Quality Magazine - March 2009 - (Page 28) BALL BAR - DUMBELL We sold one of our $800.00 Ball Bar Kits to a young man who was having problems with his C.M.M. | BR A I N T E AS E R S | Capability Analysis of Thickness of Critical Layer X = 0.22144 Nominal= 0.200 9 LSL= 0.100 LNPL= 0.15827 USL= 0.300 UNPL= 0.28460 8 X : 0.22144 7 SDx : 0.02105 Cp : 1.58316 Cpk : 1.24377 6 See: www.precisionballs.com One week later, he call to discuss the results. His C.M.M. had just been serviced and certified by the Frequency 5 4 3 2 machines manufacturer at a cost of $700.00. OUR Ball Bar found that the “Z” axis of this machine was out of square with the “X” axis 0.009” (0.23 mm). The “Z” to “Y” and “X to Y” Axii were perfectly square. After adjusting “Z”-“X” the entire machine was measuring within 0.0003” (0.076mm). He wondered if we had any suggestions on how he might improve this accuracy. As this was an older manual machine, we told him that we thought this was an acceptable accuracy. A simple, inexpensive Ball BarDumbell system provides legal certification according to ANSI B 89.4.1 or VIM-VAR 260 chart, the process behaves predictability at an average thickness of 0.22 millimeter with a moving range of 0.024 millimeter and a standard deviation of 0.021 millimeter that give natural process limits of individual values of 0.1583 millimeter to 0.2846 millimeter. The capability analysis gives values of Cp and Cpk of 1.58 and 1.24, respectively. See the graphs, “Thickness of Critical Layer” and “Capability Analysis of Thickness of Critical Layer.” The process average is not on target because Cpk is lower than Cp. The capability analysis shows that the process average is higher than the target. Q: What is the potential for reducing the average thickness for this layer of the product? A: The LNPL (Lower Natural Process Limit) is 0.1583 millimeter. This number is 0.0583 divided by 0.021 = 2.77 standard deviations above the lower specification limit. Also, the process average is on the high side of the nominal and could be lowered. Q: If Russell recommends that production reduce the average thickness of this layer, what concerns must be addressed to ensure that the thickness meets the minimum specification? A: One way to ensure that the thickness meets the minimum specification is to target a Cpk that is greater than 1. A typical target in such cases is a Cpk of 1.33. To achieve this value and yet reduce the material cost for this critical layer, Russell recommends lowering the process average to a point that is 4 standard deviations above the lower specification limit. This gives a target average of 0.10 + 4 x 0.021 = 0.184 millimeter. Dr. Sophronia Ward is a continual improvement specialist and Six Sigma Master Black Belt and coach. Brain teasers are now incorporated in the new training programs, Six Sigma and Lean Six Sigma training for Champions, Black Belts and Green Belts, offered by Dr. Ward and her associates at Pinnacle Partners Inc. For more information, call (865) 482-1362 or visit www. pinnaclepartnersinc.com. 0.3200 0.0800 0.1000 0.1200 0.1400 0.1600 0.1800 0.2000 0.2200 0.2400 0.2600 0.2800 0.3000 0.0900 0.1100 0.1300 0.1500 0.1700 0.1900 0.2100 0.2300 0.2500 0.2700 0.2900 0.3100 1 Bal-tec Division Micro Surface Engr., Inc. 1550 E. Slauson Ave Los Angeles, CA 90011 323-582-7348 www.precisionballs.com Remember to visit ONLINE (www.qualitymag.com) Quality Quick Clicks 409 at qualitymag.com Please visit us at WESTEC, Booth 4101 28 QUALITY | March 2009 www.qualitymag.com http://www.precisionballs.com http://www.pinnaclepartnersinc.com http://www.pinnaclepartnersinc.com http://www.qualitymag.com http://www.precisionballs.com http://www.qualitymag.com http://www.qualitymag.com
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