Opens in a new tab

Glossary: Tolerances

Reading time
  • Tolerance transfer: Deriving the tolerance to be applied to one characteristic based on the tolerance required for another.
  • Margin of error: the portion of Y’s tolerance that is set aside to account for what the model does not explain, as well as measurement uncertainty.
  • Residual standard deviation (S): the dispersion of Y that remains after the effect of X has been removed. This is the price of not knowing the model.
  • Sensitivity (∂Y/∂X): how much Y changes when X changes by one unit. The higher the sensitivity, the tighter the tolerance applied to X.
  • R²: the proportion of the variation in Y explained by the model. A high value does not necessarily mean it is sufficient: it is S that determines the tolerance, not R².
  • Worst-case scenario: the arithmetic sum of the deviations, assuming that everything happens simultaneously in the most unfavorable way.
  • Statistical accumulation: the sum of variances, taking into account the fact that adverse deviations do not all occur at the same time.
  • Inertia: a single metric combining skewness and dispersion. Maximum inertia gives the supplier the choice between being centered and dispersed, or skewed and concentrated.
  • Cp / Cpk: capability with and without consideration of offset. Cp indicates whether the variation falls within the tolerance, while Cpk indicates whether the production actually does.
  • Type D model (ISO 22514-2): a model in which the position and dispersion vary from one batch to another. This is the model simulated by the Monte Carlo simulation.
  • Contribution to variance: the portion of the variability in Y attributable to a factor. It is divided between within-batch variation and between-batch variation.
  • ppm: number of nonconforming parts per million produced.
  • Functional requirement: a dimension of the assembled product that must remain within a specified range—clearance, overrun, or center-to-center distance.
  • Dimension: a dimension to be manufactured that contributes to one or more requirements.
  • Coefficient α: how an odds value is factored into a chain—+1 if it is added, −1 if it is subtracted.
  • β weighting: the relative weight of a rating in the allocation of the tolerance budget. A difficult rating is assigned a higher β value.
  • Fixed tolerance: a tolerance imposed in advance, which dictates the distribution rather than resulting from it.
  • Propagated severity: The severity of requirements applied to the dimensions associated with them, in proportion to the budget consumed. It refers to the dimensions to be included in the inspection plan.
  • Situation: a possible state of the process—off-center positioning and dimensional variation. A point on the compliance triangle represents a situation, not a part.
  • Exception: A fixed tolerance based on measured values, used to determine whether a batch that falls outside the tolerance range can still be assembled.
  • Conformance Triangle: The region of the plot (mean, standard deviation) within which the target capability is maintained.