{"id":49,"date":"2022-07-21T15:17:33","date_gmt":"2022-07-21T15:17:33","guid":{"rendered":"https:\/\/pressbooks.palni.org\/spcleanmanufacturing\/?post_type=chapter&p=49"},"modified":"2022-11-02T11:01:16","modified_gmt":"2022-11-02T11:01:16","slug":"chapter-11a","status":"publish","type":"chapter","link":"https:\/\/pressbooks.palni.org\/spcleanmanufacturing\/chapter\/chapter-11a\/","title":{"raw":"Chapter 11: Is the Process Capable?","rendered":"Chapter 11: Is the Process Capable?"},"content":{"raw":"

Chapter 11: Is the Process Capable?<\/h2>\r\n\r\n
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A Capable Process<\/h2>\r\n[h5p id=\"35\"]\r\n\r\n \r\n\r\n[h5p id=\"49\"]\r\n\r\n \r\n\r\nIn order to understand capability, it is first important to understand the primary goal in SPC and the inherent nature of variability.\u00a0\u00a0\u00a0 We know that all manufacturing processes will produce parts which vary in any given dimension.\u00a0 This normal variation is called common cause or \u201csystem\u201d variation.\r\n\r\nOur goal is SPC is to create statistical control charts which are based on this system variation and allow us to visually see when the process changes or shifts.\u00a0 So create the charts, we must first study the normal process by doing the following:\u00a0 gather sufficient data for each specified measurement on the product, create a histogram of the data.\u00a0 This histogram data identifies the range of sizes produced by the normal process.\u00a0\u00a0 We previously learned that the peak of the histogram curve represents the center of the data (and the process).\u00a0 And the range between + 3 sigma and -3 sigma from this peak represents 99.73% of all the possible parts produced under normal production running.\u00a0 In manufacturing, we calculate the + 3 sigma and -3 sigma values and call them the process upper and lower control limits.\r\n\r\nEngineering typically establishes the required dimensions to create a product that fits and functions as desired by the customer.\u00a0 These dimensions are called the product specification limits.\r\n\r\nSo, to determine if a process can consistently meet the customer requirements, we can compare the histogram data to the product specification limits.\u00a0 This comparison determines the Process Capability.\u00a0 It is a comparison of \u201cwhat the process actual produces\u201d to \u201cwhat is needed\u201d for a quality product.\r\n\r\nIdeally, a buffer zone will be established between the control limits and the specification limits. This means that the upper specification limit is greater than the upper control limit and the lower specification limit is less than the lower control limit.\u00a0 Why is this important?\u00a0 We will use the control limits to construct run charts for manufacturing and rules for adjusting the process.\u00a0 However, if the specification limits and the control limits are identical, the process could still produce a random part .27% of the time which is an out-of-specification part (100-99.73% = 0.27%).\u00a0 Also, if the process shifts at all, it will begin making out-of-specification product.\u00a0 If there is a sufficient buffer zone then small shifts can be identified and corrected BEFORE out-of-specification product is ever produced.\u00a0 This is what makes Statistical Process Control a PREVENTION TOOL.\r\n\r\nThe size of the buffer zone is a strong indictor of how capable the process is in producing in-specification parts. The larger the buffer zone, the greater the process capability.\u00a0 The buffer zone impacts scrap, rework, and sorting for bad material costs (COPQ). The buffer zone and process capability in general also determine equipment and tooling needs.\u00a0 In addition, as process capability increases inspection frequency can be reduced. to long-term sampling rates.\r\n\r\nThere are several capability indices commonly used to communicate process capability.\u00a0 Cp and Cpk are the most common.\r\n\r\nTo determine Process Capability:\r\n
    \r\n \t
  1. Estimate sigma or standard deviation.<\/li>\r\n \t
  2. Calculate the process control limits at +\/- 3 sigma.<\/li>\r\n \t
  3. Calculate Cp and\u00a0Cpk.<\/li>\r\n \t
  4. Evaluate Cp and Cpk to the required goals.<\/li>\r\n<\/ol>\r\n

    Calculations:<\/h3>\r\nCp = (Total Allowed Variation)\/ (Actual Process Variation) = (Upper Specification Limit \u2013 Lower Specification Limit) \/ (Upper Control Limit \u2013 Lower Control Limit)<\/strong>\r\n