This information provided by Bryan R. Fischer, of TDP360 LLC.
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Features of Size
Features of size are important. Cylindrical holes, pins, and shafts, and keys and keyways may be features of size. These features are very common in machinery and are used to connect parts, transmit power, and for many other uses. In many cases, it is important for these features to have clearance or a precise fit. Fasteners, pins, and shafts must fit into holes, shafts must fit precisely with bearings and bushings, and keys must fit into keyways. GD&T provides a simple way to control the size and form of such features and to manage its fit with mating features. The form and size of a regular feature of size may be controlled by a directly-toleranced dimension. The size limits establish the allowable variation in size and form for a regular feature of size
Important: To be considered as a regular feature of size, a feature must have a certain shape; it must be a cylinder, sphere, or two opposed parallel planes (like the sides of a key or keyway); and it must be defined by a directly-toleranced dimension (a dimension with +/- tolerances or a limit dimension). Note that a size dimension that uses a title block size tolerance is also considered as a directly-toleranced dimension.
Regular Features of Size – Metric and Inch
Size Limits of Features of Size
Say we have a Ø1.000 cylindrical hole and apply a ±.005 tolerance as follows: Ø1.000 ±.005. The size limits are .995 and 1.005. Size limits are defined by a directly-toleranced dimension or limit dimension. Features of size have size limits: least material condition (LMC) and maximum material condition (MMC).
LMC: Size of a feature of size (FOS) where the part contains the least amount of material, such as the largest internal FOS (e.g. largest hole) or the smallest external FOS (e.g. smallest shaft).
MMC: Size of a feature of size (FOS) where the part contains the maximum amount of material such as the smallest internal FOS (e.g. smallest hole) or the largest external FOS (e.g. largest shaft).
Part has less material Part has more material
Nominal Size, LMC and MMC
Rule #1 (Envelope Principle)
Rule #1: Unless otherwise specified, the size tolerance applied to a feature of size controls its size and its form. Rule #1 only applies to features of size. The size tolerance also controls the shape of a feature of size. Thus, a feature of size produced at MMC must have perfect form. For the Ø20 ±1 shaft below, the dimension and tolerance define a Ø21mm perfect form boundary – the shaft must fit on or within a theoretically perfect Ø21mm cylinder. For the Ø22 ±1 hole below, the dimension and tolerance define a Ø21mm perfect form boundary – the hole must fit on or around a theoretically perfect Ø21mm cylinder. MMC for an external feature of size (e.g. shaft) is the largest size. MMC for an internal feature of size (e.g. hole) is the smallest size.
Features of Size, Limits of Size, and MMC Perfect Form Boundary (Envelope Principle)