What instrument is used to measure the size of interior features such as holes?

The instrument specifically designed to measure the size of interior features, such as holes, is the telescope gage. Telescope gages are valuable tools that consist of a pair of telescoping arms that can be expanded to fit inside a hole. Once the arms are widened to touch the walls of the internal feature, the gage can be removed and measured using a micrometer or caliper, allowing for exact dimensions of the hole's diameter or other internal attributes.

In contrast, taper gages are used to measure tapers or angles and are not intended for internal measurements. Calipers can measure various dimensions, including the external and internal sizes of objects, but they are not as specialized as telescope gages for specifically measuring internal features like holes. Thread gages are designed for measuring the pitch and diameter of threads, which is another specific type of measurement and does not apply to general hole sizes.

Understanding the functionality and specialization of these tools is essential for accurate measurements in metal cutting and machining applications.