Process capabilities

Process capability measures how well a process can perform to the specification set for it. The measures of process capability are statistical. To many, dealing with statistics is a challenge. This overview, plus a simplifying book by Warren Bmssee, should help. However, being aware of the language of variation is no longer optional for anyone involved in SCM. 

The specification is an important part of the buyer-seller relationship. Too tight a specification means the seller must go to extraordinary means to meet the specification. It could also lead to scrap and excessive material cost. Sometimes, specifications are set without consideration of the manufacturing or other processes capabilities. Too loose a specification, on the other hand, spells trouble when the seller s components go into the buyer s product. 

The statistics for quality have been around for a long time. Common performance measures include Pp, Pp, Cp, Cp first-time capability (FTC), line speed, and defective parts per million (ppm). This section provides an overview of common process capability terms and addresses their importance in supply chain management. 

The standard deviation takes the difference between each observation and the observation mean, squares it to remove plus and minus values, then divides by the number of observations, and finally takes the square root of the result. Both means and standard deviations can be calculated with spreadsheet programs. 

However, this kind of reliability is not enough when one considers that there are many processes required to produce a product. Even a three sigma process capability will produce 3000 defects in a million tries. With many processes required to make a product, at three sigma levels there will be many defective products. A six sigma capability, on the other hand, means that only 3.4 outcomes in a million attempts are out of specification. 

We ll illustrate the concepts with an example of a part that must be produced to a certain specification. The concepts described also apply to attributes. Attribute measures are yes, it passes or no, it doesn t pass  

The shapes take on the familiar normal distribution, or bell curve. In this example, A has the widest distribution as shown by its fat curve. The fatness is expressed statistically by the standard deviation. Higher standard deviations reflect fat distributions, which increases chances a part will be defective. Normal practice is to assure that the variation in the process is low enough so that a six standard deviation spread is well within specified tolerances. 

For example, let s assume the six standard deviation spread extends from 0.85 to 1.15 inch, a spread of 0.3 inch. From this information, we can produce a performance measure called C. In this example it is 0.2/0.3 or 0.67. C, is 

Sometimes only a small sample is available. This happens in the case of a new component or a new process. In these cases the sample may be small and the process rmtested under high-volume conditions. So managers will calculate and using pilot production samples. In these cases, a higher thresh- 

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The process capability for FPSOs is up to some 100,000 barrels per day, with storage capacity up to 800,000bbls. 

From Synthesis Gas. A rhodium-catalyzed process capable of converting synthesis gas directly into acetaldehyde in a single step has been reported (83,84). 

Isobutyl alcohol [78-83-1] forms a substantial fraction of the butanols produced by higher alcohol synthesis over modified copper—zinc oxide-based catalysts. Conceivably, separation of this alcohol and dehydration affords an alternative route to isobutjiene [115-11 -7] for methyl /-butyl ether [1624-04-4] (MTBE) production. MTBE is a rapidly growing constituent of reformulated gasoline, but its growth is likely to be limited by available suppHes of isobutylene. Thus higher alcohol synthesis provides a process capable of supplying all of the raw materials required for manufacture of this key fuel oxygenate (24) (see Ethers). 

Examine the processing equipment to determine what processing capabilities are possible, and write phases to accomphsh each possible processing capability. 

Have several final product parameters been shown to have process capability (Cpk) >1 ... 

Process Capability (Cp) The ratio of the specification range to six standard deviations of the process. 

Statistical Process Control (SPC) The use of statistical techniques (such as control charts) to analyze a process and take appropriate action to maintain statistical control and improve process capability. 

Variation is an obvious measure for quality of conformance, but it must be associated with the requirements set by the specification to be of value at the design stage. Unfortunately, difficulty exists in finding the exact relationship between product tolerance and variability. Approximate relationships can be found by using process capability indices, quality metrics which are interrelated with manufacturing cost and tolerance (Lin et al., 1997). ... 

The first concern in designing process capable products is to guarantee the proper functioning of the product, and therefore to satisfy technical constraints. Dimensional... 

It is recommended at this stage of the text that the reader unfamiliar with the basic concepts of variation and process capability refer to Appendix I for an introductory treatise on statistics, and Appendix II for a discussion of process capability studies. 

From the above discussion, it follows that the quality and conformance to tolerance of the product characteristics should be designed in and not left to the process engineer and quality engineer to increase to the required level. In order to do this, designers need to be aware of potential problems and shortfalls in the capability of their designs. They therefore need a technique which estimates process capability and quantifies design risks. 

In the next chapter, we introduce the concepts of component manufacturing capability and the relationships between tolerance, variability and cost. The Component Manufacturing Variability Risks Analysis is then introduced, the first stage of the CA methodology, from which process capability estimates can be determined at the design stage. The development of the knowledge and indices used in the analysis... 

Figure 2.1 Example of process capability improvement (adapted from Leaney, 1996a)...

Initially, knowledge of the chosen manufacturing process is required. If the type of material is stated on the process capability map, assume m =1, otherwise proceed to A. 

Values of B relating to material compatibility for those processes where a material Is not defined on the process capability maps given (2) ... 

There is a material to process compatibility risk for impact extrusion, cold forging, cold extmsion, sheet metalworking, machining and powder metal sintering processes because their respective process capability maps relate to the ideal material case. 

SAND CASTING PROCESS CAPABILITY MAP FOR ALUMINIUM AND MAGNESIUM ALLOYS... 

IMPACT EXTRUSION PROCESS CAPABILITY MAP (RADIAL TOLERANCES ONLY)... 

A target process capability value, Cp = 1.33, is aligned to the risk value at = 1.7. Values for "A greater than 1.7 indicated on the maps continue with the squared... 

Figure 2.11 Employment of the tolerance data In the generation of a process capability maps...

Constructing process capability maps from manufacturing data... 

Figure 2.13 Process capability map construction flow chart...

The critical characteristic on each component was analysed, calculated from the analysis and the value obtained was plotted against the process capability indices, Cpk and Cp, for the characteristic in question. See Appendix V for descriptions of the 21 components analysed, including the values of Cp and Cp from the SPC data supplied. Note that some components studied have a zero process capability index. This is a default value given if the process capability index calculated from the SPC data had a mean outside either one of the tolerance limits, which was the case for some of the components submitted. Although it is recognized that negative process capability indices are used for the aim of process improvement, they have little use in the analyses here. A correlation between positive values (or values which are at least within the tolerance limits) will yield a more deterministic relationship between design capability and estimated process capability. 

Note that the squared relationship which was initially used to model the degree of difficulty in obtaining more capable tolerances for a given manufacturing route and product design is being returned by the power law. Similarly, a relationship between the process capability index Cp and q for the components analysed is shown in... 

A standard for the minimum acceptable process capability index for any component/characteristic is normally set at = 1.33, and this standard will be used later to align costs of failure estimates. If the characteristics follow a Normal distribution, Cp = 1.33 corresponds to a fault probability of ... 

While 30 ppm may be acceptable as a maximum probability of occurrence for a failure of low severity, it is not acceptable as severity increases. An example table of FMEA Severity Ratings was shown in Figure 2.20. In the definite return to manufacturer (a warranty return) or violation of statutory requirement region (S = 5 or S = 6), the designer would seek ways to enhance the process capability or else utilize some inspection or test process. Reducing d will reduce occurrence, as indicated by equation 2.11, but inspection or test is of limited efficiency. 

Determine the potential process capability associated with the component and/or assembly characteristics... 

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