Referred to as “an improvement in gages,” indicators were introduced in the late 19th century with the first patent dated May 15, 1883. John Logan, a watchmaker from Waltham, MA, saw a need for a precision instrument to measure small distances and angles with an uncomplicated value reading, and so he invented the indicator. One would imagine after some 130-plus years, indicators have changed. Indeed, indicators have evolved and many different types have been introduced such that there now exists a whole world of indicators.
The dial type drop indicator was the first introduction of this precision instrument. Measurement with this type of indicator is obtained by the movement of a spindle multiplied with the reading of the dial in association with the needle. From this known point, one will be satisfied or adjust the manufacturing process to achieve the desired measurement. Usually held by the stem in a fixture, the indicator enables easy measuring to determine runout, height, flatness, and/or distance. Whether used in machining, setup, or for quality control, a drop indicator has many application uses. In 1945, the National Institute of Standards and Technology (NIST) set forth the American Gage Design (AGD) identifying standard classifications for the outer diameter of the indicator to facilitate an easy choice based on space in a specific application.