GageGuide
by Drew Kopplemann
Dial Indicators for Every Need
If you think something could possibly be
measured with an indicator, it probably can.
Dial indicators are jacks-of-all-trades. Available in a wide variety of
standard styles and readily customized, they can be used to measure the
majority of dimensional attributes and a great many nondimensional characteristics
as well.
By itself, a dial indicator is not a gage. Dimensional gages require some
means to position the workpiece relative to the indicating device. This
can be as basic as a comparator stand, which holds the indicator above a
fixed reference contact for height measurements. Or it can be as specific
and highly engineered as a fixture gage that measures a production part,
with six or more indicators positioned to check several features simultaneously.
The common element, however, is the indicator.
Indicators vary in their measurement ranges and levels of magnification.
When equipped with secondary dials or revolution counters, they are capable
of making absolute (as opposed to comparative) measurements of 3" or
more. Discrimination values (i.e., the dimension indicated by one "tick"
on the dial) typically range from 0.0001" to 0.001". Of course,
both inch and metric versions are available, as are dial faces that show
both sets of units.
Continuous-reading dial faces are for unidirectional measurements, in which
all readings appear as positive values above zero. In contrast, a "balanced"
dial face shows positive values on one side of zero and negative values
on the other, to read plus or minus deviation from the mastered dimension.
Positive values can be to either the left or right of zero. Needle movement
can also be clockwise or counterclockwise.
Indicators can differ in their mechanical engineering for different applications.
Most indicators are sprung in the down, or closed, position, but some are
sprung to return to the up, or open, position and must be pushed down to
contact the workpiece. Contact action is normally parallel to the plane
of the dial face, but some indicators have perpendicular action (from the
rear), promoting more convenient mounting and easier viewing in some applications.
Indicators can be specially engineered to withstand wet or dusty working
environments, or even to provide readings inside of processing ovens up
to 600° F.
Indicator faces are easily adapted for special functions. One option is
"stoplight" gaging, in which the dial face is segmented into green,
yellow and red sections, to show the operator if a part is good, borderline
or out-of-tolerance. Dials can be segmented into any number of wedge-shaped
sections to help operators sort parts by size category. And whole sections
of the dial can be masked, so that out-of-tolerance measurements can't be
read at all.
Movement of the indicator's sensitive contact can be made to read as a fraction
or multiple of that movement through the use of a special-ratio indicator
face. For example, an indicator could be used to gage a part to coarse increments
of 0.01", by including a 10:1 motion-transfer mechanism between the
part and the indicator contact, and an indicator face that reads 0.01"
for every 0.001" of movement at the contact. Special ratios are also
useful for measuring dimensions as a function of angular features (e.g.,
measuring the diameter of a countersink as a function of its depth).
Even nondimensional units can be measured with dial indicators. For example,
direct relationships exist between temperature and the deflection of some
materials. By measuring material deflection with an indicator, it is therefore
possible to derive a temperature reading. But instead of making the gage
operator perform the conversion with a calculator, the indicator can have
a special dial calibrated in degrees of temperature. Dial indicators have
also been used to measure Rockwell hardness, cable tension, spring and impact
forces, thread pitch and foot-lbs. of torque.
Accessories enhance the adaptability of dial indicators. Moveable "flags"
on the dial face allow the operator to set tolerance limits. Contact points
and anvils are available in a range of stock and custom sizes and shapes.
Weights may be added to the mechanism to ensure that compressible materials
are measured under a consistent amount of force. Lifting levers allow the
sensitive contact to be retracted for easier part insertion.
Every one of these variations could be the subject of more detailed description
and applications advice. Dial indicators are used in a surprising range
of applications. If you think something could possibly be measured with
an indicator, it probably can.
About the author
As applications manager, gaging products, at Federal Products Co. in Providence,
Rhode Island, Drew Koppelmann provides dimensional gaging applications assistance
to companies in a wide range of industries. He can be reached by fax at
(401) 784-3246.
