| Statistical process control can help identify | | | | light, less than 4 ounces, the electrical properties |
| problems, suggest corrective actions and reduce | | | | of the substrate could adversely affect |
| overall costs while preventing potentially serious | | | | component performance, leading to failures in the |
| issues. | | | | field-a very expensive, time-consuming problem. If |
| To assess pulmonary health, physicians routinely | | | | the substrate is too heavy, the material |
| order electrocardiograms (EKG) for their patients. | | | | "giveaway" will cost the company more money |
| Electrodes are attached to the patient's chest and | | | | than necessary. The financial risk of a substrate |
| signals from the heart are "listened to" by a | | | | being too light is greater than that of it being too |
| machine. A paper printout plots heart rhythm | | | | heavy, but both pose financial risks to the |
| over time, allowing doctors to identify peaks, | | | | manufacturer. |
| valleys and patterns in the heart's electrical | | | | Every 15 minutes, a single substrate is weighed |
| impulses. Using the printout, physicians can assess | | | | by operators. |
| heart health and uncover previously unknown | | | | Control charts typically comprise two different |
| problems. After issues are uncovered, doctors | | | | charts. The top chart shows the individual weights |
| can prescribe medications, suggest corrective | | | | through time. The bottom chart is called a moving |
| actions, and prevent or eliminate potentially | | | | range chart. This chart plots the raw deviations |
| serious health issues. | | | | between any two weights, thereby revealing |
| Now imagine applying an EKG to a company's | | | | weight variation. |
| most critical manufacturing processes. What if | | | | The initial control chart made during study one |
| electrodes could be attached to an assembly line, | | | | exposes useful information about weights: |
| or to the fill heads of a bottling process? Doing so | | | | 1. The moving range chart shows much less |
| would allow manufacturers to assess machine | | | | variation in weights between subgroups 26 and |
| health, know when it is "sick" and prevent | | | | 58. |
| potentially expensive problems. This technology is | | | | 2. Weights naturally cycle from high to low several |
| available for manufacturers today, and it is called | | | | times during the day. |
| statistical process control (SPC). | | | | 3. There is a slow, gradual trend in weights |
| Statistical Process Control | | | | between subgroups 25 and 60. |
| SPC is a data-driven methodology for analysis and | | | | 4. The overall average (mean) weight is 4.31 |
| improvement that quality professionals have been | | | | ounces. |
| using since the early 1900s. This scientific method | | | | 5. Because the expected weight is 4 ounces, on |
| of analysis involves graphing important | | | | average, nearly one-third of an ounce per |
| characteristics to determine trends that indicate | | | | substrate is "given away" in the study. |
| potential quality issues. In the past 20 years, | | | | The control chart patterns-cycles, trends, for |
| manufacturers have implemented automated SPC | | | | example-helped the implementation team better |
| systems so they can quickly and easily control | | | | understand start up, warm up and other issues |
| product and process quality on the shop floor. | | | | affecting the manufacturing process. After |
| SPC's prime objective is to provide information | | | | speaking with operators, engineers and hardware |
| about manufacturing processes in real time. | | | | vendors, the team made machinery modifications |
| Quality features and data values are plotted in | | | | and integrated procedural changes aimed at |
| time series on a chart. Like an EKG's printout, the | | | | minimizing weight fluctuation and variation. |
| result is a simple, visual graph that provides crucial | | | | After making the changes, more data was |
| information to an operator. With a better | | | | gathered during a second study. The |
| understanding of their production processes, | | | | post-improvement results reveal this information: |
| operators are able to make fact-based decisions | | | | 1. No clear patterns or trends are present. |
| to control their machines. Ultimately, this powerful, | | | | 2. No weights fall outside control limits. |
| real-time information allows better control of | | | | 3. The lack of patterns, trends and points outside |
| processes, prevents off-quality product from | | | | control limits indicates that no statistically |
| being produced and reduces overall costs. | | | | significant changes in weights have occurred in the |
| Control Charts | | | | second study. |
| At the heart of SPC technology is a graph called | | | | 4. The overall mean has been reduced to 4.16, a |
| a control chart. Control charts are used by | | | | savings of 0.15 ounce per substrate. |
| operators on the shop floor to help identify when | | | | 5. The variability in weights has been greatly |
| corrective actions are required. Control limits, or | | | | reduced compared to the first study. |
| decision limits, clearly indicate when corrective | | | | The second control chart made during study two |
| action is needed and, conversely, when it is not. | | | | indicates dramatically improved process control. In |
| Control limits should not be confused with | | | | addition, because thousands of substrates are |
| specification limits. Specification limits define a | | | | made each week, overall manufacturing and |
| window of acceptability based on customer needs | | | | material costs are significantly reduced. |
| or engineering requirements and are an indicator | | | | Controlling manufacturing processes is tough |
| of whether a product is good or bad. | | | | work, particularly when data analysis tools are |
| Case Example | | | | unavailable. However, SPC was specifically |
| A high-tech electronics manufacturer makes | | | | designed to help companies control processes |
| substrates for use in military applications. A new | | | | with simple, graphical methods of analysis. Like an |
| production line has just been installed for | | | | EKG for manufacturing, a robust SPC software |
| manufacturing 4-ounce substrates. Because of the | | | | can help identify problems, suggest corrective |
| extremely high cost of the materials used in the | | | | actions and reduce overall costs while preventing |
| substrate, the company wishes to carefully | | | | potentially serious issues. |
| control substrate weight. If the product is too | | | | |