Dial indicators are instruments used to accurately measure a small distance. They may also be known as a Dial gauge, Dial Test Indicator (DTI) or as a "clock".

The definition of small obviously depends on the observer however a range between 1mm (0.040") and 50mm (2") may be thought of as typical with a travel of 10mm (approx 0.5") being perhaps the most common.

Probe type

They typically consist of a graduated dial and needle (thus the clock terminology) to record the minor increments ,with a smaller embedded clock face and needle to record the number of needle rotations on the main dial. They may be graduated to record measurements of between 0.01mm (.001", which is not a direct unit conversion) down to 0.001mm (.00005") for more accurate usage. The probe (or plunger) moves perpendicular to the object being tested by either retracting or extending from the indicators body.

The dial face can be rotated to any position, this is used to orientate the face towards the user as well as set the zero point, there will also be some means of incorporating limit indicators (the two metallic tabs visible in the right image, at 90 and 10 respectively), these limit tabs may be rotated around the dial face to any required position. There may also be a lever arm available that will allow the indicator's probe to be retracted easily.

A situation that utilizes all these features would be in the inspection department of a manufacturing facility. The inspection department would have the Dial Test indicator (DTI) set up in a fixture (possibly a magnetic base) which would secure the DTI and allow its adjustment to read zero at the optimal size of a sample part, the two limit tabs would be set to the extremes of the parts tolerance, finally the lever arm would allow the probe to be retracted when loading the sample to be tested, between the DTI and base. This lever arm reduces the chance of applying undue force to the probe or DTI, possibly upsetting its accuracy.

The tip of the probe may be interchanged with a range of shapes and sizes depending on application.

Lever type

A Lever arm test indicator or Finger indicator has a smaller movement range (perhaps 1mm depending on the model) and measures the deflection of the arm, the probe does not retract but swings in an arc around its hinge point. The lever may be interchanged for length or ball diameter and permits measurements to be taken in narrow grooves or small bores where the body of a probe type may not reach. The model shown is bidirectional, some types may have to be switched via a side lever to be able to measure in the opposite direction.

Digital type

With the advent of electronics and LCDs the clock face and analog display has been replaced with digital displays, these have the added advantage of being able to record and transmit the data electronically to a computer. This process is known as Statistical process control (SPC) and involves a computer recording and interpreting the results, this also reduces the risk of the operator introducing recording errors. Digital indicators can also be switched between imperial and metric units with the press of a button, thereby increasing the DTI's versatility.

Applications

• As outlined above, they may be used in a quality environment to check for consistency and accuracy in the manufacturing process.
• On the workshop floor to initially set up or calibrate a machine, prior to a production run.
• By toolmakers (moldmakers) in the process of manufacturing precision tooling.
• In metal engineering workshops, where a typical application is the centering of a lathes workpiece in a four jaw chuck. The DTI is used to indicate the run out of the work piece, with the ultimate aim of reducing it to a suitably small range by small chuck jaw adjustments.
• In areas other than manufacturing where accurate measurements need to be recorded, eg:- physics.

References

Dial & Electronic Indicators & Gages from the manufacturer Starrett.

Mitutoyo product literature from the manufacturers (Mitutoyo) site.

Non authoritative links

Practical metal and woodworking applications.