While using a Dial Gauge or a Dial Test Indicator a common error that many may not realize, can occur: The Cosine Error.
What is the Cosine Error?
Cosine error occurs in measuring instruments when there is an angular misalignment between the Dial Indicator and the measuring workpiece. The line of axis of the Dial Indicator and the measuring object should be aligned on the same line.
To illustrate how a Cosine Error may occur we can look at the following examples:
Cosine Error while using a Dial Test Indicator:
If we take a 0.05 mm feeler strip. The stylus is aligned parallel to the measuring surface in the first Lever Dial. In the second Lever Dial, the stylus has been tilted to 45°. Now let’s measure the feeler strip. First Dial reads 50 microns and the second dial reads 60 microns which means there is a 10-micron deviation.Â
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This effect can be minimized by setting the stylus to a minimum angle θ. If needed the dial readings can be compensated for the actual θ value by using the results given in the table below.Â
Cosine Error while using a Plunger Dial Gauge:
Once again if we take a 0.05mm feeler strip. In the first Dial, the Plunger Dial and the measuring object have been aligned on the same line of axis. In the second plunger dial, the line of the axis has been misaligned with the measuring object. Upon measurement, the first Dial reads 49 microns and the second dial reads 52 microns.Â
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If both dial gauges are aligned on the same axis line, then both the dials will read 49 microns.Â
While measuring with the plunger Dial Gauge make sure that the line of the axis is aligned, tilt the stylus to a minimum angle from the object, and then proceed for measurement. If needed the error can be compensated.Â
For more details on how the cosine effect impacts the accuracy of a Dial Gauge or Dial Test Indicator do not miss the following video