dial gauge

Dial indicator

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". They are named so because the measurement results are displayed in a magnified way by means of a dial.

Dial indicators may be used to check the variation in tolerance during the inspection process of a machined part, measure the deflection of a beam or ring under laboratory conditions, as well as many other situations where a small measurement needs to be registered or indicated. The definition of small depends on the observer however a range between 1 mm (0.040") and 50 mm (2") may be thought of as typical with a travel of 10 mm (approx 0.5") being perhaps the most common.

General Classification

  • part revolution type.
  • one revolution type.
  • few revolution type
  • parts

Applications

  • To check for runout when fitting a new disc to an automotive disc brake. Runout can rapidly ruin the disc if it exceeds the specified tolerance (typically 0.05mm or less).
  • 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 lathe's workpiece in a four jaw chuck. The DTI is used to indicate the run out (the misalignment between the work piece's axis of rotational symmetry and the axis of rotation of the spindle) of the work piece, with the ultimate aim of reducing it to a suitably small range using small chuck jaw adjustments.
  • In areas other than manufacturing where accurate measurements need to be recorded, eg:- physics.

Probe indicator

Probe indicators 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. The tool may be graduated to record measurements between 0.01 mm (.001", which is not a direct unit conversion) down to 0.001 mm (.00005") for more accurate usage. The probe (or plunger) moves perpendicular to the object being tested by either retracting or extending from the indicator's body.

The dial face can be rotated to any position, this is used to orient 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.

Dial Test Indicator

A Dial Test Indicator, also known as a Lever arm test indicator, or Finger indicator has a smaller measuring range than a dial indicator and therefore has the ability to measure in smaller increments. A test indicator 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 and 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.

These indicators actually measure angular displacement and not linear displacement. If a force is perpendicular to the finger, the linear displacement error is acceptably small within the display range of the dial. However, this error starts to become noticeable when the force is as much as 10 degrees off the ideal 90. These indicators are used to compare two surfaces and alert the user when they are at the same position relative to the body of the indicator. In this application the force can be applied at almost any angle since you are looking for the same reading on two different surfaces.

Digital indicator

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 sometimes 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.

Probe (tip) types

The tip of the probe may be interchanged with a range of shapes and sizes depending on the application. However if the tip is not the same length as the one that is designed for the indicator, an offset (multiplier or correction factor) is required for a proper measurement reading. The shape is usually chosen to give point contact, the shape of a small sphere allows for consistent measurements as the tip moves through an arc, while the size may be changed to allow the tip to enter a small hole. The tip may also be cylindrical or flat as the need arises. It should be noted that the contact angle of the tip is 90 degrees (see above) in relation to the surface that is being measured. If the angle is not 90 degrees, an offset (multiplier or correction factor) must be applied to achieve a correct non-zero measurement. This factor is not needed if the indicator is being used for reference only. It is needed if the indicator is being used for a precision measurement instrument.

Some cheaper models of test indicators may come with a pear shaped probe in an attempt to compensate for cosine error and a multiplier or correction factor will not have to be used. Contact points of higher end test indicators such as Mitutoyo, Brown & Sharpe, TesaTast and Interapid come with a standard carbide sphere at 1, 2 or 3mm diameter. More usable materials are available for contact points such as ruby, teflon and PVC. These are more expensive and not available OEM, but, are extremely useful in applications where surfaces are easily scratched or damaged.

References

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