26 BELL SYSTEM TECHNICAL JOURNAL 



frequent wire breaks. The noted figures are 45-50% of the wire breaking 

 strength based on A.W.G. reduction steps. Die pull is a cumulative check 

 of the following die characteristics: contour as correlated to the reduction 

 angle and bearing length, and the polish of the drawing surfaces. In 

 conjunction with the die diameter, it affords a means of grouping the dies in 

 matched balanced sets and obviates actual try out of these dies in the 

 machines. 



Inspection Methods 



Suitable inspection methods and equipment to check the die requirements 

 contribute much to good die performance. Microscopes have been used 

 with good results by some manufacturers of fine wires and by several of the 

 more progressive die manufacturers for some time for this purpose. The 

 Western Electric Company has adopted this practice. The binocular wide 

 field microscope has proved to be an indispensable tool for inspection. It is 

 used to the complete exclusion of loupes. The stereoscopic effect, the wide 

 field and the long focal distance features of this instrument make satis- 

 factory examination possible. We have found a magnification of SOX to 

 be most practical for the previously noted die sizes. Good agreement to 

 observations by different persons can be had by the use of this apparatus. 

 This is a rare occurrence in the case of loupes. It is surprising that this 

 inexpensive tool which removes much of the guessing and so-called art in 

 diamond die manufacture has not been universally utilized by wire drawers 

 and die manufacturers. 



Frequent examination of finished dies and dies in the various stages of 

 recutting is expediently accomplished by microscopic observation. Routine 

 die contour checks are visually made in this manner. Periodic checks of 

 die angles and die contour are accurately made with a contour projector. 

 Shadowgraphs of lOOX magnification are made of die impressions formed of 

 a soft metal. 



When polished to the proper degree, it is possible to examine a mounted 

 die under a microscope with a suitable light source and discern the internal 

 flaws in the stone. The examination of stones removed intact from the 

 mounting will also check this point. Mountings of metals having a low 

 reflectivity will require a light source from above to illuminate the stone. 



The die pull is checked on a commercial instrument shown in Fig. 3. 

 Auxiliary equipment and gauges have been added to it to make it more 

 versatile for this use. The device consists of a piston which fits snugly into 

 a hydraulic chamber fiUed with a suitable liquid. The piston is recessed at 

 the outer end to hold the die. The die pull causes the movement of the 

 piston which builds up the registered pressure. The wire is reduced one 

 A.W.G. step and the pressure against the die holder is noted as indicated on 



