Untermaß

Untermaß is a term used in metrology and manufacturing to designate the lower bound of a dimension within a tolerance range. It describes how much a produced part may be smaller than the nominal size (Nennmaß). Together with Übermaß (the upper bound), Untermaß defines the permissible size interval for a feature such as a hole, shaft, or external dimension. A part is acceptable when its actual dimension lies between the Untermaß and the Übermaß.

In practice, tolerancing specifies a nominal size and a tolerance zone. The zone is bounded below by the Untermaß and above by the Übermaß. The exact values depend on the standard (for example DIN or ISO) and on the type of feature. Untermaß is commonly used in discussions of fits and allowances, where engineers must ensure that assembled parts will fit together despite small variations in manufacturing.

Examples help illustrate the concept. A hole with nominal diameter 100.0 mm and a tolerance of -0.20 mm to 0.00 mm has an Untermaß of 99.80 mm (the smallest permissible hole). The corresponding Übermaß would be 100.00 mm, establishing a permissible range for the hole size. Similar constructions apply to shafts and other features, where the Untermaß sets the minimum permissible size within the specified tolerance zone.

Terminology related to Untermaß includes Nennmaß (nominal size), Übermaß (upper deviation), Toleranz (tolerance), and Passung (fit). The concept is central to quality control and manufacturing planning, helping ensure interchangeability and functional fit of parts.