When we think about comparative indicators, we usually are referring to dial indicators. However, test indicators also provide comparative measurements. The difference is that while dial indicators sense displacement that is parallel to the axis of the spindle, test indicators are designed to sense and measure displacements that are perpendicular to the shaft of the contact point.
Test indicators are referred to by their dial configurations, the most common of which are the front mount, top mount and side mount. There are also two different dial diameters (1.1" and 1.5"). Selection of a test indicator is dependent on how and where it will be used.
The probe (contact point) of the indicator is part of a lever mechanism that transfers the motion to the working part of the indicator. At the point where the contact is attached, a swivel allows the contact to be positioned anywhere within a 180-degree arc.
The use of test indicators is straightforward, but there are two important points to remember about the contact point.
The first is length. The indicator is based on a lever transfer, so length is critical. A one-to-one ratio is set up with a standard indicator. Changing to a longer or shorter contact changes this ratio and can result in measurement errors.
The second issue is the relationship of the contact point to the surface being measured. When the contact arm is not set parallel to the part, the contact tip is also displaced across the part surface, causing cosine error. The steeper the angle, the greater the cosine error.
As shown in the figure below, the arm should be set so that contact tip movement is essentially perpendicular to the part as the part dimension varies. This is usually easy to arrange, as the arm is held in place by a friction clutch, and can be adjusted readily, even if the body of the test indicator is at an angle to the part.
The failure to recognize and correct for cosine error can result in rejecting good parts and/or accepting bad. However, at shallow angles, cosine error is usually small enough to ignore. For example, at a 5-degree angle and an indicator reading of 0.010", the difference is only 15 microinches—far below the ability of most mechanical test indicators to resolve or repeat.
There are an endless number of applications for the test indicator. However, they can be broken into two distinct categories: making dimensional measurements, or checking alignment or position during machine setup. Here are a few examples:
Aligning a bore to a machine spindle. This is most commonly used for centralizing a rotating machine tool’s spindle to the bore on the part. The test indicator’s contact can access even hard-to-reach areas while allowing convenient observation of the dial.
Aligning machine tool fixtures or a part to the axis of motion. When setting up a part, squareness and parallelism are critical for proper geometric machining. Before chips are cut, the test indicator is used to ensure that both part and fixture are properly located.
Exploring geometry of closely-located surfaces. This should be used when geometric characteristics such as concentricity or run-out of several closely-located surfaces have to be observed in relationship to a common datum. In these situations, test indicators permit fast verification.
However, the most common application of the test indicator is as a data transfer mechanism. It is used to transfer height dimensions from a reference piece to the object surface. This is a basic operation in surface-plate-based measurement. Also, when surfaces are recessed or hidden, test indicators are generally preferred and may actually be the only possible way of making the measurement.