Internal dimensions, like holes, are at their MMC when they are at what part of their tolerance zone?

The concept of Maximum Material Condition (MMC) refers to a state where a feature contains the maximum amount of material permissible within its specified tolerances. For internal dimensions such as holes, this condition is met when the hole is at its smallest permissible size.

When considering holes, the "lower limit" represents the minimum allowable size of the hole—essentially, the point at which it contains the most material in terms of the wall thickness surrounding the hole. If a hole is at its lower limit, it means that it has the least internal space, thus maximizing the surrounding material. This is crucial in manufacturing and assembly processes where the fit and tolerance significantly affect the performance and functionality of the parts being joined.

The upper limit would imply a larger hole, which does not correspond to MMC as it contains less material. The average limit and minimum tolerance do not accurately define the condition for MMC in the context of holes either, as they do not focus explicitly on the smallest permissible dimension necessary for maximum material presence.

Therefore, the scenario where internal dimensions are at their MMC is indeed when they reach their lowest limit within the tolerance zone.