Process Capability | Cp, Cpk Analysis | Statistical Process Control | Quality Control Tools | Total Quality Management | Lean Six Sigma Tools
CHAPTERS
0:00 Introduction
1:08 Learning contents
2:11 Process capability
3:35 Control limits and specifications
4:17 Limits, specifications, and distributions
5:55 Process width and specification width
7:18 Process capability w.r.t. specification limits (Cp)
8:18 Process capability w.r.t. target (Cpk)
9:32 Calculating Cp and Cpk
12:07 Varying Cp and Cpk
19:59 Conclusion varying Cp and Cpk
21:35 Corrective action
22:31 Process improvement
#cp #cpk #spc #processcontrol #processcapability #leansixsigma #quality #qualitycontrol #qualitymanagement
PROCESS CAPABILITY
Process capability implies, how capable your process is to meet the target value and stay within specification limits.
Control limits are established as a function of the averages. In other words, control limits are for averages such as:
• Upper control limit (UCL) =mean + 3 standard deviation (σ)
• Lower control limit (LCL) = mean – 3 standard deviation (σ)
Specifications, Upper specification limit (USL), and Lower specification limit (LSL) on the other hand are the permissible variation in the size of the part and are, therefore, for individual values. The specification or tolerance limits are established by design engineers to meet a particular function.
The control limits, process spread or PROCESS CAPABILITY, distribution of averages, and distribution of individual values are interdependent. They are determined by the process, whereas the specifications have an optional location.
Process Capability is the integral part of quality control, lean six sigma, operational excellence, quality excellence, statistical process control (SPC), and statistical quality control (SQC), for the analysis of varying Cp and Cpk values.
PROCESS WIDTH
Process Width is the difference between the Upper Control Limit and the Lower Control Limit (UCL - LCL).
Mathematically; If UCL is at +3σ and Lower Control Limit is at -3σ then,
• Process Width = UCL - LCL = +6σ.
SPECIFICATION WIDTH
Specification Width is the difference between the Upper Specification Limit and Lower Specification Limit.
Mathematically;
• Specification Width = USL - LSL
PROCESS CAPABILITY w.r.t. SPECIFICATION LIMITS (Cp)
Process Capability (Cp) is the ratio of Specification Width to Process Width
• Mathematically; (USL - LSL)/(UCL - LCL) or (USL - LSL)/6σ
• If, Cp is greater than 1; the process is capable of meeting specifications.
• If, Cp is less than 1; the process is not capable of meeting specifications.
• The de facto standard is Cp = 1.33.
• In the case of Six Sigma quality, Cp = 2.00
• Larger the value of Cp implies better quality.
PROCESS CAPABILITY w.r.t. TARGET (Cpk)
Cp does not measure process performance in terms of the target value. Here we need Cpk. Cpk measures the closeness of the process average with the target line. It is an indicator of the off-target process.
• Mathematically; Cpk = Cp(1-k), where, k = Shift/0.5SW, Shift = Absolute value of (Mean - Target)
• The de facto standard is Cpk = 1.00, but some organizations consider Cpk = 1.33, or 1.67 for a good process.
VARYING Cp and Cpk
• The Cp value does not change as the process center changes.
• Cp = Cpk when the process is centered.
• Cpk is always equal to or less than Cp.
• A Cpk value greater than 1.00 indicates the process conforms to specifications.
• A Cpk value less than 1.00 indicates that the process does not conform to specifications.
• A Cp value less than 1.00 indicates that the process is not capable.
• A Cpk value of ZERO indicates the average is equal to one of the specification limits.
• A negative Cpk value indicates that the average is outside the specifications.
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