Page 23 - Personal Study Notes (Engineering Metrology - 22342)

Page 23 of 176

Construction and Working:

Figure 6.4 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.

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.

It is primarily used to compare work pieces 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. 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 work piece is set below
the contact point; the difference in dimensions between the master and the work piece
can be directly read on the dial scale.

Requirements of Good Dial Indicator:

1. It should give trouble free and dependable readings over a long period.

2. The pressure required on measuring head to obtain zero reading must remain
constant over the whole range.

3. The pointer should indicate the direction of movement of the measuring plunger.

4. The accuracy of the readings should be within close limits of the various sizes and
ranges.

5. The movement of the measuring plunger should be in either direction without
affecting the accuracy.

6. The pointer movement should be damped, so that it will not oscillate when the
readings are being taken.

Applications:

1. Comparing two heights or distances between narrow limits.

2. To determine the errors in geometrical form such as ovality, roundness and taper.

Engineering Metrology (Only For Personal Use) 15