Advanced Process Capability

Process CapabilityWhat it is • Process Capability looks at short term capability and long term performance of a process with regard to customer specifications. • Standard indexes are used: Cp, Cpk, Pp and Ppk. These can be converted into ppm. A simple view of Process Control and Process Capability • process control - refers only to the “voice of the process” - looking at the process using an agreed performance measure to see whether the process forms a stable distribution over time. • process capability - measures the “goodness of a process” - comparing the voice of the process with the “voice of the customer”. The voice of the customer here is the specification range (tolerance) and/or the nearest customer specification limit. The objective is to get as close to the theoretical best that your process can achieve by eliminating special causes of variation, so that only common (natural) causes are acting on the process, and then to reduce these to a minimum, whenever possible. 1 Process Capability v1.ppt no reference to specs ! In Control (Special Causes Eliminated) Out of Control (Special Causes Present) Process Capability In Control and Capable (Variation from Common Lower Spec Limit Causes Reduced) Upper Spec Limit In Control but not Capable (Variation from Common Causes Excessive) 2 Process Capability v1.Process Capability What it is Process Control Note .ppt . ppt .Process Capability What it is 1 2 CAPABILITY PERFORMANCE Short Term Long Term In Control Not in Control Pooled std dev Total / overall std dev compares How spec range a much (tolerance) Cp Pp variability to process width how close process How b centred centre is to Cpk Ppk nearest spec limit Cp or ENTITLEMENT is the theoretical best hat a process can achieve. 3 Process Capability v1. 1 2 CAPABILITY PERFORMANCE Short Term In Control Long Term Not in Control Not (necessarily) in Control: common compares Pooled std dev Total / overall std dev causes and special causes acting How spec range a much (tolerance) variability to process Cp Pp (possible drifts or shifts in the process) width how close process How b centred centre is to Cpk Ppk nearest spec limit long term 600 550 500 450 400 350 300 Time short term In Control: only common causes acting (no sudden drifts or shifts in the process) 4 Process Capability v1. .ppt . Process Capability What it is The 4 capability Capability Indexes: explaining the terms . . f.) ] Total (or overall): sT = sqrt [ sum (x . Process Capability What it is The 4 capability Capability Indexes: explaining the terms .ppt . *s2) / sum (d. . . 1 2 CAPABILITY PERFORMANCE Short Term Long Term In Control Not in Control Pooled std dev Total / overall std dev compares How spec range a much (tolerance) Cp Pp variability to process width how close process How b centred centre is to Cpk Ppk nearest spec limit 600 550 500 450 400 350 300 Time Pooled or total standard deviation .used in manual calculations: Pooled (or combined): sP = sqrt [ sum (d.f.x) 2 / (n-1) ] 5 Process Capability v1. . .ppt . 1 2 CAPABILITY PERFORMANCE Short Term Long Term In Control Not in Control Pooled std dev Total / overall std dev compares How spec range a much (tolerance) Cp Pp variability to process width how close process How b centred centre is to Cpk Ppk nearest spec limit LSL USL Cp. It is the ratio of the specification width to 6 x process standard deviation. Cp = USL-LSL Pp = USL-LSL 6sP 6sT sP = pooled standard deviation sT = total standard deviation 6 Process Capability v1. Pp compares specification range (tolerance)to process width regardless of where the process is centred. Process Capability What it is The 4 capability Capability Indexes: explaining the terms . It is (difference between the process average and the nearest specification) (3 x the process standard deviation). . Cpku} 3sP 3sP Ppkl = X-LSL Ppku = USL-X Ppk = min {Ppkl. Ppku} 3sT 3sT 7 Process Capability v1. Cpkl = X-LSL Cpku = USL-X Cpk = min {Cpkl. Process Capability What it is The 4 capability Capability Indexes: explaining the terms . Ppk measure how close the process centre is to the nearest customer spec.ppt . . 1 2 CAPABILITY PERFORMANCE Short Term Long Term In Control Not in Control Pooled std dev Total / overall std dev compares How spec range a much (tolerance) variability to process Cp Pp X width how close process How b centred centre is to Cpk Ppk nearest spec limit LSL USL Cpk. 33 .5 sigma.67 4.1. Capability studies for non-normal data (Weibull) do not give Cpk values 8 Process Capability v1.33 1.67 > 5.1. since it combines the indexes with a graphical representation of the process: actual sample.67 1.1.00 . Cpk MUST be seen as part of a package of information.5 Yellow (OK) 1.33 .33 < 4.33 > 1. estimated short term capability and estimated long term performance • Also use as a rule of thumb the following chart Cp Cpk Pp Ppk Sigma Red (Bad) < 1.5 (Cpk x 3) +1.5 .Process Capability How to use it • Use Minitab output whenever possible.00 .00 < 1.ppt .67 > 1.33 1. (industry standard).5 = sigma Caution: Cpk is an index much misused: It is only applicable to normally distributed variable data that is measured in the short term. Conversions from Cpk to ppm and sigma assume that all defectives are occurring at one tail of the distribution with an average sigma shift of 1.33 < 1.1.33 > 1.5.5 Green (Good) > 1.00 < 1. 56 CPU 2.00 PPM > USL 79256.49 Cpk 0.47 PPM > USL 100000.62 CPL 0.000 ST Target * LT LSL 420.00 PPM > USL 0.09 9 Process Capability v1.67 PPM < LSL 69795.67 PPM Total 69795.7176 StDev (LT) 59.833 Sample N 30 StDev (ST) 10.44 PPM Total 474659.49 Cpm * 250 350 450 550 650 Overall (LT) Capability Observed Performance Expected ST Performance Expected LT Performance Pp 0.28 PPM < LSL 366666.09 PPM Total 466666.24 Ppk 0.000 Mean 435.81 PPU 0.ppt .44 PPM < LSL 395402.6888 Potential (ST) Capability Cp 1.43 PPL 0.Process Capability Example Using Minitab: Process Capability Analysis for Data LSL USL Process Data USL 520. 00 Expected LT Performance 0 5 10 15 20 25 PPM < LSL 150164. with non-normal data Process Data USL 6.09 PPL 0.00 PPM > USL 400000.Process Capability Example Using Minitab.09 Observed LT Performance PPM < LSL 120000. NO values for Cpk are given. and LONG TERM metrics only are given.00 PPM Total 520000.58256 Scale 6.ppt .35 Ppk 0.00000 Target * Process Capability Analysis for Weibull LSL 2.01 PPM > USL 396235.71 PPM Total 546399. because the data is non-normal 10 Process Capability v1. data must have positive values.65406 Sample N 50 LSL USL Shape 1.29977 Overall (LT) Capability Pp 0.00000 Calculations Based on Weibull Distribution Model Mean 5.72 Note: For Weibull Capability Analysis.19 PPU 0. 5. 8. • It indicates what our short term and long term situation looks like. 8.7 : SPC (and non-SPC control) • ASG “seniors” aTQ training module 1.1. • Recognise however that our processes need to be stable (in statistical control) as well as capable. we must have faith in our measurements. 3.3.5 : (MSA) Measurement System Analysis • 4.4. • A Process Capability study reflects what our customer sees. • Before anything.2. 4.5 : Capability 11 Process Capability v1. hence the need for a measurement system analysis.5 : Process Capability • 8.6.ppt . References • ASG aTQ training modules: • 3.4.3. 4. 4. 3. 8.Process Capability How it helps • A Process Capability study is the end result of the analysis done on process output data. 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