In controlling runout, you must use what type of references?

• A. Pass/fail metrics
• B. Measurement tape
• C. Datum surfaces
• D. Calipers

Answer: (C)

Controlling runout requires defining a specific reference or origin from which the runout measurement can be effectively evaluated. Datum surfaces serve this purpose by providing a stable and known point or surface on the workpiece that ensures measurements are taken consistently and accurately relative to the part's geometry. This reference is essential because runout is a measure of how much a feature can deviate from its ideal rotational path, and the datum establishes the baseline from which this deviation can be assessed.

Using datum surfaces allows manufacturing and quality control personnel to ensure that parts meet specified tolerance levels and perform correctly in their intended application. Conversely, alternatives like pass/fail metrics, measurement tape, or calipers do not provide the necessary precision and control required for assessing runout effectively. They lack the specific framework needed to define the relationship between features on the part, which is crucial in GD&T practices.