Indicators are mechanical instruments , which can sense and measure distance variations. The mechanism of the indicator converts the axial displacement of the measure spindle into rotational movement is mechanically amplified and then displayed by a pointer rotating over the face of a circular dial with evenly spaced graduations. Some indicators have hands that can be moved around the face to indicate the permissible tolerance being measured.
Digital indicators are of the alphanumerical type that display the process in numbers and letters. There are many different types of such indicaotrs such as, the seven-segment neon number, liquid crystal, plasma, and vacuum fluorescent.These indicators must be picked for the application conditions. Some of them must be read directly in front; this limits their use where the display must be read at angles from directly in front of the instrument. Sunlight and interior lighting can adversely affect the readability of some of these digital indicators.
Dial indicators, the magnification of a dial-type indicator is obtained by means of a gear trainare .This type of indicator is most commonly used because its magnification accuracy meets the large majority of requirements. In operation, a sensitive contact or point is attached to a rack that transfers the motion to the rack gear. A train of three to five gears, depending on the magnification desired, magnifies and transmits the movement of the contact to the pinion gear on which the indicator hand is mounted with a hairspring and take up gear to eliminate backlash.
Test indicator. Test indicators sense and measure displacements that occur in a direction perpendicular to the shaft of the contact point. Magnification is obtained by gears and levers. Because they are small, test indicators are particularly used in setup inspection and toolroom work.
Reed-type indicators .In this type of indicator, the red magnifying mechanism uses various combinations of flat steel reeds to obtain mechanical amplification. The sensitive spindle is mounted on a block that floats on reeds connected to a fixed block that is attached to the mounting. Extending from the top of the two blocks is a vertical member consisting of a pair of reeds, one of which is attached to each of the fixed and the floating blocks; a hand or pointer is mounted on this member. Slight motion of the sensitive contact flexes the reeds and moves the hand along a vertical arc.