u8 



NA TURE 



[July 22, 1922 



The Action of Cutting Tools. 



By Prof. E. G. Coker, F.R.S. 



ENGINEERING activity is so largely dependent on 

 1 he action of cutting tools, that it is not surprising 

 to find a very large amount of research work has been 

 devoted to its study, and at the present time there are 

 in England two committees actively pursuing researches 

 in this field, in addition to private investigators. At 

 the suggestion of the Cutting Tools Research Com- 



-Steel tool pi; 



trip of 1 



the area in" front of the tool, and a similar state of 

 tensional stress behind it. 1 Measurements of the 

 principal stresses and their inclinations show that 

 this is approximately what obtains, and a very fair 

 idea of the stress conditions in the material can be 

 obtained from the photograph if radial lines are drawn 

 in all directions from the point of the tool to intersect 

 the colour bands. The outer bands pass through all 

 points at which the stress is 1150 pounds per square 

 inch, for the sharply defined boundary between the 

 purple and the blue, and the succeeding ones reckoning 

 inwardly mark stresses of twice and three times this 

 value. The fourth band indicates possibly a somewhat 

 different stress intensity than its numerical order 

 warrants, owing to its close proximity to the black 

 area in which intense plastic stress is developed at 

 and near the cutting edge of the tool. An interesting 

 feature is the partial recovery of the material, for the 

 black area is met with only at this place. The shaving 

 again shows brilliant colour effects after it has finally 

 left the tool, but later becomes obscured, again owing 

 to the further curl developed due to contact with the 

 parent material. 



Careful measurements show, however, that the 

 actual state of stress, excluding the plastic field, is 

 somewhat more complicated. The stress is never 

 quite radial, and the isoclinics are therefore not straight 

 lines, but are always curved somewhat, as indicated in 

 Fig. 2, which shows a set of isoclinics obtained from 

 a disk of about six inches in diameter when subjected 

 to the action of a turning tool with a somewhat acute 

 cutting angle. The lines of principal stress confirm 

 this, and the valves of the minor principal stresses 

 appear to be small in the cases examined so far, and to 

 a first approximation the distribution of principal 



ISOCLINIC LINES 



mittee of the Institution of Mechanical Engineers, of 

 which Sir John Dewrance is chairman, some experi- 

 mental studies of a preliminary character have been 

 made recently at University College, London, on 

 transparent bodies subjected to the action of cutting 

 tools, and the double refraction produced by stress 

 has been used to measure the stress distribution in the 

 cut material. Similar experiments have also been 

 carried out on some glass-cutting tools used for turning 

 and planing operations. 



The photo-elastic method has many advantages over 

 direct experiment on metals, as up to and, in fact, 

 well beyond the elastic limit of the trans- 

 parent nitro-cellulose used, the stress distri- 

 bution produced can be measured with 

 considerable accuracy at all points in 

 a disk or flat plate under the action of a 

 tool. The optical effects give, at once, a 

 measure of the difference of the principal 

 stresses at a point, the lateral contrac- 

 tions afford a measure of their sum, and 

 the isoclinic lines map out the directions 

 of the stresses. Existing literature shows 

 how very difficult it is to obtain similar 

 information from the metal itself when 

 under the action of a tool. Since the 

 distributions are similar up to the elastic 

 limit of each material, owing to the 

 absence of elastic constants in the funda- 



mpn+al pnimtinn —7 4 -., — n tk.„ , „ Fig. 2. — Isoclinics and lines of principal stress in a ilisk under the action of an edge-turning 

 IIRllUtl equation *X — °J tnere are tool. From the Proceedings of the Institution of Mechanical Engineers, by permission of 



obvious advantages in a study of the thc Council - 



characteristics of cutting tools by these means. The 

 general phenomena observed when a tool is cutting 

 are shown in Fig. i , where a steel tool is planing a plate 

 of nitro-cellulose in a circularly polarised fieldof light. 

 It will lie observed that colour bands spring from the 

 cutting edge of the tool and curve round in approxi- 

 mately circular arcs to meet the boundary, indicating 

 the existence of variable radial compression stress in 



NO. 2751, VOL. I 10J 



stresses RR and 68 may be taken to be of the type 



RR=(-2P/7r).cos(9/r 

 where 00 is small, and the angles are measured from 

 the radial black brush dividing the tension from the 

 compression area. The position of this latter brush 



1 " An Account of some Experiments on the Action of Cutting Tools," 

 (",. Coker and Dr. K. C Chakko, Proceedings of the Institution 

 of Mechanical Engineers, 1922. 



