As chapter 5 argues, margins can be seen to have an element of buffer and an element of excess or contingency. The concepts mentioned in the interviews also can be thought of as either buffer or excess concepts, but the division is not always clear. Safety margins and tolerances are both buffers which are added to deal with uncertainty. Safety margins deal with uncertainty in use and, in particular, uncertainty arising from misuse of products. Tolerances are catering for uncertainty arising from manufacturing variability. In trucks an element of buffer is already incorporated in the requirements, as they include the variability in ordinary and intended use that will be covered by a system or component.
Room for growth is an explicit excess concept, which allows the designers to meet higher requirements in the future. Overdesign can be seen as a both an excess and a buffer, as the designers see their colleagues raising requirements as an uncertainty against which they protect themselves through overdesigning a component. Once the uncertainty of knock-on effects of changes is removed the overdesign moves from a buffer into an excess. Later in the process overdesign is seen as a form of costly waste, i.e. a form of excess that can be eliminated. Given the fact that the engineers’ decision regarding the design margins may vary, depending at which stage of the process the decision is made, it was decided to use a decision tree to model a specific situation that a designer or an engineer might have to deal with. Decision trees are a simple, yet powerful form of multiple variable analysis. They are produced to identify various ways of splitting a data set into branch-like segments (chapter 2). Decision trees attempt to find a strong relationship between inputs and targets in a group of observations that form a data set.
A strong input-target relationship is formed when knowledge of the value of an input improves the ability to predict the value of the target. For example, if designers have to select a material, knowing its strength improves the ability to predict how much load the material can carry. The general form of this modelling approach which was used to build the decision tree is shown in Figure 6-13.
Figure 6-13 Design strategies based on margins state
Once the margin is known, the analysis moves to identifying how much of that design margin is considered as a buffer and how much of it is considered as excess. Then the investigation
156 starts by evaluating different possibilities (inputs for each node) and considers the decision actions that need to be taken (target), as shown in Figure 6-13.
The left hand branch in Figure 6-13 considers when the margin is a buffer. When he uncertainty is very high, engineers consider a range of parameter values to compensate for those uncertainties. However, engineers want to have control over those uncertainties and be able to predict their probability of occurrence. As described in Chapter 2, the uncertainty can be endogenous or exogenous. If the uncertainty is endogenous, it means that engineers gave some control over the probabilities they are facing. Therefore, they can conduct further tests and simulations and try to reduce the buffer. If the buffer is reduced then that part moves into excess, otherwise the engineers can wait until they get more feedback from the physical test. In both cases, the decision made at this level regarding the design margin is more accurate. Even if there are buffers which are not immediately usable in design but offer the potential that they can be reduced and release tangible and usable margins. These offer options for meeting higher levels of functional requirement or optimising product parameters against existing functional requirements.
The right hand branch of the decision tree in figure 6.13 considers when the margin is excess. When the value of the margin is known and clearly exceeds requirements, it is an excess. The excess part of a margin provides design flexibility; it is the part that designers can make use of, for example to deal with changes or increasing requirements. In this case designers need to decide whether or not to design with that excess (surplus). As soon as the requirements are known the role of design margins is to reduce uncertainty in the design process. At the early phases of the development process design margin are seen as a positive way of managing design effort. However, as soon the product reaches the production stage, design margins are seen as a source of extra cost and engineers look for excess to remove. Yet, if engineers have a global
view over the design margin state throughout the PDP, they can see that for specific scenarios, introducing design margins as a form of deliberate planning for a number of potential changes, can reduce the cost of making changes from scratch. For these reasons it is important to follow and update the model of design margins across all phases of the product development and update the design strategies based on the margin state.