What means CP, CPK and PPM?
The meaning of Cp and Cpk
Cp and Cpk are capability index. They both characterize the performance of a process by representing the ratio of specification limits (SL) to process capabilities. Both indices assume ±3 Sigma manufacturing process capabilities:
Cp = SL / (3·sigma)
Cpk = MAX [(Mean-LSL) / (3·sigma), (USL-Mean) / (3·sigma)}
They represent the number of times the spread of the process can fit into the tolerance width. The higher the value of Cp, the better the process. Some indication of their values and corresponding interpretations:
- Cpk < 1.0 : Indicate the manufacturing process cannot reliably meet the design goals
- Cpk > 1.33 : Good performance of the process
- Cpk > 3 : Loose specification of limits
The difference between the process capability index is that unlike Cp, Cpk takes into account the possibility that the process mean may be shifted from the mean of the design limit range.
Cp and Cpk in Enventive tolerance analysis
In Enventive Concept, all the contributors have their respective input Cp values, while the output of the study is given in term of Cpk.
When calculating Cp and Cpk, Enventive Concept treats all contributors as if they were at their process limits. The contributors are in turn used to calculate the resultant Sigma range for the statistical analysis. As a result, it is important to verify that the tolerances on all contributors are greater than or equal to the process limits.
add a screenshot of the report here
What is the notion of Target CP into Enventive?
In Enventive, the Stack Target Cp values are automatically set according to the upper/lower Severity settings. The Stack Target Cp values can be set directly for individual parameters by changing the type from Default to User Defined. You can set defaults for the Cp values that are applied to dimensions and contributors in the “Set Model Default Tolerance” table.
The Stack Target Cp represents your objective for the analysed dimension. It is what you want to achieve to meet client’s requirements for example.
What Cp should we use when we have no information on our process capabilities?
To be completed
The meaning of PPM
The PPM (Parts per Million) is a quality performance indicator extensively used in the manufacturing industry. The failure rate is the number of times per million the design will be out of tolerance. The report displays the failure rate, in parts per million (ppm), at a specific Sigma value.
HOW ARE PPM DEFINED?
At the beginning of any project, the customer expectations are turned into measurable data. This data obtained during the functional analysis phase is stored in the FMEA. From the FMEA an objective PPM rate is set for each functional criteria. The PPM defines how many Parts per Million are tolerated to be outside specification by the customer.
So the use of PPM assumes a statistical approach on the product and process dispersion contributors. We refer to it as Part Per Million but we focus on the product functionnal criteria outside specifications not only on parts.
The higher the stack severity is the lower the PPM must be.
[Paul: I think you wrote that. Please review] We can also note that in some industries, depending on how much outside specification the function is, it can addtionnal PPM requirements. E.g.: On a single functional criteria we may have multiple conditions. For example we may allow a PPM rate of 100 on 10+/-0.1 because it has an aesthetical impact and then a PPM of 10 (out of the 100) above 10+/-0,5. Because above 10,5 or below 9.5, not only it is less aesthetical but it also because dangerous for the user.
ARE PPM ALL IDENTICAL FOR CUSTOMERS?
Not necessarily. Two final customers can purchase products that are 95% identical with the same expected usage. However, they can end up with different expectations on the functions severity and as a result in terms of PPM. So a PPM can slightly vary from customer to customer even if the product is the same in term of usage.
ARE PPM EXPECTATIONS NECESSARILY IDENTICAL ON THE UPPER AND LOWER SPECIFICATION LIMITS ?
Not necessarily. To be completed
WHAT IS THE CONNECTION BETWEEN SPC TOOLS AND PPM
SPC (Statistical process control) tools use statistical techniques to control a process or production method. They can help you monitor a process and find solutions for production issues. SPC tools commonly used include Control Chart, Pareto chart and Histogram.
SPC tools are key to confirm that the expected level of PPM on the upper and lower specification limits of each functional criteria are achieved.
To be completed
Picture to be added
PPM usage in Enventive variation analysis
Enventive Concept characterizes the performance of the process by publishing the “Actual Cpk”, the “Failure rate” and the “% in tolerance”. The failure rate is the number of times per million the design will be out of tolerance. It is expressed in “ppm” (Parts Per Million). The percent in tolerance is simply the opposite of the parts per million failure rate (1,000,000 – Failure Rate = Percent in Tolerance).
Does changing a Target Cp impact a ppm rate?
No. The target Cp is what you want to achieve. It’s an input set by the user. Fail Rate (ppm) – as well as % in Tolerance and Cpk are output values calculated by Enventive.
To be completed
What CPK is acceptable for my functional condition?
That’s exactly the type of question you are able to answer with Enventive Concept report. By levergaing the ppm, Target CP,… info you are able to.. To be completed
The meaning of Cpm
Cp and Cpk capability indices have already been defined. They are the most commonly used, but other indices like Cpm also exist. The Cpm index automatically takes into account the bias from the target. The bias is simply the difference between the process average and the process target.
We should probably expand this section even if not used in Enventive
Cpm usage in Enventive variation analysis
The Cpm indice isn’t displayed in Enventive Concept report because it is used less frequently
It could be easily manually added in Enventive’s spreadsheet using the standard deviation reported by Enventive.
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