What is so special about the Special Characteristics?
The burden with Special Characteristics in the FMEA. Special Characteristics (SC) are a constant nerve wrecker in the company. You can often see two fractions. One fraction favours SC and would like to use them to secure the results for the customer in the best possible way. The other avoids SC wherever possible. On the question of what can a reasonable balance of these extreme positions look like, the focus is on the meaningfulness of specifications and decisions. What is necessary to be in control of the risk to the company without losing the competitiveness? What are the legitimate SC requirements and what is anyway so special about a special characteristic?
Technology decides Special Characteristics
Special Characteristics are a quality issue. After all, their specification and pursuit is required in quality norms, not in technical norms. The question of meaningfulness is thus not asked, rather, is taken for granted. The VDA mentions two important points in its statements for delineation of Special Characteristics from normal characteristics:
- Not every characteristic is a special characteristic.
- Special Characteristics are needed as product and process characteristics
FMEA as a decision-making method for Special Characteristics?
One approach is the specification of SC by the FMEA. This correlation is described in many companies as a part of the FMEA standard. Some require the deduction of SC from the FMEA internally and by its subcontractors. The exact consideration of how to proceed with a FMEA shall not be explained in detail here. Instead of it, we shall directly look at the possible results of an FMEA to specify the SC:
- If the decision is made based on the meaning alone, there are important and critical characteristics, and those across all levels from system up to process parameters.
- A decision is made by evaluating the appearance on whether it is no longer necessary to run a characteristic and the possible error as SC. The specification is no longer the driver here, it is the reaction.
The meaning of a characteristic is necessary and significant information, to be able to specify Special Characteristics meaningfully.
Technological Criteria
Special Characteristics make clear what is important for the customer. This can relate to subjective expectations as well as objective requirements. At the latest when the technical, haptic and visual requirements are laid down such that they can be evaluated and reproduced, we speak about technological know-how.
Technologists know the correlations between cause and effect. You can answer two key questions from experience or with the help of statistical evaluations of the present data:
- What additionally influences the result?
- How much is the influence?
It can be determined from each cause whether their effect is big or small, reproducible or just incidental. If the functional correlation of cause and effect is essential for the function and is clear, then this cause is a main characteristic and hence justified by a technological characteristic. And only when the consequence is a special one does the characteristic become a main characteristic.
Only a main characteristic can become a Special Characteristic.
A third role is played by whether Special Characteristics have been requested by the customer at all. If that is not the case, the supplier can specify SC, but does not have to. Therefore a third condition is:
Special Characteristics have to fulfil a Special Characteristic at a higher level.
What must be the ability of a Special Characteristic be?
The work must be worth determining the main characteristics with the developers for the product as well as process characteristics. The quality requirement that they must fulfil must be clarified for at least these main characteristics:
Such quality classes allow the expense for the next level to be greatly lowered in comparison to a comprehensive cpk-requirement without risking the result. The costs for Special Characteristics will then be lowered; the acceptance of Special Characteristics will rise.
Help by Standards
The technological differentiation of the cause-effect-relationships from the design and process FMEA by main and normal characteristics is fairly expensive. Even the qualified allocation of quality classes means expense. But, this expense can be standardised for recurring topics and process steps. The written sets of rules similar to the ISO and DIN norms are an option for standards.Standard FMEAs are closer to the application in everyday operation. These FMEAs can get the status of a norm in the company and bindingly illustrate the technological specifications in the company. In the product development, standard FMEAs lend themselves for product families. Individual product steps or full production sequences can be described in this way for process planning.
Dr Uwe-Klaus Jarosch is senior expert for QM methods and tools in the global quality management of the Benteler Automobiltechnik GmbH, Paderborn. In the FMEA info and tips in our FMEA Think-tank, he describes an example along a specific problem definition for Special Characteristics.