hardness tester

How Good is My Measurement System?

You just completed your ANOVA Gage R&R analysis. The results indicate that your measurement system is responsible for 40% of the process variance (GRR%). You look up the guidelines on the internet and this is what you see:

  • Less than 1%: The measurement system is acceptable.
  • Between 1% and 9%: The measurement system is acceptable depending on the application.
  • Greater than 9%: The measurement system is not acceptable and should be improved.

You are in trouble! 40% and it can’t be greater than 9%! What will your customer say? Should you...

test method

Gage R&R and Process Variation

This is the second in a four-part series on Gage R&R. The first blog explained what a Gage R&R study is . This blog examines the relationship between the Gage R&R results and the process variation and answers the question:

Is the measurement system capable of telling the difference between the parts or samples taken the process?

This is just another way of asking if the measurement system can be used to control the process. The basic equation describing the relationship between the...

What is a Gage R&R Study?

This is the first of a four-part blog on gage R&R. It answers this basic question:

What is a Gage R&R study?

If you google this question, you will get several answers including this one from the www.isixsigma.com dictionary:

“Gage R&R, which stands for gage repeatability and reproducibility, is a statistical tool that measures the amount of variation in the measurement system arising from the measurement device and the people taking the measurement.”

This statement is...

What is the Purpose of a Control Chart?

This is the third in a four-part series introducing control charts. The first blog addressed the question of what a control chart is. The second blog explored the relationship between variation and control charts . This blog begins to answer the following question:

What is the purpose of a control chart?

We will continue with the driving to work example shown in the last two blogs to explore three different purposes for a control chart....

common and special causes on a control chart

Variation and Control Charts

This is the second in a four-part series introducing control charts. The first blog addressed the question of what a control chart is . This blog will answer the following question:

What is variation and how does it relate to a control chart?

Understanding variation is the key to effectively using a control chart. A control chart provides a method for your process to communicate with you – to tell you if the process is doing what you designed it to do (only common causes of variation are...

What is a Control Chart?

This is the start of a four-part blog on control charts. The blogs will answer the following questions:

  • What is a control chart?
  • What is variation and how does it relate to a control chart?
  • What is a control chart used for?
  • Where is a control chart used?

Control charts are at the heart of statistical process control (SPC). So maybe we should start with answering what a process is. A process is simply what we do. It can be filling out an expense report, checking a person into a hospital, driving to work, filling a prescription, etc...

Cpk vs. Ppk: Who Wins? [video]

Cpk vs. Ppk: Who Wins? [video]

This week’s blog concludes our series on process capability and asks which is better: Cpk or Ppk?

Your supplier has sent you the process capability chart you requested. Looks like your supplier is really performing for you. You note that his process has a Ppk = 1.14 and a Cpk = 2.07. Why are those different? Well, it doesn’t matter. The Cpk is above 1.33, which is what you asked the supplier for. You just missed a very important piece of information about your supplier’s performance. Know what it is?

The accompanying video explores Cpk and Ppk in more detail.

The...

in control range chart

What is Ppk?

Our first two blogs in this process capability series answered two questions: What is process capability? – and - What is Cpk? This blog answers the next question: What is Ppk?

The answer is quite simple. Just to refresh your memory, Cpk is expressed as the following:

Cpk = Minimum (Cpu, Cpl)

Cpu=(USL- X )/3σ

Cpl=( X -LSL)/3σ

where Cpu is the capability based on the upper specification limit (USL), Cpl is the capability based on the lower specification limit (LSL), X...

What is Cpk?

What is Cpk?

Our first blog in this process capability series explored the simple question “what is process capability?”. There were two parts to the answer:

Process capability is the range of values that you can expect to get from the process over an extended time period.

A process is capable if the range of expected values fall within the specification limits.

Since you are comparing two ranges – the range of output from the process to the range of the specs - it is natural to ask questions about how much these two ranges are different or overlap. This gave rise to numerical...

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