CPK – HOW TO CONTROL THE CAPACITY OF A PROCESS

 

If you are interested in CPK values, it means that your end customer is expecting you to produce with a certain constant level of quality over time, with no surprises.

In fact, through this method you can determine how to control the capacity of a process in order to prove, first to yourself and then to your end customer, that you have control over the process of statistical quality control.

If you fail to introduce this measurement method, you will not know how your production process is behaving. And if your are one of the suppliers to the automotive industry, you are likely to incur incalculable damage.

A supply of components machined to the customer’s specifications that turn out to be non-conforming, can lead to damages of a few hundred million euros.

IF YOU SUPPLY COMPONENTS TO THE AUTOMOTIVE SECTOR WITHOUT HAVING IMPLEMENTED A STATISTICAL PROCESS CONCERNING QUALITY,

YOU ARE AT RISK OF INCURRING LOSSES FOR MILLIONS OF EUROS.

Here is a practical example which you have certainly heard or read about somewhere: when a carmaker is forced to recall a large number of cars in order to replace a component that may cause problems, the reason for this is often a CPK value that a supplier failed to comply with.

The operations concerning the mass resolution of non-conformity problems cost the carmakers millions and millions of euros, which the carmaker immediately charges back to the suppliers that produced the non-conforming product.

This is why you need to implement this type of statistical check if want to become a supplier to the automotive sector.

The trend I have noticed in recent years is that even companies outside the automotive sector have started to demand these CPK values in order to avoid problems with the final product and improve quality.

I must say that this system works very well.

Once it is implemented, it actually keeps the process under control, providing numerical data that allow you to understand right away how you are producing and if something is about to take a turn for the worse.

It is a system that can warn you, almost in real time, that you are falling outside certain quality parameters.

By implementing this system, you avoid producing thousands of pieces only to find out later on, during the assembly or final test phase, that problems occurred during production.

Although this method is usually disliked during the initial phase, it is nevertheless capable of significantly reducing the cost of quality, since it allows you to work on the process right away in order to quickly correct the problem, thus avoiding higher costs resulting from having to scrap thousands of pieces in addition to problems with your customers, which in some cases entail the payment of production penalties.

WHAT IS THE CAPACITY OF A PROCESS?

 

After this initial introduction, let’s get into the details in order to better understand what exactly is “the capacity of a process”.

As usual, since I am a very curious person by nature, the first question I asked myself years ago was this: why should I bother studying the capacity of a process?

At first I was a little suspicious that this was one of those temporary fads thought up by some career-oriented Quality Manager who, in order to climb up the corporate ladder, was willing to make things difficult for the production department with impractical “university” methods not applicable to the real production world.

The doubt did not concern at all the validity of the system, but rather whether or not it could be applied, at a low cost, to the production department of small and medium size companies.

In fact, there was not doubt about the method being suited to aerospace companies or the like, as I had read examples of its success.

With these doubts in my head, I ran into an article by Ben Gunter (1997) written for the Quality Progress, in which he underscored how a company’s need is to understand how important and necessary continuous improvement is for the company itself.

“Continuous quality improvement” is interpreted as the continuous improvement in the quality of a company’s production processes.

In the mind of most individuals, the concept of “quality improvement” is associated with the quality of the product, and it is believed that this improvement can be obtained by using better materials, more services, and so on.

THIS WAY OF LOOKING AT QUALITY ACTUALLY CREATES A WRONG VIEW!

When we talk about “continuous quality improvement”, we refer to the improvement of production processes, the processes that produce your product, always with the same quality level, but improve the way in which you produce it, eliminating steps or simplifying the process itself, so that they do not generate costs but allow you to save money!

Why do they allow you to save money?

Because they derive from a reduction of that which, for a company, is considered “superfluous”, or MUDA for LEAN MANUFACTURING fans, and ultimately it costs less.

Some examples?

  • THOUSANDS OF NON_CONFORMING PRODUCTS PRODUCED
  • ASSEMBLIES THAT ARE COMPLETED TO THEN DISCOVER THAT THEY ARE DEFECTIVE
  • MOVEMENTS OF MATERIALS TO VARIOUS AREAS DUE THEIR BEING NON-CONFORMING
  • DISASSEMBLY OF UNITS IN ORDER TO FIX MISTAKES
  • LATE DELIVERIES TO CUSTOMERS
  • PENALTIES APPLIED BY CUSTOMERS
  • LOSS OF IMAGE ON THE MARKET

How much does all of this cost you? I am sure you are aware of this because, like everyone else, you probably already faced this type of problem, so you know that the cost is quite high.

WHAT ARE PROCESS CAPABILITY INDICES?

First of all, let’s define what we mean with the word CAPABILITY: “the ability in carrying out a task or achieving a result which is defined by the specifications requested by the customer”.

So, everything starts from the market, and this is something that I personally like a lot, since the market is truth, the compass to be followed, leaving aside many different theories or philosophies. In fact, the question that every serious company asks itself before taking on or accepting a job for a client is this: “Are we capable of meeting the needs of our customers?”

If you fail to ask yourself this question as a company, and your first instinct is to accept as many orders as possible, I have some bad news for you…

However, since you are reading this article, I will take for granted that you are not that type of company, also because here we are not referring to ISO standards or CPK values, but just common sense!

In any event, to this question that seems to be quite easy, the difficulties encountered have resulted in an avalanche of research and debates all over the world.

In fact, after searching the web I found out that this topic is deeply rooted in the past.

The first leader to use quality control and continuous improvement was the company Bell Laboratories which, in collaboration with Walter A. Shewhart (1931), considered to be the pioneer of quality control, introduced the Shewhart control charts.

THE FIRST LEADER IN QUALITY CONTROL WAS THE COMPANY BELL LABORATORIES (1931)

The Shewhart control charts are one of the first control tools developed from statistical theory for the purpose of monitoring the variability of production processes.

More recently, specifically in the 1980s, the FORD Motor Co. started to use CPKs to measure the performance of its processes.

In fact, it was in that period that the automotive industry experienced a huge improvement in terms of its quality control system.

In 1991, the three major American car manufacturers (General Motors, Chrysler, Ford Motor Co.) started to use at full capacity, for themselves and for their suppliers, process control and, consequently, capability indices.

These three carmakers worked together to draw up a reference manual for quality control, in order to standardize it regarding format, reporting and technical nomenclatures.

The result of this manual is called “AIAG Fundamentals Statistical Process Control Reference Manual”, a reference manual that, to this day, is still considered a guide in the automotive sector.

As you can see, indices are becoming more and more a standard for information concerning the quality of a company process and the company itself, crucial if a company wants to become and remain a qualified supplier.

As confirmed by these car manufacturers through the years, the success of quality indices can be measured in terms of millions of dollars that have been saved.

QUALITY INDICES LEAD TO SAVINGS IN THE RANGE OF MILLIONS OF DOLLARS!

Now that I have proved to you that the right path to follow in order to make your company grow is to map the process through the CPK method, even if you are not operating in the operating sector but are nevertheless interest, keep reading below, since I will explain the meaning of this value.

The following values are used as a guideline in order to understand the percentage of non-conformant products for a given value of the CPK index:

CPK 0.25 = 16 NON-CONFORMANT PARTS OUT OF 100

CPK 1.00 = 7 NON-CONFORMANT PARTS OUT OF 100

CPK 1.33 = 13 NON-CONFORMANT PARTS OUT OF 10,000

CPK 1.67 = 1 NON-CONFORMANT PART OUT OF 1,000.000

CPK 2.00 = 16 NON-CONFORMANT PARTS OUT OF 1,000,000,000

As you can gather from the table, the more the CPK value increases, the less scrap you can produce. On average, looking at many drawings of various components, one always starts from a basic CPK of 1.33. I have rarely seen any requests under this limit.

If CPK 1.33 is the starting point, CPK 1.67 is starting to be requested more and more, while in rare situations I have seen a request for CPK 2. So, wouldn’t it be nice if your company, on a lot of 10,000 pieces, would also be able to produce only 13 scrap pieces?

Or, better yet, by adopting a CPK of 1.67, only one scrap piece out of 1,000,000 pieces produced? This means 1 ppm (1 part per million!)

NOW THAT YOU HAVE FINALLY UNDERSTOOD WHAT CPK 1.33 AND CPK 1.67 MEAN, HOW CAN I APPLY IT?

As you can imagine, in order to achieve these objectives, one basic requirement is to have production machines that are reliable and, even more importantly, repeatable.

If the machine is repeatable and constant in time, it means that once the fine-tuning is carried out through offset, your piece will be constantly produced within tolerance.

The MULTICENTER possesses these characteristics: it is a very repeatable machine that allows you to apply this method with great satisfaction, so that you can bring home new customers! A few weeks ago, we tested a MULTICENTER machine at our facilities.

According to the contract, we had to comply with a CPK of 1.67 on a BOSCH piece made of aluminium (remember that this means only 1 scrap out of 1,000,000).

Well, we managed to satisfy BOSCH and even exceeded expectations, recording a CPK of 1.85!

Now it’s your turn.. All you have to do is to write down the answers to these 3 questions, because I know that, so far, you have not written them down many times!

If you don’t write your answers down, you will quickly forget them. Remember, the results you achieve will be comparable to your actions.

WHAT WOULD CHANGE AT YOUR COMPANY IF YOU INSTALLED A MULTICENTER IN ORDER TO PRODUCE WITH A CPK OF 1.33/1.67?

…AND WHAT HAPPENS IF YOU KEEP USING THE SAME MACHINES YOU HAVE ALWAYS USED IN THE PAST?

 

WHAT WOULD BE THE RISKS FOR YOUR COMPANY IF YOU DID NOTHING AND SIMPLY POSTPONE YOUR DECISIONS?

 

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Maurizio Porta
Expert in Flexible Production