Use Statistical Process Control Run Charts to Reduce Manufacturing Scrap and Rework
Statistical process control (SPC) run charts, also called pre-control charts (PCC), are an important quality tool to prevent the shop floor operator from producing scrap and rework. By plotting and monitoring quality data on a run chart, you can quickly see whether the manufacturing process is producing product within the customer specifications. This simple but important technique is not a replacement for more sophisticated control charts or capability charts. Instead, the run chart tool can help you build a positive long-lasting reputation for your ability to deliver products that satisfy customers.
What Does the Pre-Control Chart, Also Called Run Charts, Monitor?
The pre-control chart monitors whether a product is being produced within customer upper and lower specification limits. Collecting this quality data helps shop floor operators manage the process so defective parts are not produced – eliminating scrap and rework. Run charts provide a visual indicator that your customer will be satisfied with the products that are being produced. In other words, the product is produced to the customer requirements.
Pre-Control Charts, Also Called Run Charts, Are Based on Tolerances – Not the Process Variation
The text below is an excerpt from The Book of Statistical Process Control by Zontec. This second edition book, written for the non-statistician, serves as a guidebook for statistical process control.
The run chart, or pre-control chart, provides a way to study the stability of a process and to detect process trends. The goal of the run chart is the prevention of non-conforming observations. It is based on tolerances — not the process variation; therefore great care must be used regarding how this chart is used and interpreted.
Because it reflects the process over time, run chart data is plotted on the chart in the order that it was produced. The X-axis of a run chart is usually a simple count of the data, with the first entry being 1, the second 2, and so on. The Y-axis shows the value of the data. Figure 4.12 shows a run chart.
All test observations are classified into one of three groups: green, yellow and red to correspond with good, questionable and poor quality parts. For this reason, run charts are sometimes referred to as stoplight control charts. Decisions to stop and adjust the process or to continue operation are based on the number of green, yellow and red data points occurring in a small sample.
To initiate the run chart for pre-control, Operators must first produce five consecutive parts in the green zone. The following rules then apply:
- Two data points in the green zone — continue to run the process
- One data point in the green zone and one data point in the yellow zone — continue to run the process
- Two yellow points in a row (same zone) — adjust the process
- Two yellow points in a row (opposite zones) — stop the process and investigate
- One red data point — stop the process and investigate
Each time the process is adjusted, before the sampling can start again, the process must produce five consecutive parts in the green zone.
Pre-control should not be used when the process shows a Cp or Cpk greater than one or a loss function that is not flat within the tolerances.
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The SynergySPC Software Run Chart Tool Gives You Peace of Mind and Real-Time Alerts
The red lines represent the upper and lower customer specifications.
Run charts are a powerful diagnostic tool because they signal when processes are unable to achieve customer specifications. SynergySPC takes this basic technique and combines it with technology to provide timely and insightful information that leads to customer satisfaction. Our software functionality gives you confidence of the actual production status in real-time. As data is entered, real-time run charts containing red, yellow, and green, color-coded visual indicators are drawn. If an out-of-spec data point is entered, real-time alerts display for the operator. This interruption requires evaluation of the data before continuing with the next measurement. At the same time, automatic emails/text messages are sent to notify select quality managers. When managers access the run charts, the color-coded data indicates overall data trends, at-a-glance, while the detail is available with one simple click. This quick access to top line and detailed information allows quality managers to visualize problems and put corrective actions in place. With these unique features, SynergySPC software gives you freedom and peace of mind.
The run chart is a tool for measuring your product conformance to customer requirements. The next steps to quality improvement include applying Control Charts and Capability Charts to optimize your processes. These SPC tools will identify more opportunities to remove waste, lower your cost of goods, and achieve higher profits.