How to dimension a circular arc correctly When a circular arc is shown in a view where its true shape is visible, which value is normally given to dimension that arc?

Introduction / Context:
Drafting standards specify how to communicate curved geometry with clarity. For circular arcs that are not complete circles, the conventional practice is to dimension by radius so the manufacturing process can reproduce the curvature accurately.

Given Data / Assumptions:

• The geometry is a partial circle (an arc) seen in true shape.
• We need a single parameter that defines the curvature.
• Standard practice for arcs differs from full circles.

Concept / Approach:
The radius uniquely defines the curvature of a circular arc and is easy to inspect or gage. Diameter dimensions are reserved for full circles. Arc length and chord length may be referenced as supplemental information but are not the primary dimension for defining an arc's curvature.

Step-by-Step Solution:

Verification / Alternative check:
Confirm manufacturability: the radius callout guides tool path creation for milling or turning and matches inspection with radius gauges or CMM checks.

Why Other Options Are Wrong:

• Diameter: Correct for full circles, not typical for partial arcs.
• Arc length / Chord length: These do not define curvature by themselves; they depend on the included angle and radius.

Common Pitfalls:
Placing a diameter note on a partial arc confuses machinists and inspectors. Always use radius for arcs unless a specific process note requires otherwise.

Final Answer:
Radius