In a bolt-circle pattern, what must be defined to ensure a true position tolerance controls the pattern's location?

In a bolt-circle pattern, what must be defined to ensure a true position tolerance controls the pattern's location?

• A. Datum references and the primary datum.
• B. Surface finish is defined.
• C. Hole diameter tolerance defines.
• D. Material type.

Answer: (A)

The key idea is that true position is meaningful only when there is a defined coordinate framework to measure against. For a bolt-circle pattern, you must establish a datum reference frame by identifying datum features and assigning a primary datum (and, if needed, secondary and tertiary datums). This frame sets the origin and orientation for the part, so the true position tolerance can actually control where the holes sit around the bolt circle. Once you have these datums defined, the true position callout for the pattern constrains each hole's location relative to that datum frame, ensuring the pattern stays in the intended place and orientation on the part. Without a defined datum reference frame, the true position tolerance has no reference to fix the pattern in space. Surface finish, hole size, or material type don’t establish that coordinate frame, so they don’t provide the necessary basis for controlling the pattern’s location with a true position tolerance.

The key idea is that true position is meaningful only when there is a defined coordinate framework to measure against. For a bolt-circle pattern, you must establish a datum reference frame by identifying datum features and assigning a primary datum (and, if needed, secondary and tertiary datums). This frame sets the origin and orientation for the part, so the true position tolerance can actually control where the holes sit around the bolt circle.

Once you have these datums defined, the true position callout for the pattern constrains each hole's location relative to that datum frame, ensuring the pattern stays in the intended place and orientation on the part. Without a defined datum reference frame, the true position tolerance has no reference to fix the pattern in space.

Surface finish, hole size, or material type don’t establish that coordinate frame, so they don’t provide the necessary basis for controlling the pattern’s location with a true position tolerance.