Hi,
Attaching somwe insite for ur refe….
Process Capability (Measuring conformance to customer requirements)
Objective:
To determine whether a process, given its natural variation, is capable of meeting established requirements or specifications of the customers.
1) Helps a team answer the question, Is the process capable?
2) Helps to determine if there has been a change in the process
3) Helps to determine percent of product or service not meeting customer requirements.
Steps to arrive at Process Capability.
Step 1.
Collect the performance data of the process. Determine the process grand average, X (Double Bar) and the average range, R Bar.
Use a stable Control chart, which means the process is stable and normally distributed.
Step 2.
Calculate the Upper Specification Limit (USL) and the Lower Specification Limit (LSL)
The USL and LSL are based on customer requirements. Recognize that specification limits are based solely on customer requirements and do not reflect the capacity of the process.
Step 3.
Calculate the process standard deviation
Process capability is based on individual points from a process under study. Information from a Control Chart can be used to estimate the process average and variation (standard deviation, s).
s is a measure of the process (population) standard deviation and can be estimated from information on the Control Chart by
s = R bar / d2 or s = s bar / C4
Where R bar and s bar are the average of the subgroup ranges and standard deviation, and d2 and C4 are the associated constant values based on the subgroup sample size. See the Table of Constants in Control Charts.
The process average is estimated simply by X double bar and X bar.
Step 4.
Calculate the process capability
To measure the degree to which a process is r is not capable of meeting customer requirements, capability indices have been developed to compare the distribution of your process in relation to the specification limits.
A stable process can be represented by a measure of its variation- six standard deviation. Comparing six standard deviation of the process variation to the customer specifications provide a measure of capability. Some measure of capability includes Cp and its inverse Cr, Cpl, Cpu, and Cpk.
Cp (simple process capability)
Cp = USL LSL
6s
Note:
While Cp relates the spread of the process relative to the specification width, it DOES NOT look at how well the process average is centered to the target vlue.
Cp is the ratio of the specification spread (upper spec – lower spec or US – LS) to the process spread (six standard deviations of the process data, i.e., 6 sigma).
So, Cp contains more information than just the process standard deviation. It tells you whether or not the process variations are well accommodated within the specification limits, i.e., it gives you info about the capability of the process to deliver sepcs. Through simple statistical formulae, you can convert a given Cp value into the process rejection rate – which you can’t, if you know only the process sigma.
Cpk, simply tells you if your process is skewed from the center of the specs so that just by readjusting (resetting) the process center, you can reduce the process rejections.
Cpk is the smaller of the two ratios: (US – process aveaage)/ 3 sigma) and (process average – LS)/ 3 sigma. You can see that it represnts the conservative value of Cp and equals Cp when the process is centered at the specs center.
Regards
Om Sharma is working for GTL Limited, India, he is B.Sc. Com Sc.), Masters in Total Quality Management, PGDQM from World Quality Council, UK, Persuing Doctorate in Quality Management and Operational Research from Portsmouth University UK. He posses ISO 9000:2000 & ISO 14000:1996 Lead auditor Certifica ion from IRCA UK, Six Sigma Black Belt, COPC Regd Coordinator from COPC Inc USA, Has implementation Experience on ISO 9K, ISO 14k, QS 9000, CMM, GPNQA, Six Sigma, QC,QA,QM etc. He has performed numerous quality audits on Quality Management Systems, Six Sigma, TQM Business excellence etc, He is a lead Six Sigma practitioner at GTL for Six Sigma implementation. Om can be reached at [email protected]
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