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LO 5-3 Identify and describe common instruments and techniques to make linear precision measurements

1. Linear Measurement

1.21. Dial Indicators

LO 5-03: Identify and describe common instruments and techniques to make linear precision measurements

 

Dial Indicators
One of the most versatile precision measuring tools used by technicians is a dial indicator FIGURE 26. Dial indicators are incorporated into a variety of precision measuring tools. Its measurement speed, ease of use, and accuracy make it a popular choice for precision measurement. Digital electronic- and mechanical-type indicators are both available. A mechanical-type dial indicator converts linear movement of a spring-loaded plunger in contact with an object into rotary movement of a pointer against the background of a dial gauge. The plunger of a quality gauge has a contact tip that is typically threaded into the plunger to enable interchangeability with tips of any length or shape that are better suited to gauge the object under measurement. For example, a sharp needle-like plunger tip may be used to make contact between surface threads while a larger, round roller or mushroom-shaped tip is better used to glide over a shaft surface to prevent it from marking its finish or detecting tiny but irrelevant surface imperfections.

 

   Fig05_26

FIGURE 05-26: Parts of a dial indicator.

Construction and Use of Dial Indicators
Inside the dial indicator, the plunger moves a toothed rack that transmits motion to a pinion gear and then through a series of gears that eventually turn the pointer around the dial face. This gear arrangement magnifies the smallest movement of the plunger to rotate the dial pointer around the dial face. Common graduations of the dial face are available for reading in inches, with increments from 0.001", 0.0005", 0.00025", and 0.0001". For metric, graduations are available in 0.01 mm, 0.002 mm, and 0.001 mm increments. To prevent errors between metric and inch measurements, dial faces are commonly painted yellow to measure metric dimensions while inch measurements are white.

Fig05_27

FIGURE 05-27: Most dial indicators are balanced, which means there are two sets of numbers or + and – indicators on either side of the zero graduation to indicate the direction of plunger travel.

  

Dial indicators can be constructed with a second, smaller pointer on the dial face. In what is called a balanced gauge , the second, smaller pointer marks the number of revolutions for each rotation of the larger dial pointer FIGURE 27. Positive and negative symbols are on either side of the zero graduations to indicate the direction of plunger movement. This helps prevent measurement error and extends the plunger’s travel distance. A second type of dial indicator, called a continuous reading gauge , uses a revolution counter for the second, smaller dial. A revolution counter requires multiplying the distance of the total dial sweep times the number of rotations, plus adding the distance of the last incomplete rotation. The continuous-type measurement extends the measuring distance even further than a balanced gauge but may not have the same precision. Dials may be built with a rubber dust guard around the dial to seal out dust and dirt but generally use a metal outer ring, called a bezel , with two small markers and a method to turn the dial face to the zero graduation mark. Turning the bezel aligns the pointer position. These features help technicians accurately measure pointer travel when the gauge is moved around to measure a variety of objects. Magnetic bases are used to locate a dial indicator on a work piece. A half-turn-type knob that changes the strength of the magnetic field enables the magnetic base to securely clamp onto or release from a steel alloy or iron work piece.

 

 

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