What do we call the acceptable ideal range of tightness that may result from the application of specific combinations of allowances and tolerances?

The concept referred to in the question is best described by the term "fit." In the realm of machine design and manufacturing, fit denotes the relationship between two mating parts, quantifying how tightly or loosely they can be assembled. It encompasses the acceptable range of variations associated with allowances and tolerances that ensure parts can work together effectively without issues.

When designing mechanical components, understanding the fit is crucial because it influences assembly, functionality, and longevity of the final product. A precise fit can lead to optimal performance by ensuring that parts maintain their intended positions and functions. For instance, in scenarios requiring a tight fit, small tolerances may be specified to accommodate the requirements of the application, such as in precision machinery.

On the other hand, allowance limit and tolerance limit are terms that individually refer to specific parameters relating to the acceptable variations in dimensions. Interference, meanwhile, pertains to a specific type of fit where parts are intentionally designed to overlap or compress, leading to a tighter assembly than what would be classified as just a fit. Understanding fit, therefore, is essential as it integrates the concepts of allowance, tolerance, and practical application into a coherent framework for assessing part interactions.