MECHANICAL COMPARATORS| DIAL INDICATOR

The dial indicator or the dial gauge is one of the simplest and the most widely used comparator. It is mainly used to compare workpieces against a master. The basic features of a dial gauge consist of a body with a circular graduated dial, a contact point connected to a gear train, and an indicating hand that directly indicates the linear displacement of the contact point.

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Fig.1 Dial Indicator – Analog type

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Fig.2 Digital dial indicator

The contact point is first set against the master, and the dial scale is set to zero by rotating the bezel. Now, the master is removed and the workpiece is set below the contact point; the difference in dimensions among the master and the work piece can be directly read on the dial scale. Dial gauges (fig.2) are used along with V-blocks in a metrology laboratory to check the roundness of components. A dial gauge is also part of standard measuring devices such as bore gauges, depth gauges, and vibrometers. Figure 3 illustrates the functional parts of a dial indicator.

The contact point in a dial indicator is of an interchangeable type and provides versatility to the instrument. It is available as a mounting and in a variety of hard, wear-resistant materials. Heat-treated steel, boron carbide, sapphire, and diamond are some of the preferred materials. Although flat and round contact points are commonly used, tapered and button-type contact points are also used in some applications.

The stem holds the contact point and provides the required length and rigidity for ease of measurement. The bezel clamp allows locking of the dial after setting the scale to zero. The scale of the dial indicator, typically referred to as dial, provides the required least count for measurement, which normally varies from 0.01 to 0.05 mm. The scale has a limited range of linear measurements, varying from 5 to 25 mm. In order to meet close least count, the dial has to be large enough to improve readability.

The dials are of two types: continuous and balanced. A continuous dial has graduations starting from zero and extends to the end of the recommended range. It can be either clockwise or anti-clockwise. The dial corresponds to the unilateral tolerance of dimensions. On the other hand, a balanced dial has graduations marked both ways of zero. This dial corresponds to the use of bilateral tolerance. Figure 4 illustrates the difference between the two types of dials. Metrological features of a dial indicator differ entirely from measuring instruments such as slide callipers or micrometers.

It measures neither the actual dimension nor does it have a reference point. It measures the amount of deviation with respect to a standard. In other words, we measure not length, but change in length. In a way, this evaluation measurement is dynamic, unlike direct measurement, which is static. Obviously, the ability to detect and measure the change is the sensitivity of the instrument.

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Fig.3 Functional parts of a dial indicator

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Fig.4 Method for designating numbers

Working Mechanism of Dial Indicators

Figure 5 and Fig 6 illustrates the mechanism used in a dial indicator in order to achieve high magnification using a set of gears and pinions. The plunger and spindle are usually one piece.

The spindle attached to the bottom of the rack is the basic sensing element. A coil spring resists the measurement movement and thereby applies the necessary gauging pressure. Thus, the application of gauging pressure is built into the mechanism rather than leaving it to the technician. It also returns the mechanism to the ‘at-rest’ position after each measurement.

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Fig.5 Working mechanism of a dial indicator

The plunger carries a rack, which meshes with a gear (marked gear A in the figure). A rack guide prevents the rotation of the plunger about its own axis. A small movement of the plunger causes the rack to turn gear A. A larger gear, B, mounted on the same spindle as gear A, rotates by the same amount and transfers motion to gear C. Attached to gear C is another gear, D, which meshes with gear E. Gear F is mounted on the same spindle as the indicator pointer. Thus, the overall magnification obtained in the gear train A–B– C–D–E is given by TD/TE × TB/TC, where TD, TE, TB, and TC are the number of teeth on the gears D, E, B, and C, respectively.

The magnification is more enhanced at the tip of the pointer, depending on the length of the pointer. A hair spring loads all the gears in the train against the direction of gauging movement. This eliminates backlash that would be caused by gear wear. The gears are precision cut and generally mounted on jewelled bearings.

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Fig.6 Dial indicator mechanism

Contact Points

Dial indicators are multipurpose instruments because their mountings adapt them to many methods of support. Interchangeable contact points adapt them to varied measurement situations. Contact points are available in various hard and wear-resisting materials such as boron carbide, sapphire, and diamond. Contact points made of hardened steel are also often used. Figure 7 illustrates some of the standard contact points.

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Fig.7 Standard contact points

The standard or spherical contact point is the most preferred one because it presents point contact to the mating surface irrespective of whether it is flat or cylindrical. However, care must be taken to pass them through the center line of the spindle. The highest reading will be the diameter. It becomes less reliable when gauging spherical components because sphere to sphere contact makes the uppermost point of contact difficult to find. Another limitation is that it can take only limited gauging pressure, as high gauging pressure will leave an indent on the work piece. A button-type contact point can be used if light contact pressure on smaller components is required.

A tapered point is convenient for component surfaces that cannot be accessed by either standard or flat contact points. The use of contact points on spherical surfaces presents some problems. Only a flat point is suitable in such cases. It gives reliable readings for cylindrical surfaces too. Paradoxically, flat contact points are not preferred for flat surfaces. On the one hand, the presence of a thin air film can lead to minor errors; on the other hand, a higher area of contact with the component may result in rapid wear and tear of the contact point.



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