If you wanted to check the straightness of an arbor, you would use:

• A. An Indicator
• B. A Micrometer
• C. A Caliper
• D. A Dial Indicator

Answer: (A)

To check straightness, you need a device that reveals small deviations along the length of the arbor as you move across it. An indicator provides that capability by translating tiny positional changes into a readable deflection on a dial or display. When you sweep the indicator along the arbor, a perfectly straight piece will show little to no variation, while any bow or bend will show up as a change in the indicator reading. This focused ability to detect lateral deviation is what makes an indicator the appropriate tool for assessing straightness.

Micrometers and calipers measure size or distance at specific points—like diameter or length—not the straightness of a long surface, so they aren’t suited to reveal bending along the length. A dial indicator is a type of indicator, and in practice is the common choice for this job because it gives clear, sensitive readings of small deflections.