How is a hole pattern positioned relative to datums?

How is a hole pattern positioned relative to datums?

• A. The centers of holes must be located exactly on a datum axis.
• B. The pattern must be sized after the DRF is defined.
• C. The pattern is oriented by the DRF but not sized.
• D. The centers of the holes must lie within the specified true position tolerance zone relative to the DRF.

Answer: (D)

In GD&T, the location of a hole pattern is defined relative to the datum reference frame (DRF) using a true position tolerance. The DRF, built from the datums, provides the origin and orientation. The true position sets how far the actual centers of the holes may deviate from their ideal locations within that frame, and this deviation is limited by a cylindrical tolerance zone coaxial with the hole axes (or oriented as specified). Therefore, every hole center in the pattern must lie inside that specified true position tolerance zone relative to the DRF. They don’t have to sit exactly on a datum axis, and the rule isn’t just about sizing after defining the DRF—the key is that the centers are permitted to be only within the defined tolerance zone in relation to the datums.

In GD&T, the location of a hole pattern is defined relative to the datum reference frame (DRF) using a true position tolerance. The DRF, built from the datums, provides the origin and orientation. The true position sets how far the actual centers of the holes may deviate from their ideal locations within that frame, and this deviation is limited by a cylindrical tolerance zone coaxial with the hole axes (or oriented as specified). Therefore, every hole center in the pattern must lie inside that specified true position tolerance zone relative to the DRF. They don’t have to sit exactly on a datum axis, and the rule isn’t just about sizing after defining the DRF—the key is that the centers are permitted to be only within the defined tolerance zone in relation to the datums.