What term describes a dimension that is exact and not directly toleranced?

What term describes a dimension that is exact and not directly toleranced?

• A. A dimension that requires a tolerance band.
• B. A reference dimension.
• C. A derived dimension.
• D. Basic dimension.

Answer: (D)

In GD&T, a dimension that is exact and not directly toleranced is called a basic dimension. Basic dimensions define the theoretically exact value used to locate or orient features, and they themselves have no tolerance attached. The actual allowable variation comes from the geometric tolerances and the datum references that accompany them, not from the basic dimension value itself. For example, the true position of a hole is defined by a basic dimension for its location, while the positional tolerance in the feature control frame restricts how far the actual hole center can deviate from that exact location. The other terms don’t fit this role: a dimension that requires a tolerance band is simply a toleranced dimension with a specified plus/minus tolerance; a reference dimension is provided for information only and isn’t used to control the geometry; a derived dimension isn’t the standard GD&T term for an exact, non-toleranced value.

In GD&T, a dimension that is exact and not directly toleranced is called a basic dimension. Basic dimensions define the theoretically exact value used to locate or orient features, and they themselves have no tolerance attached. The actual allowable variation comes from the geometric tolerances and the datum references that accompany them, not from the basic dimension value itself. For example, the true position of a hole is defined by a basic dimension for its location, while the positional tolerance in the feature control frame restricts how far the actual hole center can deviate from that exact location.

The other terms don’t fit this role: a dimension that requires a tolerance band is simply a toleranced dimension with a specified plus/minus tolerance; a reference dimension is provided for information only and isn’t used to control the geometry; a derived dimension isn’t the standard GD&T term for an exact, non-toleranced value.