30 BELL SYSTEM TECHNICAL JOURNAL 



medium takes place. The steel lapping pins are accurately ground to a 

 definite straight included angle and circular arc on a special grinder. The 

 ripped die surface should be a smooth, dull matte surface free of deep rings 

 and scores. 



In polishing, the die contour has already been established and it is ex- 

 pedient to have the lap of a material which will rapidly shape itself to the 

 contour to be polished to obtain maximum contact. Here the loading of the 

 lap will hold the diamond dust in contact with the work. A viscous medium 

 for the dust will also help. On the wire polishing machines, the springback 

 of the wire on flexing is important. It was found that a wire resiliency 

 tester will aid to evaluate the desired wire properties. Diamond dust of a 

 uniform and fine grain size is vital to the polishing operation. High RPM 

 in the case of revolving die mounts is beneficial provided it does not "throw 

 out" the diamond dust from the working area. 



Sizing of the bearing requires a spindle RPM about 100% higher than 

 that used in ripping. Other factors in this operation are the diamond dust, 

 frequency of its addition and the frequency of the pin grinds. 



All die recutting machines should be periodically checked and maintained 

 to minimize vibration. 



Diamond Dust 



Another important factor in the recutting operations is the diamond 

 dust abrasive. This is especially true in the finishing operations of polish- 

 ing and sizing where no appreciable breakdown of the original dust takes 

 place. Uniformity of particle size range and grain size determine the 

 efl&cacy of the abrasive. Diamond dust graded by air flotation is now being 

 used with good results. It was found that this material was more satis- 

 factory for our use than that obtained by sedimentation in liquid media. 

 Recent investigations in the separation of microscopic size dry powdered 

 material have been actively carried out in the ceramics and powdered metal 

 industries. The Journal of the American Ceramic Society and the Bureau 

 of Mines publications contain noteworthy papers describing practices which 

 could be applicable to diamond dust grading. Stokes^ law of fall for micro- 

 scopic size particles will also hold true for the separation of diamond dust 

 by means of air. 



Die Life 



In theory and practice, die life resolves itself into a problem of wear. The 

 wear is due to the movement of the wire surface over the drawing die area in 

 contact with the wire. In turn the wear is proportional to the forces acting 

 on these surfaces, their area, the condition of these surfaces and the co- 

 etiicient of friction. This is dependent on the nature of the wire, the die 



