992 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1953 



coated surface uniformly to reduce the coating thickness. An adjustment 

 of 15 per cent increase may be obtained by this means. After adjustment, 

 resistors are given a protective coating, code marked and subjected to 

 ten accelerated aging cycles over a temperature range of —50° C to 

 85** C. 



3.43 Use of Control Charts 



Since the manufacturing processes are essentially common within 

 certain ranges of resistance it has been found possible to maintain con- 

 trol charts associated with the three-cell method for combinations of 

 different values of resistance. In order to handle such combinations the 

 characteristic under control is expressed as a percent deviation from 

 the specified nominal. The control chart for averages then consists of 

 averages of these percent deviations and the range chart is expressed 

 in similar terms. This procedure permits a single chart to represent a 

 large number of resistance values. 



Control charts are used at two points in the manufacture of the 200- 

 type resistors. The first chart is applied to the resistance value after 

 adjustment either by rubbing, in the case of lower values, or by the 

 combination of helixing and rubbing used for the higher resistance 

 values. 



Estimates based on preliminary studies predict that at half life this 

 type of resistor will age upward by an average of 0.5 per cent. Man- 

 ufacturing requirements for the nominal resistance values are set 0.5 

 per cent low in order to compensate for this condition. 



The second chart is kept on the finished product after the accelerated 

 aging cycles. This chart is also kept in terms of the percentage deviation 

 from nominal. A comparison of the charts for the product after cycling, 

 which represents the units which are candidates for stock, with those 

 charts at the earlier resistance adjusting operation will show up changes 

 in value due to processing. Appropriate changes in processing or adjust- 

 ment bogies are made to obtain a more nearly centered value. 



3.44 Shipment of Product 



Although resistor production consists of a series of batch operations 

 with each batch readily segregated and identified, the small quantities 

 involved have resulted in the decision to package all of the product in 

 accordance with the three-cell method. 



The use of control charts after adjusting and after cycling has resulted 

 in a more uniform product for the 200A resistors than would have been 



