THE CAPABILITY AND THE QUALITY LEVEL OF THE PRODUCTION PROCESSES

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Hubert Paul
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1 THE CAPABILITY AND THE QUALITY LEVEL OF THE PRODUCTION PROCESSES FLOAREA VASILE ICPE Bucharest, 313 Splaiul Unirii, Sector 3, , Bucharest, Romania, Process control and process management are constant concern for engineers and quality specialists that working in this area. To approve / homologate process capabilities involves analyzing its behavior in the activities of product development and to stabilize that behavior within acceptable margin variation. The paper presents practical ways in order to validate any product or service development process efficiency. Keywords: process, statistical control, improvement. INTRODUCTION Competition and competitiveness on the global market cause producers to find solutions for lowering cost products. The development approach of electrical machines was always done for improving their technical characteristics. The reducing of electrical machine gauge was done taking into consideration as main machine characteristics the values of power/mass parameters, especially for servomotors. The differences occur from the data solutions given by the researchers and designers, but also from the processes objective-function based on which those products are achieved. Any product or service can be achieved in several aspects of quality, determined by customer requirements and technological solutions that are applied. Also, the technological solution implementation is influenced by the type, performance and process control. In order to determine the process capability, it is considered the assessment of the extent to which a process is able to satisfy a certain technical specification. Power analysis process consists of: - capability measurement process, to determine the extent to which satisfies the prescribed tolerances; - identifying reasons for which a capability process does not succeed in satisfying the prescribed tolerances. CAPABILITY APPROVAL FOR A PROCESS Capability approval/homologate for a process is based on measurable characteristics of the analysis generated in that process. It can be defined as a combination of two characteristics: - measurable characteristic: the proportion of non-conforming products; - conferring feature; the state of statistical stability of the process. A process cannot provide the same results all the time, so it is needed a methodology that can determine the extent to which observed variability should be corrected. To study the processes of manufacturing variability, W.A. Shewhart created the control charts ( X, R), with which it investigates the behavior of the two parameters of the process: centering (with an estimated average) and accuracy (expressed by amplitude) [4, 5]. These parameters are quantitative tools that highlight the existence, or absence, the state of the statistical stability. 557

2 The stages that approval process passes are: - the process stability state validated or invalidated by the center and the precision parameter, actuated from the ( X, R) chart; - potential determination process validated or invalidated by the coefficient of potential C p. Approval steps are exemplified results obtained by analyzing the processes of execution for electrical machine components, designed and made at ICPE SA [1]. Process Stability The stability entail is made through the control sheet ( X, R) see Figure 1. It is considered, in general case, that the measurements are made on a lot of N = m n pieces of equal size, grouped into m samples, each with n components. Experimental results are supplemented with the ration of following sizes [, 3]: - the average value X of each sample measurement: X i X = (1) n - R magnitude of the variation of each sample size: R = X MAX - X MIN () - the average of the average of all sample: - average amplitudes: X j X = (3) m R j R = (4) m With these results the control limits are calculated [, 3]: For the average: For the amplitude: LCS X = X +A R LCI X = X - A R (5) LCS R =D 4 R LCI R =D 3 R (6) The constants A ; D 3 ; D 4 are taken from standard SR EN ISO 858 +SR C1/1999 Shewhart control sheets [4]. These values are the central margins of control boundary. Using the values, is graphically represented the two components of the control sheet, for the averages and for the amplitude R, by the central values and control margins that are calculated and this are analyzed the trajectories that are indicating the stability process. 558

3 Case study: The characteristic diameter shaft: Φ17 0 0, 011mm, the revolving rate of the brushless servomotor, type G1: ACM n 00 made in Center 3, ICPE SA were analyzed. Specified tolerances are T i = 16,989 mm, T s = 17 mm respectively, in this case tolerances are asymmetric toward the target value. Measurements were carried out on a batch of 0 pieces, divided into m = 5 samples of the actual n= 4 pieces each, the results are presented in Table 1. Table 1. Average - Amplitude Measured values [mm] X R 16,990 16,99 16,991 16,99 16,990 16,991 16,989 16,993 16,991 16,99 16,99 16,990 16,991 16,993 16,990 16,991 16,991 16,99 16,993 16,991 16,991 16, , ,995 16,9915 0,00 0,003 0,00 0,00 0,003 The control margins have the following values [, 3]: For the average: For the amplitude: LCS X = X +A R =16, ,577 0,004=16, LCI X = X - A R = 16, ,577 0,004=16,998 (7) LCS R =D 4 R =, =0,005 LCI R =D 3 R = 0 because D 3 = 0, for n<7 (8) The values obtained were represented graphically by the two components of the control sheet, due to central values and control limits calculated see Figure 1 [1]. 559

4 Figure 1. The process Shewhart sheet The pathways highlight a stable process because: - no point does not exceed the process margins; - the trajectories are independent, no one flows the other course; - a single point on the trajectory of the average is placed near the upper limit of control. The Process Potentiality The process potentiality is expressed by the coefficient (the marker) of potentiality C p. This is calculated based on the process tolerances and control limits determined above [, 3]. ITS C p = ITN ITS = T S T i ITN= LCS X LCI X (9) The process is potential if the coefficient is over-unitary C P >1, as the value is greater so the process has the necessary potential to provide convenient product variability that process is part of. Case study: For the considered case see Chapter.1, the coefficient of potential is [1]: where: ITS 0,001 C p = = =4,03 ITN 0, ITS = T S T i = 17-16,989=0,

5 ITN= LCS X LCI X = 16,998-16,990066=0,0734 (10) It notes that indeed C P >1 which means that the process is carried out in acceptable limits and has the need potential to provide convenient product variability for the process that is part of. The Process Capability The process capability is expressed by the capability indexes C pk superior and inferior, compared to determine the process tolerances and control limits determined above: C pk(inf) = X T i ITN C pk(sup) = X T s ITN (11) The capability indexes C pk show the process positioning capability of the limits specified T i and T S. The index C pk should be over-unitary in order to have a stable process. If both values are over-unitary, the double feature is tolerated. In this situation, the index capability C pk is chosen as the most unfavorable value: C pk = min{ C pk(inf, C pk(sup) }; C pk >1. (1) To quantify the performance process is used the process capability chart that shows the position of the point particle (C pk, C p ) relative to the first bisectix. The first bisectix graph is the minimum level of performance: C pk = C p. The process is considered quality stable if the point particle is above the point of first bisectix. Case study: For the considered case see Chapter.1 and Chapter., the coefficients of capability are [1]: 16, ,989 C pk(inf) = 0,00734 =1,79 16, C pk(sup) = =6,5 (13) 0,00734 Because the process is double permissive, the value is chosen as the coefficient of the minimum capability value C pk =

6 Process analysis is able to generate the features Φ17 0 0, 011mm with an optimum level of quality. In Figure is shown the considered capability process diagram. The coordinates point (1.79; 4.03) is placed in the performance statistics of the process. C p 4,03 1 C pk 1 1,79 Figure. The process capability graph CONCLUSIONS The paper widely presents the method of approval for a given process capability using the sheet control. The acquired results underline the necessity of evaluating the capability for all products processes that complete on achieving a product. Only this can be determined and improved the bad processed to ensure product quality. Thanks to our results, we can make technological choices in electrical machines production with greater objectivity. Acknowledgements This paper was achieved as part of project: RO-0054: Integrated micro CCHP Stirling Engine based on renewable energy sources for the isolated residential consumers from South-East region of Romania (m-cchp-se). REFERENCES 1. Vasile, F. (009), Quality of Electric Servomotors: Characteristics and Performance Indicators, ELECTRA Publishing House, Bucharest.. Maniu Isac, Al. and Voda, V.Gh. (1997), Quality Manual, Economic Publishing House, Bucharest. 3. Petrescu, E. and Voda, V.Gh., (00), Process Control Charts. Theory and Case Studies, Economic Publishing House, Bucharest. 4. SR EN ISO 858+C1/1999: Shewhart Control Charts. 56

A Unified Approach to Uncertainty for Quality Improvement

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