What a dial indicator is

A dial indicator, also called a dial gauge, is an analog comparison gage used in engineering for high precision measurement of small displacements. While a micrometer gives the overall dimension of a part, a dial indicator shows the deviation of the actual size from its nominal value, amplifying that deviation so it becomes easy to read.

The hand movement is driven by a rack-and-pinion and a train of gears, which is what magnifies the small motion of the spindle. Dial indicators are available in analog and digital forms. Lever type test indicators are used for centering machine tools, checking eccentricity (runout) and checking out of roundness.

A dial indicator reveals lengths that cannot be discerned by eye, for example a small height difference between two flat surfaces, a slight loss of concentricity between two cylinders, or other small physical deviations.

With the appearance of an ordinary clock, dial indicators are also used to measure tolerances on machined parts, to check the flatness of bars and, more generally, for any measurement that involves small deviations.

Technical characteristics

A dial indicator is defined by several technical characteristics:

• Its measuring range, or travel (0-5mm, 0-10mm, 0-30mm and so on)
• Its graduation, or resolution (0.01mm, 0.001mm and so on)
• Its size (the bezel diameter, Ø40mm, Ø58mm and so on)
• The number of revolutions of the main hand per mm of travel
• The revolution counter, a small second hand that counts the revolutions of the main hand
• Whether it is shockproof (a more robust internal mechanism for longer service life)
• Whether it is waterproof, with an IP protection rating
• Balanced dial for comparison measurements (for example a reading of ±0.4mm)

Applications

• Checking the runout of brake discs, typically 0.05mm or less.
• Quality control in production.
• In the machine shop, for calibrating and setting up machines (for example indicating a milling cutter).
• Centering the workpiece in a lathe chuck.
• A wide range of precision applications outside the machine shop, such as geology, physics and so on.

Stem mounting (flat back type)

Lug mounting (rear lug)

Why orientation matters

When the measurement is taken with the indicator horizontal, or even inverted, the measuring force at the contact point is naturally lower. A dial indicator should in fact not be used upside down, because its preloaded spindle is not strong enough to overcome its own weight. In these positions the repeatability of the measurements must be verified.

Maintenance

• Do not use lubricants (oil) on the moving spindle. Oil tends to collect dust and hampers the motion.
• If the motion is not smooth, clean the spindle with a cloth and alcohol.
• Before taking any measurement, make sure the spindle moves smoothly up and down.

Cosine error (the cosine effect)

The measurement will be in error if the direction of the intended measurement does not coincide with the direction of the spindle. This error is minimized by setting the spindle so that the angle θ is as small as possible, preferably within 15°.