An important success factor of the continuous tolerancing work within the general car development process is coordination of all stakeholders involved in the process and interdisciplinary communication.
The common objective should be satisfaction of final customer’s requirements for the whole vehicle product in terms of functionality, reliability and appearance, all that with due regard to cost effectiveness.
This chapter provides an exemplary illustration of how the process of tolerancing geometric features in the general vehicle development work could look like.
Step 1: Selection of qualitative characteristics
The research starts from the vehicle as a whole. Relevant geometric features having a significant effect on functionality, reliability and appearance of the entire vehicle are identified.
Those features will be then grouped under the umbrella term “qualitative characteristics”. In addition to such functional aspects, also the testing process should be taken in account so that a qualitative characteristic may be measured at a later time in the series manufacturing.
The qualitative characteristics so identified will be then documented in a suitable manner, e.g. in a characteristic catalogue.
Step 2: Tolerance analysis of a complete vehicle
The complete vehicle will be decomposed into large system components – ideally such that they correspond to assembly groups. For the complete car, this could be for instance the rough design of the bodyworks, engine, chassis, lamps, driver compartments, doors, roofing system of the front compartment, rear module, etc.
Using tolerance simulations at the complete vehicle level, tolerance specifications for qualitative characteristics of individual components will be now fine-tuned. The components as such will be further examined only as “black-boxes”.
To do that, we need the tolerance specifications identified in Step 1 on the one hand, and components’ datum systems as well as information about assembling and coupling concepts for the components in the complete vehicle on the other.
Already at this stage there is a significant potential for optimization through balanced fine-tuning of tolerance specifications for qualitative characteristics of components, and through possible improvement of the assembling and coupling concepts. Results, together with appropriate tolerance simulations, and the underlying assembling and coupling concepts are documented in a suitable form.
Schematic representation of system boundaries of a complete vehicle:
complete
Figure 130: System boundaries of a complete vehicle
Example of a part and a component in a complete vehicle:
Step 3: Tolerance analysis at the component level
Step 3 will be carried out by different persons who are responsible for the relevant components. They may be system suppliers.
The tolerance specifications identified in Step 2 for qualitative characteristics of components must be verified through the tolerance simulation.
It often happens that tolerance specifications of a component characteristic cannot be met.
In such case, the first step is to look for optimization possibilities before the problem is escalated to the complete vehicle level for resolution.
In addition to externally specified tolerances at system boundaries, there is, naturally, a range of qualitative characteristics within a component that require a review by the person responsible for the component.
Figure 131: A part and a component in a complete vehicle
Step 4: General dimensioning concept
The tolerancing process is closely interrelated with other processes involved in the general car development.
They are particularly the following :
- Preparation and approval of a gap and radius plan
- Documentation of datum systems/reference point systems in drawings - Documentation of geometric tolerances in drawings or CAD models - Identification of special characteristics in drawings
- Designing manufacturing and assembling tools
- Planning prototype, pilot run ad series manufacturing tests - Arrangement of the testing process
The objective of Step 4 is to ensure loss-free, insofar as possible, assessment of the acquired learning for the purposes of the above development processes. It requires intensive information sharing with relevant entities involved in the process.
Losses of information are as lesser as better are the entities participating in the development engaged in the Simultaneous Engineering work. The objective of the tolerancing process systems, which is pursued across all project stages, can be best attained with suitable team structures and actively cooperating partners within the frame of Simultaneous Engineering.
Step 5: Evaluation of results for prototype and series manufacturing
Where conspicuous deviations of prototype and pilot series testing results from simulation results occur, it is necessary to check whether the toleration simulations were not based on wrong or adverse assumptions. Where appropriate, the assumption must be corrected so that the derived experience may be applied to further projects.
Where appropriate, decisions concerning approval of geometric deviations of part measures must be adopted within the frame of the manufacturing process and product approval procedures (“Produktionsprozesse- und Produktfreigabe” (PPF)). The learning derived from the tolerancing process may provide an important guidance here. Such knowledge may be also used in the analysis of problems in the series manufacturing in relation to dimensional deviations.
12.3 Tolerance Assessment in FMEA
In the Failure Modes and Effects Analysis (FMEA) a potential failure is often described only in qualitative terms, e.g. “the insulating coat thickness is too small”.
Particularly with a flat quality characteristic curve the potential consequence of a failure strongly depends on the quantitative measure of the failure, e.g. “little significance if the insulating coat thickness is 5% lesser, but great significance if the insulating coat thickness is 5o% lesser”.
Therefore, quantitative description of a failure should be preferred (e.g. “the coat thickness is 5% to 30% lesser”).
Determination of classes of characteristics is an integral component of the quality planning process and it reflects the classification of failures related to a qualitative characteristics.
A general classification of failures is focused on the consequences of failures, and it classifies failures into three classes: “critical failure”, “major failure” and “minor failure”, which may be complemented by further sub-classification.
However, there is no supporting generally accepted or standardized text or interpretation on which the classification of characteristic failures could be built.
Today, estimations of process or product risks for functional characteristics or other qualitative characteristics are typically based on system FMEA ratings.
If the classification of characteristics has been completed, for the sake of expediency we will further focus on the classification of risks.
If the classification by risk class has been already done as part of the system FMEA of a product, tolerances of functional characteristics or qualitative characteristics will follow such classification.
Logically, as the risk is reduced, tolerances will be set, with due regard to manufacturing capacities, at such levels that decisive factors in the tolerance selection exercise are cost-efficiency considerations and that small tolerances are set only in instances where the existing risk is proportionally high.
12.3.1 Example for Assignment of FMEA Ratings to Characteristic Classes
FMEA Rating Class of characteristics
Rating Critical characteristic typical requirements
Cpk > 1.67
No value of the characteristic out of the area of tolerance is
permissible (0 ppm)
A rate of characteristic values outside the area of tolerance of
up to 62 ppm is permissible
A rate of characteristic values outside the area of tolerance of
up to 2700 ppm is permissible
Severity/significance 1 - 4
Probability of occurrence ---
Probability of detection ---
Table 30: Assignment of FMEA ratings to characteristic classes