Monitoramento de processo seis sigma por gráficos de controle de Shewhart

• Autor(a) principal: Marques, Caio Augusto Nunes
• Data de Publicação: 2013
• Tipo de documento: Dissertação
• Idioma: por
• Título da fonte: LOCUS Repositório Institucional da UFV
• Texto Completo: http://locus.ufv.br/handle/123456789/4073
• topic: Qualidade; Alarme falso; Alarme verdadeiro; Quality; False alarm; True alarm; CNPQ::CIENCIAS AGRARIAS

Developed at Motorola in 1987 the Six Sigma methodology seeks, by reducing the variability of key-processes, obtain critical to quality characteristics (CTQs) with defect probabilities close to zero. It has a Six Sigma process when the distance between the CTQ s target value (VN) and its nearest specification limit, is equal or greater than six standards-deviations (σ). In practice, despite the big attention being paid to the process, the average of the CTQ s probabilities distribution is able to shift until 1,5σ from the target value which even so, the process will be considered Six Sigma. So there is an interval between 4,5 and 6σ in which the process can vary without losing the quality level considered as world class . Thus, in this study, aimed establishes recommendations for planning the Shewhart control charts ̅ and R for monitoring Six Sigma processes. To do so, it was established a reference performance in which it was assumed the joint probability of false alarm equal to or less than 0.01; and the joint probability of true alarm growing according the reduction of the process Sigma level, from 0 in 6σ processes to 0.10 in those 5σ, reaching 0.90 at 4.5σ processes until reaches the unit for 3σ processes and inferior. Accordingly, it were investigated plannings with combinations between n = 2, 3, 4 and 5 and k = 2.5, 2.6, 2.7, 2.8, 2.9 and 3.0. It was identified that the pair of graphs in question performed well when the process was only under the effect of average displacement and lost performance occurred the increase of the variation as the only disturbance present or when the two anomalies were acting. It was possibly identify that the average displacement is the most observed problem, the simultaneous occurrence of both anomalies is less frequent and exclusive presence of increased variation is rare. Therefore, it was recommended that planning with n = 5 and k = 2.9 for monitoring Six Sigma Practical processes (ie, with sigma level between 4.5 and 6σ), which performed well only when the process was mainly under the effect of the average displacement. However, it is expected a good performance of this planning when the process is mainly under the effect of the average displacement. Thus, it is likely that the processes quality level falls without any signal from the control charts in question to indicate quality loss due to the increase of the variation, with or without the presence of the average displacement.