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Volume 07 - Powder Metal Technologies and Applications Part 12

Tham khảo tài liệu 'volume 07 - powder metal technologies and applications part 12', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | capability index. The first measure Cp is based on the relationship between the upper and lower specification limits and the standard deviation process as follows _ USLjt-LSLx ib The minimum acceptable value for Cp is considered to be one. However because it is possible to achieve a Cp of greater than 1.0 while still producing a significant proportion of parts that do not meet specification limits a revised version of the process capability index Cpk has been widely adopted. This index sometimes called the actual process capability index imposes a penalty for deviation of the process mean from the nominal value of the specification. The minimum value considered acceptable for Cpk is usually 1.33 although some industries are now beginning to demand 2.0 or better. The Cpk index is defined as the difference between the process mean and its closest specification limit divided by 3-. To calculate this index first the relationship between the process mean . and the specification limits in the units of standard deviations is determined USL X USL----------- LSL - X ZLSL - Ợv Then the minimum of these two values is selected Zmin - min ZusL -Zlsl The Cpk index is then defined by dividing this minimum value by 3 Commonly Cpk must be 1.33 but higher limits are also specified. For example Fig. 4 shows a process capability study of the overall length of a fully processed bevel gear as heat-treated . The Cp of 1.75 shows that the inherent process variation is less than the maximum acceptable range for the product. The Cpk of 1.71 demonstrates that the process is properly targeted to produce an adequately centered finished product dimensional distribution. Slgina Stu.Dev USL 31.95000 N5L 31 75000 LSLi 31.55000 Mean 31.74633 Sigma o.oaaig -SSigma 31.63176 3Sigira 31 .-66090 Cr 0.57286 Cp 1.74564 CpK 1.71384 Skewness 0.40785 Kirtosis 2.10138 Est. Out Of Spec X AỒDVỂ 0.00000 X Below 0 00000 X Total 0 00000 Zupper 5.33293 Zldwer 5 14091 Gearg-Tast 0.07 Chi-squared 0.34 Lil liefors 0 .