In various contexts of science, technology, and manufacturing (such as machining, fabricating, and additive manufacturing), an indicator is any of various instruments used to accurately measure small distances and angles, and amplify them to make them more obvious. The name comes from the concept of indicating to the user that which their naked eye cannot discern; such as the presence, or exact quantity, of some small distance (for example, a small height difference between two flat surfaces, a slight lack of concentricity between two cylinders, or other small physical deviations). The classic mechanical version, called a dial indicator, provides a dial display similar to a clock face with clock hands; the hands point to graduations in circular scales on the dial which represent the distance of the probe tip from a zero setting. The internal works of a mechanical dial indicator are similar to the precision clockworks of a mechanical wristwatch, employing a rack and pinion gear to read the probe position, instead of a pendulum escapement to read time. The side of the indicator probe shaft is cut with teeth to provide the rack gear. When the probe moves, the rack gear drives a pinion gear to rotate, spinning the indicator "clock" hand. Springs preload the gear mechanism to minimize the backlash error in the reading. Precise quality of the gear forms and bearing freedom determines the repeatable precision of measurement achieved. Since the mechanisms are necessarily delicate, rugged framework construction is required to perform reliably in harsh applications such as machine tool metalworking operations, similar to how wristwatches are ruggedized. Other types of indicator include mechanical devices with cantilevered pointers and electronic devices with digital displays. Electronic versions employ an optical or capacitive grating to detect microscopic steps in the position of the probe. Indicators may be used to check the variation in tolerance during the inspection process of a machined part, measure the deflection of a beam or ring under laboratory conditions, as well as many other situations where a small measurement needs to be registered or indicated. Dial indicators typically measure ranges from 0.25 mm to 300 mm (0.015in to 12.0in), with graduations of 0.001 mm to 0.01 mm (metric) or 0.00005in to 0.001in (imperial/customary). Various names are used for indicators of different types and purposes, including dial gauge, clock, probe indicator, pointer, test indicator, dial test indicator, drop indicator, plunger indicator, and others.
General classification There are several variables in dial indicators:
Analog versus digital/electronic readout (most are analog) Dial size. Typically referred to be American Gauge Design Specification (AGD):
Accuracy Range of travel Number of dial revolutions Dial style: balanced (e.g., −15 to 0 to +15) or continuous (e.g., 0 to 30) Graduation style: positive numbers (clockwise) or negative numbers (counterclockwise) Revolution counters, which show the number of revolutions of the principal needle.
Principles Indicators inherently provide relative measurement only. But given that suitable references are used (for example, gauge blocks), they often allow a practical equivalent of absolute measure, with periodic recalibration against the references. However, the user must know how to use them properly and understand how in some situations, their measurements will still be relative rather than absolute because of factors such as cosine error (discussed later).
Applications In a quality environment to check for consistency and accuracy in the manufacturing process. On the workshop floor to initially set up or calibrate a machine, prior to a production run. By toolmakers (such as moldmakers) in the process of manufacturing precision tooling. In metal engineering workshops, where a typical application is the centering of a lathe's workpiece in a four jaw chuck. The dial indicator is used to indicate the run-out (the misalignment between the workpiece's axis of rotational symmetry and the axis of rotation of the spindle) of the workpiece, with the ultimate aim of reducing it to a suitably small range using small chuck jaw adjustments. In areas other than manufacturing where accurate measurements need to be recorded (e.g., physics). To check for lateral run-out when affixing a new rotor to an automotive disc brake. Lateral run-out (lack of perpendicularity between the disc surface and the shaft axis, caused by deformations or more frequently by a lack of proper cleaning of the mounting surface of hub. This run-out can produce brake pedal pulsations, vibration of the vehicle when brakes are applied and can induce uneven wear of the disc. The lateral run-out can be caused by uneven torque, damaged studs, or a burr or rust between the hub and rotor. This variation can be tested with a dial indicator, and most times the variation can be more or less cancelled by reinstalling the disc in other position, so that the tolerances of both the hub and the disc tend to cancel each other. To reduce the run-out, the disc is mounted and torqued to half the specified torque (as there is no wheel to distribute stresses) then a dial Indicator is placed against the braking surface and the face of the dial is centered, the disc is slowly rotated by hand and the maximum deviation is noted. If the maximum run-out is within the maximum allowed run-out specified in the manual, the disc can be installed at that position, but if the technician wants to minimize the total lateral run-out, other around the clock positions can be tried. Excessive run-out can rapidly ruin the disc if it exceeds the specified tolerance (typically up to 0.004 inches (0.10 mm) but most discs can attain less than 0.002 inches (0.05 mm) or less if installed at the optimum position).
Probe indicator
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