by Drew Koppelmann
Digital or Dial Indicators?
Until fairly recently, mechanical dial indicators were the standard indicating
device for most dimensional gages. Dial indicators are rugged, reliable,
easy to use and inexpensive compared to air gaging or electronic gage heads
Now, however, gage users can choose between dial indicators and digital
indicators. Both types have their particular advantages, so choosing one
isn't easy. Most dial indicators still cost less than digital indicators,
but the cost difference isn't great-and digitals can be more economical
The greatest advantage of a digital indicator is its ability to output data
automatically for statistical process control. After a workpiece is placed
in the gage, an operator simply presses a button or footswitch to export
the measurement to a computer, printer or data logger. In some applications
where gaging is primarily performed for the purpose of SPC, and not to inspect
dimensions on individual parts, the operator doesn't even need to read the
display. Throughput can, therefore, be very rapid.
Compare this to the procedure typically done with a dial indicator. The
operator places the workpiece in the gage, reads the dial and records the
measurement on a worksheet. Then, a page full of work-sheet data is keyed
into a computer. Errors can creep into the system when reading, recording
and inputting the data.
As analog devices, however, dial indicators offer some advantages. In many
gaging applications, the user doesn't need to know exact measurements-it's
only necessary to determine if a dimension is over, under or within tolerances.
A glance at a dial indicator is often sufficient to tell the story. Does
the needle fall between the two set-points? In comparison, digital-indicator
users must read the number on the display, then compare that number with
the upper and lower tolerance limits.
Analog dials excel at revealing trends. Experienced users recognize when
a series of measurements is creeping in one direction or another. Such movement
is not so apparent with a digital display. The user is also able to "split
grads" (i.e., to estimate measurements that fall between two "ticks"
on the dial face). Digital indicators lack this capability because the last
digit in a reading is always absolute. Of course, this can also be viewed
as an advantage because it eliminates judgment calls and approximations.
A common source of error concerning mechanical indicators occurs when an
operator fails to notice that the needle has made a complete revolution
of the dial and returned to zero. The part may be grossly out of tolerance,
but to the inattentive operator, the needle appears to be exactly where
it should be. Digital displays never return to zero when a part is out of
tolerance-the numbers just keep getting bigger. However, special single-revolution
dial indicators exist that avoid this problem.
Recognizing the benefits of analog displays, manufacturers have managed
to incorporate analog-style features into electronic indicators, including
quasi-analog bar-graph or fan displays. Almost all digital indicators have
tolerance flags that alert operators to out-of-tolerance conditions. These
supplementary LCD or LED elements register at limits preset by the user
and are readily noticed by even the most careless of operators.
There is at least one electronic indicator on the market that can be equipped
with a remote transducer. This is particularly helpful in gaging complex
parts-when many dimensions must be checked simultaneously. Multiple transducers
can be closely spaced together in a fixture gage, with some of them virtually
buried in inaccessible locations. Meanwhile, the readouts are remotely rack-mounted
for easy viewing. Such an arrangement would be impossible with dial indicators.
While digital indicators seem to have a functional edge in many gaging applications,
dial indicators still play an important role in many shops. The choice primarily
depends on the gaging task and the operator's capabilities.
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, including
automotive, aerospace, packaging and electronics. He can be reached by fax
at (401) 784-3246.