Threads, Fasteners & Springs — The axis of a screw is the longitudinal centerline that passes through the fastener, serving as the reference for diameters, pitch, and concentricity.

Introduction / Context:
Thread geometry is referenced to the screw axis, a central line running along the length of the fastener. This axis is critical for defining diameters (major, minor, pitch), measuring runout, and ensuring alignment between mating parts.

Given Data / Assumptions:

• A cylindrical fastener (bolt/screw) with helical threads.
• Dimensional references use the longitudinal centerline.
• Functional requirements: alignment, runout control, and fit.

Concept / Approach:
The axis is a geometric ideal used for measurement and tolerance schemes. Concentricity, straightness, and circular runout are assessed relative to this line to ensure reliable assembly and rotation if required.

Step-by-Step Solution:
1) Locate the geometric center of the cylindrical shank.2) Extend this centerline along the length—this is the axis.3) Reference diameters and thread features to the axis for inspection.4) Confirm the definition matches standard practice.

Verification / Alternative check:
Inspection setups (e.g., V-blocks with dial indicator) always reference the part’s axis to assess runout and straightness.

Why Other Options Are Wrong:
Edges/crests/roots are circumferential features and do not define the central datum; torque direction is irrelevant to the geometric definition.

Common Pitfalls:
Confusing the helix path with the axis; using off-axis features as datums leads to measurement errors.

Final Answer:
Correct