The dial indicator
is the second most common precision measuring tool, after the micrometer, that a serious mechanic is likely to encounter. Yet dial indicators (and the related test indicators) do not really measure the actual size of anything! Unlike rules, slide calipers, or micrometers, they do not gauge a dimension directly; rather, they are comparison instruments--they indicate the amount by which a certain dimension varies from some reference. This should become clear as we describe the nature and use of the device.
The instrument comprises a circular housing with clock-like dial face and pointer, plus a plunger that extends out from the housing. The most common has the plunger protruding downward; these are termed â€śside plungerâ€?designs. In the alternative â€śback plungerâ€?design, the plunger extends out from the back of the instrument. At the end of the plunger there is a No. 4-48 screw thread for the attachment of various interchangeable of various interchangeable contact points having flat, round , or pointed shapes. The screw thread also allows the fitment of extension rods, to increase the â€śreachâ€?of the instrument. Typically, these come in 1-,2-, and 4-inch lengths, and since each has corresponding male and female threads at opposite ends, they can be stacked to give extensions in 1-inche increments from 1to 7 inches.
Within the case, the plunger drives a rack that meshes with a gear, which in turn drives other gears, ultimately connected to the pointer. By this means, the liner movement of the plunger is amplified, and the distance it has traveled is thus indicated on the gauge face .
Both the face and the â€śgutsâ€?of a dial indicator resemble those of a clock. The instrument comes by this likeness honestly: The origins of the Waltham Clock Co. of Waltham, Massachusetts, perhaps sometime in the 1930s. The dial indicator came into its own in World War II; It made possible the gauging of the precision gear teeth needed for high-speed rotating machinery.
As in much quality â€śclock-work,â€?jeweled bearings are sometimes used in dial indicators, although this is a matter of dispute among various manufacturers. One eminently respectable maker promotes this as a significant feature; another equally reputable firm insists on the superiority of its own bronze bushed internal works.
Although there are â€ślong-rangeâ€?dial indicators with a plunger travel of as much as 12 smaller rotary indicator inset into the main dial that tells the user which revolution the main pointer is on. If a dial gauge you decide to purchase offers this feature as an option, by all means go for it . Also, while inch reading dial gauges may have white, black, or red (or occasionally green) faces, metric ones are always yellow.
Note that AGD specs call for a minimum of 2/3 revolutions, not exactly 2 (and most manufactures make it 2 1/2). Note also that the rest position of the pointer is at â€? câ€™clock .â€?The extra part revolution and this offset in the pointer allow the instrument to be â€śpreloadedâ€? to take up any backlash in its internal gear train.