A strategic assessment of six sigma

Six Sigma as a quality improvement framework has been gaining considerable attention in recent years. The hyperbole that often accompanied the presentation and adoption of Six Sigma in industry could lead to unrealistic expectations as to what Six Sigma is truly capable of achieving. In this paper, some strategic perspectives on the subject are presented, highlighting the potential and possible limitations of Six Sigma applications particularly in a knowledge‐based environment. Without delving into the mechanics of the subject in detail, the points raised could be useful to those deliberating on the appropriateness of Six Sigma to their respective organizations. Copyright © 2002 John Wiley & Sons, Ltd.     

Confidence intervals and sample size determination for Cpm

The capability index, C_pm, sometimes called the Taguchi index, has the desirable characteristic of being sensitive to both dispersion and deviation of the process average from the engineering target. As a result, the proposed estimators of C_pm have a sampling distribution that is dependent on the non‐central chi‐square distribution. Hence, constructing confidence intervals, performing hypothesis testing or estimating sample size requirements necessitates manipulation of a rather complex functional expression, typically beyond the capabilities of practitioners who need readily available tools. Here, a simple graphical procedure is proposed and illustrated for obtaining exact confidence intervals for C_pm. The graphical procedure allows the user to simply enter the graph with an estimate of the index and a value of the non‐centrality parameter for a given sample size to arrive at end‐points of 90%, 95% or 99% one‐sided or two‐sided confidence intervals. Detailed tables are also provided to assist the user for a wider range of sample values and sample sizes. In addition, a procedure is also presented for determining the minimum sample size required for attaining a pre‐specified level of accuracy of the C_pm. Extensive tables are provided for the user with a simple example illustrating the facility of the technique. Copyright © 2001 John Wiley & Sons, Ltd.     

The Role of Statistical Design of Experiments in Six Sigma: Perspectives of a Practitioner

Six Sigma has now been well recognized as an effective means of attaining excellence in the quality of products and services. It entails the use of statistical thinking as well as management and operational tools to bring about fundamental improvements. This article explains, in a nonmathematical language, the rationale and mechanics of design of experiments as seen in its deployment in Six Sigma. It also outlines the way in which the design of experiments has been utilized in the past for quality improvement, culminating in its important role in Six Sigma. An appreciation of the changing scope of experimental design applications over the years and in the future would provide useful perspectives on the significance of Six Sigma in an organization's quest for quality excellence.     

Obtaining confidence intervals for Cpk using percentiles of the distribution of Ĉp

?_pk is used as an estimate of process capability and can reflect performance degradation due to both shifts in the process mean and variability. Exact upper and lower confidence limits for the actual parameter value are elusive. Objections to existing estimators focus on two areas: the difficulty of executing the needed computations and the excessive widths of those confidence bounds. The CPE estimator (so called since it relies on ?_p ) which we derive in this paper is formed by simply multiplying ?_pk by a value from the inverse chi‐square distribution. A PASCAL computer program is supplied which generates those percentiles. Our CPE is compared via simulation to other estimators and is shown to be the preferred selection in most cases by providing the requisite probability coverage and narrow interval widths. Copyright © 2001 John Wiley & Sons, Ltd.     

Quality evaluation of internal cylindrical grinding process with multiple quality characteristics for gear products

Gears are among the most crucial components in the transmission systems of machine tools. Gear manufacturing includes a number of processing procedures. The grinding process is an important procedure involving high precision and fairly small grinding surfaces. For this reason, this study aimed at developing a quality assessment model for the internal cylindrical grinding process of gears. The Six Sigma quality indices (SSQIs) were used to directly assess the quality of the internal cylindrical grinding process due to their ability to directly reflect quality level and process yield. Since the process may include nominal-the-best (NTB), larger-the-better (LTB) and smaller-the-better (STB) quality characteristics, so we used the variable transformation method to normalise the specifications of each quality characteristic for the convenient and effective management and analysis of process performance for multiple quality characteristics. We then constructed a multi-characteristic process quality analysis chart (MPQAC) to simultaneously assess the quality levels of various quality characteristics. Furthermore, the MPQAC can provide references for process improvement. This ensures the quality of internal cylindrical grinding and enhances the quality of gear and machine tool products. Finally, a real-world application and numerical experiments demonstrate the effectiveness and practical applicability of the proposed method.     

Process True Capability Evaluation with the Consideration of Measurement System Variability and Expected Quality Loss

Process capability indices are widely computed under the assumption that the measurement system is free from errors. However, measurement variability is unavoidable and has a significant impact in process capability evaluation. From an economic point of view, Taguchi loss function is an effective tool to measure the quality loss of a product characteristic deviated from target value that is extensively used without taking into account the effect of the measurement system. This paper investigates the influence of measurement system variability on the process capability analysis through the calculation of process capability indices. A new quality loss function, integrating the measurement system errors, is developed to compute the optimal true process capability regarding to the expected mean value of the Taguchi loss function and the loss resulting from the control of the true process capability. Copyright © 2016 John Wiley & Sons, Ltd.     

Application of Taguchi methods to surface mount processes

This paper describes the implementation of a Taguchi experiment for the placement of surface mount components as part of a process at Mars Electronics. The justification for the experimental work, the practical problems in its implementation and the significance of the results obtained are described. The experiment is seen as part of a programme and the implications of the results obtained for subsequent experimentation are discussed.     

Application of X̄ control chart with modified limits in process control

The conventional Shewhart X̄ chart is developed based on the assumption that the within‐sample variation is due to the inherent process variation, and any significant variation between samples is attributed to the existence of assignable causes. In the manufacturing industry there are processes where there is variation between samples due to the inherent process variation. A straightforward application of the conventional Shewhart X̄ chart would thus result in more frequent false alarms. The problems associated with various Shewhart X̄ charts in monitoring such a process are discussed using a real data set from an integrated circuit (IC) assembly process. A Shewhart X̄ chart with modified limits is adapted for such a process. In addition to the usual ability to signal for assignable causes, the X̄ chart with modified limits is also developed as a tool to signal the need for adjustment of controllable process variables for improving the process capability. Practical application of this chart in monitoring an IC assembly process is discussed. Copyright © 1999 John Wiley & Sons, Ltd.     

An alternative method to test for randomness of a process

The objective of this paper is to revisit an old technique for testing the randomness of a process. This method uses the mean square successive difference approach to estimate variance, compares this estimate with the usual variance estimate, and tests whether the two variance estimates are significantly different. If the test result is significant, it is concluded that the process is not random (i.e. not in control). The non-random results are addressed to accomplish quality improvement.     

支持自动生成大型企业过程可支持资源的资源能力建模

为了支持更广范围的过程自动化分析与优化,提出了一种基于活动的资源能力建模方法。它可根据企业中现有的大量活动抽象出活动粒度层的能力模板;在资源建模时,可通过选取模板来定义资源的能力型;在资源模型实例化时,实例化资源的能力;通过资源能力与活动能力需求的耦合来生成过程支持资源。最后给出系统实现。该资源能力定义方法支持自动地生成大型企业过程的可支持资源。     

Capability Indices and Conformance to Specification: The Motivation for Using Cpm

Proposed is a perspective on quality that is different from but consistent with the current Six Sigma thinking. The focus is still squarely on the customer, but the concept of a “defect” is reevaluated. This new perspective can be used to help define key characteristics, process capability, and evaluate overall product quality using a loss function. This article focuses on the concept of process capability using this perspective. Central to the proposed quality perspective is ability to meet the customer's product specifications rather than ability to meet the component feature specifications. Current product and component quality metrics can drive the wrong decisions and wrong behaviors. This can add significant cost to internal processes and suppliers, and inadequately protect the customers from defective products. The new perspective defines component and process quality based on the probability of meeting the product specifications rather than the individual component feature specifications. Cpm is a simple process capability metric that can indicate the probability of meeting the customer's product specification. Cpm can be used to better estimate the cost of poor quality than can be estimated by using Cpk, and thus can be used to better manage product quality to the customer.     

Application of a hot-melt granulation process to enhance fenofibrate solid dose manufacturing

Evaluation of hot-melt granulation of fenofibrate and croscarmellose sodium and its cooling time for the molten mass in a ratio of 55:45 was conducted to assess the manufacturing process capability to produce an acceptable granulation which flows well on Korsch PH300 tablet compression machine. The formation of the drug-polymer eutectic mixture was investigated by differential scanning calorimetry, scanning electron microscopy and X-ray powder diffraction. The physical properties of the hot-melt was determined by examining the milled blocks after solidification and milling after cooling periods of 10, 20 and 30 d. The milled material was assessed for the effect of hold time of the blend on the solid dose compression characteristics. The impact of cooling on the processing of the blocks was assessed after 10, 20 and 30 d of cooling. The study suggests that after the hot-melt formed the fenofibrate crystallized independently and a solid solution with croscarmellose sodium was not formed. The age of the blocks determined the hardness of the crystals, changing the processing nature of the granules with respect to compression and powder flow characteristics. The blocks processed after 20 d and beyond produced granules with a characteristic suitable for holding the blend for 14 d in the bin with no impact on flow properties and compressibility of the blend. There was no chipping, capping, sticking or picking observed and a higher compression speed was achieved.     

A new finite element integration scheme. Application to a simple shear test of anisotropic material

Finite element simulations involving large strains and large displacements imply the setting up of a precise kinematics for the integration scheme. Moreover, anisotropic mechanical behaviours have to be expressed in an appropriate local reference frame. In the present paper, a computational procedure is derived from a particular velocity gradient definition. Its implementation in a finite element code is described. Application to a simple shear test is proposed. For anisotropic materials, a sinusoidal behaviour of the shear stress component is emphasized. This is due to the anisotropic yield locus and appears in the case of a saturated isotropic hardening law. Copyright © 2007 John Wiley & Sons, Ltd.