Cement, Energy and Environment

comply with six-sigma control. Here the process range is influenced by the hardness of the material, grinding media, separator etc. and the specification limits are 2900 to 3050. With 6-sigma control the rate of rejection even with a shift of 1.5 sigma distance from the mean will be 3.4 ppm. Load: In a cement plant, cement bag weight is controlled to 50 kgs as per the standard. The variation is restricted to a unilateral tolerance of + 200 grams. Hence the specification limits for the cement bag are 50.000 to 50.200 kgs. Process average is 50.100 kgs and the standard deviation is 16.67. The process range is marked by the variations in cement filling . The set points on the packing machine are adjusted such that 6-sigma control is exercised; it means that the rate of weight violations is restricted to 3.4 ppm despite a 1.5 sigma shift. Here, 6-sigma control is built in the packing machine. Time frame: In order to attend an important meeting of plant executives at a short notice, an average of about 15 minutes are normally required to reach the conference room from the place of work. A standard deviation of 2.5 is worked out from the frequency distribution data. When a notice period of 30 minutes is allowed for holding such a meeting in the premises by the chief executive, the participants can comfortably arrive at the venue without a need for rushing. It demonstrates the 6-sigma situation, in which process range is 15 minutes and the specification limit is set at 30 Sigma Value Class of company Cost of quality 2 non-competitive 3 Normal 20 to 35 % of sales 4 Global 15 to 20% of sales 5 5 to 10 % of sales 6 World class Less than 1 % of sales Each sigma shift can provide about 8% improvement in net income. With improvement in quality rejections decrease, the sales turnover will increase. minutes, leaving a sufficient margin for unforeseen events to the extent of 1.5 sigma shift from the mean. The rate of non– participation due to chance occurrence is 3.4 ppm. Money: The publicity budget of a company varies from Rs. 10,000 to Rs. 70,000 per month. Rs. 10,000 per month is the fixed element. The average monthly expenditure is Rs. 35,000 and the standard deviation is 5000, as worked out from the distribution data. Here, the specification limits are Rs. 10,000 and Rs. 70,000. The process range is marked by the changes in the advertisement expenses caused by industrial fairs, festival sales etc. Under 6- sigma control, the rate of budget violations should not occur in excess of 3.4 ppm, despite a 1.5 sigma shift. The aforesaid illustrations should assume random occurrence of the frequency distribution data resulting into a normal curve; in case, it fo llows other than a normal curve, the conclusion drawn on the basis of six-sigma shall not apply and the results will be incorrect. Test can be applied to check whether the data are normally distributed or not, before using six-sigma approach. The Expected Gain: Six-sigma control is believed to have produced profits by zeroing the defects and by lowering the cost of quality. Approximate figures are shown as follows. The Concept: The spread of +1- 3sigma, as measured from the process mean in the normal distribution curve, covers 99.73 % of the items/characteristics, which forms the area under the normal curve. Thus, 0.27 % or 2700 defective pieces/parts per million fall outside the specification limits, which face the risk of rejection. If we extend to cover +1- 4 sigma distance from the mean, the rate of rejects reduces to 63 ppm for 5 sigma it further reduces to 0.58 ppm. Likewise, if the limit is fixed at 6 sigma, there will be only 2 defects per billion opportunities and the area under the normal curve is almost 100 % covered, as seen from the table on the last page. It is a situation of almost zero-defect. Hence 6 sigma ensures defect-free quality. Under 6 sigma, even a shift of the process mean to the extent of +/-1 .5 sigma, will result in a rate of defect to 3.4 ppm, and if there is no variation in the 16 I ~-