32 BELL SYSTEM TECnNICAL JOURNAL 



Strains in Diamond Stones 

 Rough diamond stones as well as those removed after use in drawing dies 

 have been examined in polarized light to determine the presence of internal 

 strains. In general, the clear flawless stones of alluvial origin were free of 

 strains. Mined stones of South African origin which are commonly found 

 in dies of French manufacture did show in several cases very definite strains. 

 These latter stones were of poor quality. Due to the high index of refrac- 

 tion of the diamond which is much higher than available high refractive 

 liquids, and to local surface strains which distort the interference figures, 

 it was impractical to establish a satisfactory procedure to investigate this 

 subject. 



Crystallographic Studies 



In several cases exceptional die performance was noted, three to four times 

 the average, which could not be accounted for by our regular inspection 

 methods. Also, several dies which chipped initially in use, consistently 

 chipped at subsequent recut sizes after enlargement had completely removed 

 the visible defect. Again and most important is that the majority of our 

 dies are rendered non-usable by the breakage of the die stone, which occurs 

 in a characteristically peculiar manner. These discrepancies are attributed 

 to the crystallographic structure of diamonds. It is well known that the 

 reticular density (atom spacing), hardness and cleavage are greater along 

 certain crystallographic planes. 



Investigations as to whether or not the relation of the planes of cleavage 

 to the direction in which the hole is drilled has any practical bearing on die 

 life were made by the Western Electric Company in 1929"*. The x-ray was 

 used to determine the orientation of the crystal planes. The results of a 

 limited test at that date, in which nineteen dies were drilled at right angles 

 and nine parallel to the plane of cleavage, showed no superiority of these 

 positions. The diamond stones used for dies then were of different origin 

 and quality than those now commonly empoyed for this purpose. Further 

 studies, with particular reference to the relation of crystal structure to die 

 breakage, are now under way for reasons noted previously, and some of our 

 results to date are cited here having a bearing on die performance. 



It is well known that hardness in a diamond varies with the crystal planes. 

 However, since the contour of the die hole consists of cylindrical and conical 

 surfaces, it is questionable if this plane property could be effectively utilized 

 in drawing dies. 



An x-ray method® on a shop production basis is now being used to orient 

 quartz plates preparatory to their cutting along certain crystallographic 

 planes incident to their manufacture into radio and carrier frequency control 

 filters. A similar technique was applied to determine the location of the 

 axis of the die drawing hole with relation to the diamond structure. The 



