24 BELL SYSTEM TECHNICAL JOURNAL 



to the reduction angle is necessary to guide the wire concentric to the draw- 

 ing cone. 



Reduction Angle 



In this angle, the most important sector of the die, the reduction of 

 diameter, except for the sizing accomplished in the bearing, is carried out. 

 Following the practice of the trade a 16° angle is used. The depth of this 

 area is designed to have the entering wire hit well within it to prevent 

 excessive chipping and undercutting in the pressure ring and t6 maintain the 

 desired die pull values. The line of contact of the entering wire and the 

 reduction area is located at 65-70% of the vertical height of the reduction 

 angle. 



In general, the ratios of the vertical heights of the reduction, approach 

 and bell angles are as follows: 1:0.7: 1.5 for dies of 20-26 A.W.G. inclusive 

 and 1:0.5:1 for the larger sizes. The dimensions of the approach and bell 

 angles are of course dependent on the stone size and the number of previous 

 recuts. The heights of the two latter angles may vary appreciably from the 

 above noted values provided that adequate lubrication and "washing out" 

 of the die can take place. 



Bearing 



A bearing length of about 40% diameter is formed. It is checked by the 

 die pull requirements. The bearing is purposely drilled to obtain a slight 

 front taper of about 2°. The slight taper aids the metal flow and reduces 

 "sucking". 



Relief Angle 



Here again, the wire vibration necessitates a smooth angle which will not 

 tear the wire leaving the die. Recutting for good reproduction of contour 

 and die pull also requires a smooth angle. Proper care should be taken to 

 prevent a cup shaped depression which is commonly formed by jamming 

 a diamond chip in the back of the die as a fast intermediate step to final 

 sizing. 



Polish and Finish 



The degree of polish of the die drawing surfaces is a most important 

 factor in determining satisfactory die performance. In wire drawing there 

 is encountered a vicious repetitive cycle consisting of (1) the wire abrades 

 due to the condition of the wire and die surfaces, (2) the resultant abraded 

 metal particles pack in the die throat and gall the incoming wire, and (3) 

 the heat and the conditions created by this action again adversely afifect 

 the wire and the die surfaces to repeat the cycle. There have been cases 



