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Center Planes as Datum
Features Constrain 3 Degrees of Freedom
(in accordance with the ASME
Y14.5-2009 standard)
PDF of This Tip
There is a new chart
in the ASME Y14.5-2009 standard that shows the
degrees of freedom that are constrained when the
feature is a primary datum feature and, where
appropriate, applies RMB. The chart also shows the
relationship between the feature type, simulator and
theoretical datum(s) that are established. The next
several monthly Tips illustrate the features shown
in the chart. Although some of the simulators are
illustrated as physical inspection equipment, these
simulators may also be simulated by "soft gaging"
optical or probing methods.
Center Plane as a
Datum Feature
A width is a feature that is made up of two parallel
planes. The datum feature identification symbol must
be associated with the size dimension. The datum
established by a width is the center plane of two
parallel planes at minimum separation. For an
external width the practical datum simulator might
be a milling vise where the jaws of the vise remain
parallel as they close in on the width feature. The
datum is the center plane between the jaws of the
vise. Since the datum is a plane, it will constrain
3 degrees of freedom.
The video for this Tip is from Don’s
new computer based training course on GD&T
Fundamentals available on DVD and the internet in
March 2010. |
