How is a hole pattern evaluated against datums A and B?

• A. Each hole axis must be within the specified true position tolerance relative to datums A and B.
• B. Each hole axis must be perpendicular to datums A and B.
• C. The hole pattern must be concentric with datum A.
• D. The holes must be drilled with a tolerance zone of diameter equal to hole size.

Answer: (A)

True position controls where the centers of features must be located relative to a datum reference frame. When a hole pattern is evaluated against datums A and B, each hole axis must lie inside its cylindrical true-position tolerance zone whose position and orientation are defined by those datums. In other words, every hole in the pattern has to fall within the specified tolerance around its intended location as measured with respect to the datum reference frame formed by A and B. If any hole’s axis lies outside its tolerance zone, the pattern fails.

Perpendicularity to the datums isn’t the requirement for true position, and concentricity with a single datum isn’t the general rule for a pattern evaluated this way. Also, the tolerance zone for true position is not simply a diameter equal to the hole size; its diameter is the numerical positional tolerance value specified, applied to the axis, not the hole’s size itself.