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With so many options it
is no wonder that many people find the differences hard to
understand. Each control has its place, although some are more
difficult and time consuming than others. By comparing sample
inspection methods, the differences may become clearer. This
month's tip will illustrate the differences between position
and concentricity. Next month's tip will continue with the
runout tolerances and profile.
Position may use the MMC
and LMC modifiers on the tolerance and Datum references. The
illustration below shows a possible gage (ignoring gage
tolerance and wear allowance) that could be used when MMC is
the modifier. If the sizes are within the size tolerances but
the part doesn't fit the gage, the features position
(coaxiality) is out of spec. This type of control works well
when the main concern is the assembly of this part to another.
The gage may usually be thought of as representing the worst
mating part.
According
to the ASME Y14.5M-1994 Standard, when no modifiers are
present, the implied condition is regardless of feature size
as shown below.
In this case, the axis of the datum feature and the
feature being controlled must be determined to find the error
in coaxiality.
Although this control may be applied to bearings and
dynamic balance applications, the job can usually be
accomplished at a lower overall cost by using one of the
runout controls.
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All of the coaxiality
controls are intended to control concentric features. For that
reason, many designers and engineers choose concentricity.
Unfortunately, concentricity ignores the size, roundness and
cylindricity of the feature. It requires that the inspector
derive a median line (see the June 1997 Tip-of-the-Month). In
a situation where you don't care about the size, roundness or
cylindricity of the feature, concentricity may be specified.
In thirty plus years of reviewing mechanical designs, I have
never found a design where this is truly the case. The closest
application, perhaps, is when dynamic balance is needed. In
such a case, measuring a part statically does not assure
dynamic balance if the material is not homogeneous. If dynamic
balance is required, a dynamic balancing note is probably in
order rather than concentricity. For that reason, I often use
the line, "When in Doubt, Use Runout."
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