SPC Control Charts: How Healthy is Your Process?

By: Ben Baldwin

March 30, 2021

SPC Control Charts

Do you know if your process is healthy? Are you using SPC Control Charts to track and monitor the behavior of your processes?

Every manufacturing process generates a lot of numbers. From equipment and materials to specifications and tolerances, numbers are generated from every action performed by workers and equipment on the shop floor. These numbers are incredibly useful to understand key information about your process.

But many of the values within the data can seem random. So how can we decipher the data and use the number values to our advantage? How can we discern valuable statistical information over time from seemingly random data points?

The answer is with the Statistical Process Control (SPC) and a control chart.

When reviewing the health and effectiveness of your operation, tools like interactive SPC Control Charts are extremely effective. They help manufacturers monitor process behaviors over time and discover trends that could negatively affect the process and product.

Why Use An SPC Control Chart?

For any manufacturing process to succeed, it needs two crucial behaviors to be considered healthy:

  • Stability: Process is accurate and variations remain within specifications and tolerances.
  • Capability: Process can be sustained and consistently creates value.

Natural variations occur within any process, but we need these to stay within a certain range. If variations are unstable, then the risk of creating defects or deviating from customer requirements is high. This leads to a process that is not capable as it cannot produce value for the customer or sustain profitability.

An example of an unhealthy variation is a loose part within a machine. As time passes and the machine is used, the part will progressively loosen. This will affect the tolerances and the product will progressively deviate from the required specification range.

SPC control chart

To observe the stability and capability of your processes, SPC Control Charts are a visual and interactive tool that enables you to gain a key perspective on your process’s natural behavior. This is done by tracking a specific action or value over time.

But the power of an SPC Control Chart lies in its ability to gauge and predict future behavior based on past data. Trends, repetitions, or patterns within specifications can all be indicators of instability. If the behavior is leading towards (or already outside) warning and specification limits, then adjustments will need to be made.

Example #1: 6 Sided Die

A simplified example of this is with a 6 sided die.

There are 6 numbers that this die should present. If you roll the die one hundred times, you can expect a fairly even distribution between all the numbers. The natural variation and healthy behavior would give you a mean average value of 3.5.

But if at one point you roll a 7, you know something has gone wrong. The die has given a value outside of the specification limits. Similarly, if you throw the die and get 4 more often than not, then you know the die may not be performing as it should.

It is the same with tracking the specifications of a manufactured product. If the product created gives a value outside of specifications (in our case, anything outside of 1-6), then it is a defect. And trends that show an uneven distribution also indicate an unhealthy process.

But with an accurate history and real-time data, you can quickly identify problems, review trends, and find solutions. Recording key manufacturing data onto an SPC Control Chart greatly minimizes the risk of costly defects and machine breakdowns.

How to Read Your SPC Control Chart

Reading your control chart is very simple. The left axis is the recorded values and the bottom axis is the time or number of productions. In the case below, for Vacuum Mixer #1, values were recorded with each batch.

SPC control chart in VKS

As we’ve discussed, an SPC Control Chart is a graph used to study and visualize how a process progresses over time. The control chart also indicates the specification limits and warning limits of the process.

  • Specification Limits: Acceptable parameters that meet tolerances, production goals, & customer requirements.

  • Warning Limits: Determines and defines the stability of the process. Is the process under control?.

With these limits in place, effectively monitoring if a process is in control or outside of the limits is made easy for you and your team.

7 Out of Control Variations

To help you identify the different types of variations that are markers of an unhealthy process, here are 7 Out of Control Conditions.

  • Cyclical Effect: The control chart is repeating the same sequence.

Cyclical

  • Points Outside of Control Limits: The process is creating defects or unwanted variations.

Points outside of limits

  • Lack of Variability: Though the data points are close to the middle, there is not an even distribution of values.

lack of variability

  • Run: Consecutive values are occurring above or below the target average.

Run

  • Excessive Variability: Values are regularity at the fringes and rarely in the center.

Excessive Variability

  • Trend: Data points are forming a trend pointing to eventually exceeding specification limits.

Trend

  • Alternating Values: Values frequently alternate between a few data points.

Alternating Values

How your control chart is deciphered will heavily depend on your production environment. Perhaps there is a natural cycle to your process that is needed or benign to the quality of your product.

Statistical Process Control Charts with Work Instruction Software

Did you know VKS has an SPC feature? You can either use it for specific work instructions or a general process across multiple guidebooks. It is a simple and effective method to empower employees with procedural knowledge and accurately monitor the health of your processes.

While following their instructions, workers are prompted to enter key data about the process. This data is then recorded over time in the control chart and valuable information can be extrapolated to fix problems and continuously improve.

VKS SPC control chart form

The SPC control chart will also send notifications to operators and management if readings are out of ‘warning limits’ or (worse still) out of ‘specification limits. This enables the appropriate people to receive real-time data and respond accordingly in the least amount of time.

The interactive SPC Control Chart is easily accessible within the instructional guidebook and under the reports tab.

You can glide your mouse over the sample points and the chart will give you the information from that specific point in time.

SPC history

Example #2: Humidity in Controlled Environment

Let's say you operate a pharmaceutical lab testing the effectiveness of live cultures in beer yeast. Keeping the environment at an optimal humidity level is crucial.

At the beginning of every procedure, your employees are required to take a reading of the humidity to verify that it is within standards. Once this information is verified and recorded, they can begin work with the yeast.

Each time an operator enters in the humidity, the data is compiled into the SPC Control Chart within VKS. With an accurate history of variations in humidity, you can see if there are trends or unseen factors that could negatively affect the testing of the yeast.

When reviewing the chart you see a trend of higher humidity at the beginning of the week and lower humidity at the end of the week. The data points are still within specifications but, oddly, this cycle has been repeating itself over the past few weeks.

You investigate this issue and find out that there are different teams of people working at the two separate times in the week. And there is a slight variation between how each team checks the humidity. This is throwing off the humidity records and giving an inaccurate reading.

By using an SPC Control Chart, you have been able to remedy issues with process compliance. Now the humidity levels are consistently and accurately at a healthier level.

Is Your Process Under Control?

For added information, take a look at this video to see a real-life use of SPC Control Charts with work instruction software for manufacturing chemical process control.

The power of data is irrefutable. Knowing more about your process enables you to make better decisions and even predict future events. By implementing work instruction software with SPC Control Charts, you have greater control over your process, your products, and your quality.

The more you know, the healthier your process will be.

With contributions from Yasmine Djafari.

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