868 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1954 



OBSERVATIONS ON WIRE STRAIGHTENING 



The straightening operation affects some physical properties of the 

 wire. Tensile strength is reduced about 10 per cent while elongation is 

 increased around 50 per cent. The diameter of the straightened Avire is 

 usually from 0.5 to 1.0 per cent greater than that of the raw wire AAith 

 commensurate loss in wire length. Both straightness and twist appear to 

 be dependent in large part upon the contours of the die blades. Thus 

 far these contours have been determined by trial and error on the ad- 

 justable die block straightener, although .general relationships, especially 

 with respect to wire size, are becoming evident. It is expected that fur- 

 ther study and experience wi\\ establish bases on which contours can be 

 calculated with accuracy. 



Twist imparted to the mre by the rotating action of the spindle has 

 been found difficult to measure. What is referred to as twist is actually 



Fig. 6 — Double spindle showing slots and conipU-uuni oi l wo sets of die blades. 



radial distortion of the wire about its longitudinal axis resulting from 

 partial release of internal stresses remaining in the "wire after straighten- 

 ing. Further release of internal stresses may occur when the wire ends 

 of the twin wire comb are formed before welding, in which event misloca- 

 tion of contacts will result. Fig. 7. This is objectionable from the stand- 

 point both of subsequent manufacturing operations and of relay per- 

 formance. The internal stresses are caused by the crank action applied 

 to the wire surface while it is passing between the die blades in the 

 rotating spindles. Internal stress which is not apparent until after its 

 release, as by forming, has been designated as "residual twist". 



A rough approximation of the amount of residual twist in wire can 

 be obtained by measuring w^hat has been termed "apparent tmst". 

 Apparent twist is the amount of visible rotation at the end of a wire 

 after leaving the straightener. It can be measured in degrees of rotation 

 per foot of wire straightened. When the apparent twist is Ioav, usually 

 the residual twist also is low. A working range for permissible apparent 



