Understanding Process Capability in Quality Management
Process capability is a crucial concept in quality management that evaluates the ability of a process to meet design specifications. It focuses on the relationship between process variation and desired standards, ensuring that products consistently match set criteria. Learn about Cp, Cpk, and other key measures that assess a process's effectiveness in meeting customer requirements.
Download Presentation
Please find below an Image/Link to download the presentation.
The content on the website is provided AS IS for your information and personal use only. It may not be sold, licensed, or shared on other websites without obtaining consent from the author. Download presentation by click this link. If you encounter any issues during the download, it is possible that the publisher has removed the file from their server.
E N D
Presentation Transcript
Process Capability The natural variation of a process should be small enough to produce products that meet the standards required A process in statistical control does not necessarily meet the design specifications Process capability is a measure of the relationship between the natural variation of the process and the design specifications
Process Capability Ratio Upper Tolerance Limit (UTL) - Lower Tolerance Limit (LTL) 6 Cp= A capable process must have a Cpof at least 1.0 Does not look at how well the process is centered in the specification range Often a target value of Cp= 1.33 is used to allow for off- center processes Six Sigma quality requires a Cp= 2.0
Process Capability Ratio In GE insurance claims process, Process mean x = 210.0 minutes and the process standard deviation is 0.516 minutes. The design specification to meet customer satisfaction is 210 3 minutes. So the upper specification (the Upper Tolerance Limit) is 213 minutes and the lower specification (the Lower Tolerance Limit) is 207 minutes. The manager wants to compute the process capability ratio. UTL - LTL 6 Cp= 213 - 207 6(.516) Cp= =1.938
Process Capability Index The simple Cp measure assumes that the average of the process variation is at the midpoint of the specification range. Often the process average is offset from the specification range. In such cases, one-sided capability indices are required to understand the capability of the process UTL X Upper one-sided index Cpu = 3 X LTL Lower one-sided index Cpl = 3
Process Capability Index Sometimes only the lower of the two one-sided indices for a process is used to indicate its capability (Cpk): Cpk = min( Cpu , Cpl ) Acapable process must have a Cpkof at least 1.0 A capable process is not necessarily in the center of the specification, but it falls within the specification limit at both extremes
Process Capability Index In a process of filling boxes of rice where we measure the weight of each box, the process average is 210 g and the specification range is between 198 g and 214 g and the standard deviation of the process is 2 g. Calculate the Process Capability Index. The process is not capable and therefore cannot meet specifications
Process Capability Index You are the process improvement manager and have developed a new machine to cut insoles for the company s top-of-the-line running shoes. You are excited because the company s goal is no more than 3.4 defects per million and this machine may be the innovation you need. The insoles cannot be more than + or 0.001 of an inch from the required thickness. You want to know if you should replace the existing machine, which has a Cpkof 1.0 New process mean x = .250 inches Process standard deviation = .0005 inches Upper Specification Limit = .251 inches Lower Specification Limit = .249 inches
Process Capability Index New process mean x = .250 inches Process standard deviation = .0005 inches Upper Specification Limit = .251 inches Lower Specification Limit = .249 inches .250 - (.249) (3).0005 (.251) - .250 (3).0005 Cpk= minimum of , Both calculations result in New machine is NOT capable .001 Cpk= = 0.67 .0015
Interpreting Cpk Cpk= negative number Cpk= zero Cpk= between 0 and 1 Cpk= 1 Cpk> 1 LTL UTL
