to correspond with 

 the writers of rejected manuscripts intended for 

 this or any other part of NATURE. No notice is 

 taken of anonymous communications.] 



Action of Cutting Tools. 



Prof. F. G. Coker has been good enough to send 

 me a copy of his paper on the above subject, together 

 with the discussion which followed its reading before 

 the Institution of Mechanical Engineers. I will 

 (with your permission) take this opportunity of 

 thanking him and of adding a few remarks to my 

 letter to Nature of August 26 of this year. 



I had not, when that letter was written, a copy of 

 the Proc. R.S. paper of 1881 at hand, and was not 

 certain as to how far the experimental processes 



Fig. 1. — Diagrammatic sketch to show the action of dynamometer used for 

 measuring the force on tool, T and T' being the tensions of the two 

 parts of the driving belt. The force acting on the sliding bar is 

 2(T - T). This is balanced by the extension of the spring. Hence 

 the movement of the bar, in conjunction with the known diameter of 

 the work, gives (when corrected for the friction of the pulley, etc.) a 

 measure of the force acting on the tool. 



preceding its production were described. On re- 

 reading, however, I see that the experimental part 

 was omitted, and I may here state that the force on 

 the tool was measured by a dynamometer of the type 

 shown diagrammatically in Fig. 1, and that it was 

 found that for cuts of similar section the force 

 required was very nearly proportional to the cross- 

 section of the strip removed. This of course is 

 equivalent to the statement that the same amount 

 of work will remove the same volume of material 

 whether the shavings are thick or thin, provided 

 that they are similar. 



This dynamometer, which recorded the force 

 automatically on paper moving with a velocity 

 proportional to that of the cut, worked satisfactorily 

 when the cutting speed was suitably chosen, though 

 I should not use the same pattern were I again to 

 embark on such investigations. 



The materials on wliich the experiments were 

 made included, besides the ordinary metals, others 

 easier to deal with in a lathe worked by foot, and 

 of these clay was found to be the most useful, for, 

 according to ' the state of dryness to which it was 

 brought, its behaviour under the action of the tool 

 could be made to resemble that of any sort of metal, 

 hard or soft, and at the same time cuts of easily 



NO. 2766, VOL. I IO] 



measurable thickness could be taken with compara- 

 tively small forces. 



While referring to the subject of material, I may 

 mention a matter which seemed to me rather sur- 

 prising. I wished to see whether it would be possible 

 to face up a speculum casting in the lathe in order 

 to save time in the preliminary grinding. Speculum 

 metal, as is well known, is very brittle, but by taking 

 a broad cut of extreme thinness with a dead-hard 

 steel tool, continuous shavings were produced which 

 looked like ribbons of grey satin. It was only while 

 the edge of the tool was perfect that the cut was 

 satisfactory, and this condition rarely lasted long 

 enough to cover a speculum two inches in diameter. 



With regard to Prof. Coker's paper, the only 

 objection I have to make is that it has no reference 

 to the action of cutting tools. The polarised-inter- 

 ference bands are evidence of elastic strain. They 

 might be maintained indefinitely when the tool was 

 stationary if the applied force was just insufficient 

 to produce further rupture, and would disappear 

 when that force was removed. 



The elastic deformation, though interesting, has 

 nothing to do with the special action of the tool, 

 the essential function of which is to cause destructive 

 strain throughout a small region near its edge while 

 having no permanent effect on the body of the work. 

 The real interest in the action of a cutting tool 

 is confined to the plane AB (Fig. 2) along wliich 

 destructive shear takes 

 place and a very short 

 length of the material 

 which forms the shaving 

 — together with that part 

 of the tool in contact 

 with it. 



The internal structure of 

 a shaving closely resembles 

 that of slate, the principal 

 plane of cleavage being 

 parallel to AB of Fig. 2, 

 and the angle which this 

 plane makes with the direc- 

 tion of the cut is modified 



by, and may be said to depend on, the angle which 

 the face of the tool makes with the same direction 

 and the mutual coefficient of friction between the 

 tool and the material on which it operates. 



The action of the tool is always discontinuous and 

 quasi-periodic, the period being determined by the 

 travel required to extend the destructive shear from 

 A to B. The period, therefore, is proportional 

 (among other things) to the depth of the cut. 



From this it may be seen that, in addition to the 

 principal planes of cleavage, secondary and slightly 

 differently inclined cleavages occur before the 

 shearing across AB is complete. This was well 

 shown in the polished and etched sections * referred 

 to in the 1881 paper, and can also be recognised in 

 the accompanying photographs (Figs. 3 and 4), 

 though not quite so clearly. 



The normal force on the tool during each period 

 tends to expand (like rivet heads) the base of the 

 strata wliich press against it, and this action causes 

 the shaving to curl : the frictional force (parallel 

 to the face of the tool) tends, on the other hand, to 

 drag the base of strata towards the cutting edge, 

 and thus to keep the shaving straight. 



The shape of the cross-sections of a shaving is often 

 rather peculiar, but is a definite function of the shape 

 of the tool and of the properties of the material from 



1 The earliest application of " etching " for the purpose of rendering the 

 structure of a metal visible was, I suppose, the " crowTiing " of twisted 

 gun-barrels, etc. 



Fio. 2— Action of a cutting tool, 

 showing the principal and 

 secondary planes of cleavage. 



