Indicator (distance amplifying instrument)
Since the mechanisms are necessarily delicate, rugged framework construction is required to perform reliably in harsh applications such as machine tool metalworking operations, similar to how wristwatches are ruggedized.
Electronic versions employ an optical or capacitive grating to detect microscopic steps in the position of the probe.
However, the user must know how to use them properly and understand how in some situations, their measurements will still be relative rather than absolute because of factors such as cosine error (discussed later).
The spring-loaded probe (or plunger) moves perpendicularly to the object being tested by either retracting or extending from the indicator's body.
Alternatively, the device can be held by the cylindrical stem that guides the plunger using a collet or special clamp, which is the method generally used by tools designed to integrate an indicator as a primary component, such as thickness gauges and comparators.
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.
If the cause of movement is perpendicular to the finger, the linear displacement error is acceptably small within the display range of the dial.
Other materials are available for contact points depending on application, such as ruby (high wear resistance) or Teflon or PVC (to avoid scratching the workpiece).
199A),[3] as well as smaller companies like Ideal Tool Co. Devices that could be used as either a lever test indicator or a plunger type were also manufactured by Koch.
Many models of digital indicator can record and transmit the data electronically to a computer, through an interface such as RS-232 or USB.
This is a great convenience to the user because of the practicality of having the ball being clear of the indicator body such that the unit may pass over a surface.
But the avoidance of mistakes of the comparative-versus-absolute-confounding type rests with the knowledge and attention of the user, rather than with the instrument itself, and thus repairers of DTIs usually will not certify the accuracy of a DTI that cannot offer an accurate absolute measurement—even if it is perfectly good for comparative use alone.
Such a DTI could still be certified (and labeled) for comparative use only, but because risk of user error is involved, gauge calibration rules in machine shops either demand a "comparative use only" label (if the users can be trusted to understand and follow it) or demand that the indicator be removed from service (if not).
