How do you monitor and improve production stability with SPC?

1 What is SPC?

SPC stands for statistical process control, which is a way of using statistical techniques to measure and control the quality of a process. SPC involves collecting data from the process, plotting it on a chart, and applying rules to determine if the process is in control or out of control. A process is in control when it produces outputs that meet the specifications and expectations of the customer. A process is out of control when it produces outputs that deviate from the specifications and expectations, indicating a problem or a change in the process.

2 Why is SPC important for lean manufacturing?

SPC is important for lean manufacturing because it helps you to identify and eliminate sources of variation, defects, and waste in your production process. Variation, defects, and waste are the enemies of lean manufacturing, as they reduce the efficiency, quality, and customer satisfaction of your products and services. By using SPC, you can monitor the performance of your process, detect any abnormal or undesirable patterns, and take corrective actions to restore or improve the process stability. SPC also helps you to prevent problems from occurring in the first place, by enabling you to design and optimize your process based on data and facts.

3 How do you apply SPC to your production process?

To apply SPC to your production process, you need to define your process and its output, determine the specifications and tolerances for your output, select the appropriate SPC chart for your output, collect and plot the data on the SPC chart, and analyze the chart and interpret the results. Specifically, you need to identify the key characteristics or features of your output that affect quality and customer satisfaction (these are called critical-to-quality (CTQ) characteristics), establish the acceptable range or limits for your CTQ characteristics based on customer requirements (these are called upper and lower specification limits (USL and LSL)), choose a chart that matches the nature and measurement of your output (e.g., p-chart or c-chart for discrete or categorical output; x-bar and R chart or x-bar and s chart for continuous or numerical output), gather samples of your output at regular intervals and measure the CTQ characteristics, plot the data points on the chart and calculate the center line and control limits (the center line representing average or target value of your output; control limits representing expected variation or range of your output), look for signs of out-of-control or non-random patterns on the chart (points outside control limits, runs of points on one side of center line, trends of points in one direction, cycles of points with regular frequency, clusters of points with low variation). If no out-of-control patterns are found, it means that the process is in control and only subject to common cause or random variation.

4 How do you improve your production process with SPC?

SPC is not only a tool for monitoring and controlling your production process, but also a tool for improving it. You can use SPC to identify and implement opportunities for process improvement, such as reducing the variation and increasing the capability of your process. This is expressed by the indices Cp and Cpk, which measure the potential capability and actual capability of your process respectively. The higher the Cp and Cpk, the less variation and more capable your process is. Additionally, SPC can be used to reduce defects and increase quality in your output, which is expressed by DPMO (defects per million opportunities) and Sigma Level (number of standard deviations that fit between the center line and the nearest specification limit). The lower the DPMO and the higher the Sigma Level, the less defects and more quality your output has. By using SPC to identify sources of variation or defects, you can bring your process or output closer to target values and within specification limits.

5 How do you sustain your production stability with SPC?

SPC is not a one-time or occasional activity, but a continuous and ongoing practice that requires regular review and updates. You need to collect and plot data on your SPC charts at frequent and consistent intervals, recalculate the center line and control limits, and adjust them accordingly. It’s important to analyze and interpret the SPC charts for any out-of-control or non-random patterns that indicate a problem or change. You should investigate and identify the root cause of the problem or change, document and communicate the actions taken, verify and validate the effectiveness of the actions, and use the SPC data and feedback as a source of information for process improvement. Applying lean manufacturing principles and tools such as value stream mapping, 5S, kaizen, pokayoke, and standard work can help you enhance and sustain your process performance.

6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?