250 BELL SYSTEM TECHNICAL JOURNAL 



are aligned with respect to the crystal plate, which is properly located and 

 held between sliding jaws as shown. Prior to the operation the silver spots 

 are burnished and fluxed. The wires in their guides are then slid into con- 

 tact with the plate. The hot blasts are raised from below so as to aim 

 directly at the work. The solder is melted and the attachment completed 

 in about 10 seconds when the bjasts are withdrawn. Slight pressure is 

 maintained on the wires during this operation by springs in the guides to 

 force the head of the wire to seat on the spot. A small fillet is obtained 

 around the head making the solder cover an area of about 3S to 40 mils 

 diameter. 



Crystal units using headed wire attachments have many advantages over 

 those made with straight or hooked wires. The pull-ofF strength is more 

 uniform and averages shghtly better than that of hooked wires, despite the 

 fact that only a fraction of the amount of solder used with hooked wires is 

 employed. With this type of attachment the cratering effects encountered 

 with previous methods have also been eliminated. The reduced quantity of 

 solder on the face of the crystal plate effects a decided improvement in the 

 temperature coefficient of the crystal unit as well as in its efficiency and 

 stability. Although the heading of the wires and the subsequent cleaning 

 and tinning operations involve more work, the process of making the attach- 

 ments is simpler and quicker, since the use of individual wires is better 

 adapted to making all the attachments in one operation. Headed wire 

 attachments are more uniform in size and shape and give a more workman- 

 like finish to the job. This type of attachment has now replaced those 

 using straight and hooked wires in virtually all designs of telephone type 

 crystal units using wire supports. While the potential advantages of a 

 headed wire type of attachment for crystal support wires had been known 

 for many years, the practical exploitation of the idea depended on finding 

 commercial means for producing the headed wires. The development of a 

 suitable machine for this purpose was carried out by the Western Electric 

 Company in close collaboration with the Laboratories. Figure 13.9 shows 

 such a machine. The fine wire is fed through the lower mechanism to a 

 die in which it is firmly clamped with a predetermined amount extending 

 above the plate. This part of the wire is then cold-worked by multiple 

 punches in the head of the machine until a conical shaped head is formed 

 in the die cavity. As the individually headed wires are foimed they are 

 cut off to a definite length and expelled as the vise is released and the next 

 wire brought into position. Cold heading of the wires is necessary in order 

 to retain the elastic properties of the phosphor-bronze springs employed in 

 the suspension. The operation of the tool is simple after the precise align- 

 ments of the die and punches have been made. 



