THE L3 SYSTEM — QUALITY CONTROL IN MANUFACTURE 



973 



as specification limits, 1-1 in Fig. 2. The margin is attained either by- 

 improvement of the manufacturing processes or by widening the specifi- 

 cation limits. This means that occasional out-of -control conditions can 

 be indicated by the control chart without the factory being faced with a 

 shut do\vn in production, provided that corrective measures are taken 

 before the magnitude of the deviations results in any significant quantity 

 of the product failing to meet the specification limits. 



The contrasting situation occurs where the spread between the maxi- 

 mum and minimum specification limits becomes equal to or less than the 

 6-sigma spread of the manufacturing process, as shown in Fig. 2. For 

 specification limits 2-2, production must be stopped every time an out- 

 of -control condition is indicated or 100 per cent inspection introduced 

 until the cause of the condition is located and eliminated. In the case of 

 specification limits 3-3 in Fig. 2, 100 per cent inspection must be continu- 

 ously applied in order to eliminate non-conforming product as shown in 

 the shaded portion under the distribution curve. 



2.2 SPECIAL STATISTICAL QUALITY CONTROL METHODS 



The special statistical quality control techniques developed for L3 

 carrier components present a special problem. Here they become an ac- 

 tual part of the product specification, rather than an aid in meeting it, 

 although conventional methods may still be useful for process control. 

 It was recognized that any attempt to express specification limits in 

 terms of a it3(r variation around the nominal would have to include 



LIMITS SPECIFIED FOR INDIVIDUAL PRODUCTS 



MINIMUM 

 1 2 



MAXIMUM 

 2 1 



g N 



Fig. 2 — The relations between a frequency distribution of individual units of 

 product and various specified maximum and minimum limits. 



