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Does Runout Equal
Concentricity?—Sort Of.
(in accordance with the ASME
Y14.5-2009 standard)
PDF of This Tip
Another
Circular Runout
 People
often ask if when they measure runout on a
cylindrical feature that is coaxial to a datum axis
are they also reading the concentricity error. Or,
should they take the runout measurement and double
it or cut it in half to determine the concentricity
error. Well, usually, what you see is what you get.
In other words, the concentricity error cannot be
greater than the measured runout. First, runout
tolerances and concentricity require that a datum
axis be established. In these figures, consider the
smaller diameter as the datum feature. Concentricity
requires deriving the median points of the featu re.
So, the concentricity tolerance
zone is cylindrical and controls the center of the
feature. That is why a diameter symbol precedes the
tolerance. One way to inspect concentricity is with
two opposed indicators, as shown. This can be very
time consuming
Runout is usually
inspected with a single indicator because it is a
surface control. Total runout is a composite of
coaxiality (for all practical purposes,
concentricity) and cylindricity. (You might want to
check out previous Tips for a further explanation of
runout and the difference between circular and total
runout.) If the feature is perfectly cylindrical,
concentricity error will account for all of the
runout variation. You do not double it or cut it in
half. Depending on the type of out-of-roundness, the
more out of cylindrical the feature is, the better
the concentricity may have to be.
Concentricity is on
the diameter. Eccentricity (which we do not have a
symbol for) is on the radius. People often confuse
the two. Runout is almost always easier to measure
and controls the form of the feature as well as the
concentricity. Concentricity ignores any form error.
So, runout is almost always a better choice.
“When in Doubt—Use Runout”.
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