260 BELL SYSTEM TECHNICAL JOURNAL 



tional compositions were then made of talc and clay with and without 

 sodium silicate and feldspar. It was found that most satisfactory 

 results were obtained with a ball clay, kaolin, and talc body in the 

 proportions of forty per cent, ten per cent and fifty per cent, and this 

 body was therefore adopted. 



Originally, attempts were made to cut the groove in the fired core 

 with a diamond tool but this resulted in excessive chipping of the 



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Fig. 2 — Ceramic core and completed filament resistance. 



groove. A chaser with alternate teeth was then tried out, with the 

 thought that the gradual cutting action would prevent chipping. 

 This also proved unsuccessful. The use of a circular saw, emery 

 wheel, and a phosphor bronze disc charged with diamond dust were 

 also considered. These methods were not completely satisfactory 

 although better results were obtained. Rolling the thread in the core 

 while in the leather hard state after extrusion was then tried with 

 good results and a suitable machine was developed for performing 

 this operation. - 



With this machine, an extruded blank of slightly oversize diameter 

 was placed on a revolving mandrel. An arm was provided to hold a 

 shaving tool ahead of a disc which formed the thread. This arm was 

 attached to a segment of a nut and the movement of the arm when 

 the nut segment was engaged with a thread integral with the mandrel, 

 shaved the core to exact diameter and carried the disc longitudinally 

 across the core forming the spiral groove. An auxiliary arm carrying 

 two knives was then engaged which cut the core to exact length and 



2 H. T. Bellamy Patent 1,384,587, "Manufacture of Composition Cores," July 12, 

 1921. 



