Process Control Saves Time and Money

While process control had already been implemented in mass and Rx production for a long time it was applied only occasionally in R&D and not at all in Marketing. In Production process control means to replace the final product inspection by a control of significative parameters of the manufacturing process, as temperature, pressure, viscosity, number of revolutions, etc..  The main benefit of such an approach is that in keeping process variables within certain tolerance limits you can avoid the situation that during the inspection at the end of the process you have to scrap faulty products and to stop the production to find the cause and to eliminate it. The condition for such a procedure is a thorough analysis of the process with special mathematical-physical tools (as the Design of Experiments and others) to identify the critical variables.

Daniel Steigelmann was a fervent advocate of the process control approach. A further advantage of this philosophy is the possibility to assess to what extent a production process is capable to respect the targeted tolerances. This sounds astonishing, but Daniel could show for example, that the traditional manufacturing process for progressive lenses was borderline with respect to the ISO tolerances. These tolerances had been fixed based on tolerances for bifocal lenses updated with the first experiences of manufacturing progressive lenses in prescription.

About the time, when Quality organization had been restructured, a new manufacturing method of spectacle lenses became commercially more and more important, digitally surfacing of complex aspherical surfaces. In the traditional production process a semifinished blank with a cast progressive  front surface is completed in the Rx lab with a ground back surface according to the prescription of the spectacle lens wearer. Ideally each individual prescription needs a specific progressive front  surface. For economical reasons this is not possible  and therefore the same semi finished blank has to be used for a range of prescriptions, optimum correction is achieved only for the central power of the range. The digital surfacing is a manufacturing method producing individual lenses on NC machines, enabling the production of any desired geometry of the two surfaces, so it is also called Free Form production. With its  introduction  it was possible to bring to an end what we had started in 1989 with our Multi Design concept, to produce the completely personalized progressive lens.

To adapt the progressive design to personal characteristics of the wearer, i.e. his specific ametropia as well as his professional and leisure activities was certainly the main advantage of the Free Form technology. But the process analysis showed also, that if the Free Form process was under control its capability was better than for the traditional production, i.e. the statistical variation of the optical parameters was tighter.

The breakthrough of the Free Form technology was a wonderful opportunity to demonstrate the benefits of the process control philosophy. Looking back, to put the Free Form process under control was a technically sophisticated problem and moreover sometimes an issue between Production/Engineering and Quality, but it was a big success.

Operations and Engineering were looking primarily for a minimum cost solution which was naturally also a goal for our quality experts, but they targeted particularly to guarantee  compliance with the “new” specifications. I call them “new” specifications because for the first time we established complete tolerances for a finished progressive lens. Until now in Rx production using a semifinished blank, the optical power of the final progressive lens was measured in only  two points, as the respect of the correct design on the front side had already been ensured by a tight control of the molds. In the Free Form production both the aspheric front side as well as the backside with the prescription power are manufactured at the same time in the Rx lab and so it is necessary to control all the lens points. To find a good compromise between precision and reasonable cost we chose a global physical quantity summing up optical deviations between realized design and theoretical values. The measurement device for such a global evaluation was a so called lens mapper. As it was used as an element of the process control loop of the FF production, it was sufficient to measure one reference lens per shift and not every manufactured lens.

The fact that the newly organized Global Quality was now reporting directly to General Management was certainly very helpful for the successful design of the Essilor FF process control. It was the basis for a fruitful and constructive discussion on equal footing between Operations and Quality resulting in an efficient process control method at moderate cost.

In an organization where Operations/ Engineering would have been responsible exclusively for the produced quality, there would have been the risk of a conflict of interests. I know that there are other philosophies as “getting quality products out to the customer is in the interest of the entire team”. This is certainly true, but sometimes the priorities of the different experts are not the same and accordingly the proposed solutions as well. To quote a rather extreme example of the recent past, one explanation for Volkswagen’s emission disaster in 2015 was a certain lack of control.