7_Process Capability

7_Process Capability - EMSE 182, 282 - Professor Jim Harris...

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EMSE 182, 282 - Professor Jim Harris -Process Capability- 1 PROCESS CAPABILITY OVERVIEW OF PROCESS CAPABILITY A measure of the ability (capability) of a process to produce acceptable parts Specifically the ability of the process to produce parts within specified tolerances Natural Tolerance Limits are often taken to be at 3 „ .5 • For normal distribution this represents 99.73% of the process output Aprocess Capability Analysis is an engineering study to estimate process capability. This may include estiation of distribution, mean and variance • May be performed with regards to specifications about the quality characteristic • May be expressed as a percentage outside quality specifications • Specifications are not necessary but often used Uses • Predicting how well the process will hold the tolerances • Assisting product developers/designers in selecting or modifying a process • Assisting in establishing an interval between sampling for process monitoring • Specifying performance requirements for new equipment • Selecting between competing vendors • Planning the sequence of production processes when there is an interactive effect of processes on tolerances • Reducing the variability in a manufacturing process
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EMSE 182, 282 - Professor Jim Harris -Process Capability- 2 MEASUREMENT OF PROCESS CAPABILITY Histogram • Based on 100 or more points - obtain sample mean and variance and histogram • Shape aids in determining distribution
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EMSE 182, 282 - Professor Jim Harris -Process Capability- 3 Probability Plots (eyeball goodness of fit tests) • Motivation (for normal distribution) Pr{X x} Ÿœ F Š‹ x . 5 F(x) where œ F x . 5 F(x) (z-value) F -1 x œ . 5 plot x vs F(x) should be linear! Ê F -1 • Procedure without special graph paper 1.Take sample x , . .., x and order smallest to largest x , . .., x 1n ( 1 )( n ) 2. use F(x ) (i) i. 5 n ¸ 3. Plot x vs Z value of (i) 5 n 4. Estimate as 50th %-ile and as slope or 84th %-ile 50th %-ile .5 • Other procedures for other distributions • Choosing distrinution form - Best guess - Use skewness and kurtosis where M ^^ É "" 12 M M M M i (x x n œœ œ 3 2 3/2 4 2 2 i1 n i i ± œ Ñ a region graph is provided (p357)
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EMSE 182, 282 - Professor Jim Harris -Process Capability- 4 Example Z Sample i i.
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7_Process Capability - EMSE 182, 282 - Professor Jim Harris...

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