970 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1953 



corn-eritioual maximum and minimum limits for the characteristics of 

 many of the components manufactured for the new repeaters. The in- 

 troduction of distribution criteria where maximum and minimum are 

 customarily specified requires a number of operations which are supple- 

 mentary to normal procedures. The relatively simple act of identifying 

 good product becomes complicated by the need of more extensive meas- 

 urements, the recording of data, computations, plotting of charts and 

 the active participation of technical personnel in the administration of 

 the procedures. 



The first step in the program was the development of practical sta- 

 tistical quality control techniques which would be applied under the 

 special circumstances attending the design and manufacture of compon- 

 ents of L3 carrier repeaters. This required careful study by Bell Tele- 

 phone Laboratories and the Western Electric Company and resulted in 

 the development of a general specification which provides procedures and 

 criteria for maintaining the average value of a quality characteristic close 

 to a nominal value and for obtaining as nearly as possible a random dis- 

 tribution of individual values around the nominal. The purpose of this 

 paper is to discuss the procedures thus developed with emphasis on their 

 relationship to manufacturing processes and to describe the problems en- 

 countered and solved in the course of application to the manufacture of 

 components of L3 carrier repeaters. Detailed mathematical derivations 

 and terms will be generally omitted since the theories underlying the 

 principles involved are covered by another article. 



1.2 PARTICIPATION IN DEVELOPMENT 



Normal practice in the creation of new product designs is for the de- 

 velopment and construction of the first models to be handled by the 

 design engineers. This work usually includes discussions with the manu- 

 facturing organization in order to minimize the costs and to utilize exist- 

 ing or most effective manufacturing facilities and various preferred or 

 stocked materials. In the case of the critical components of the L3 carrier 

 amplifiers, a design change in one component resulting from the transi- 

 tion from development to production requires especially close study and 

 may require an adjustment in other components in order to compensate 

 for the one being (jhanged. Knowledge of the behavior of regular manu- 

 facturing facilities and methods used in the fabrication of preproduction 

 units provides considerable assistance in establishing specification limits 

 which are compatible with the produc^t design and manufacturing process 

 capabilities. 



