|
 Tie Those Interrupted Features of Size Together
(in accordance with the ASME
Y14.5-2009 standard)
PDF of This Tip
Another
Feature of Size Tip
In
previous Standards, it wasn’t clear how to tie
together interrupted features of size if they were
to become the primary datum feature. The 2009
Standard now gives us two ways to accomplish this.
We can use a position tolerance without a datum
reference or the new continuous feature symbol.
 On
this part the 2X Ø16 features need to act as a
single feature in the assembly. They are also used
as the primary datum feature because they will
constrain the most degrees of freedom of this part,
once assembled (a topic for another Tip). On the
first drawing a position tolerance of zero at MMC
has been applied to the pattern of two features.
This creates a perfect form boundary of Ø16.02 that
the features may not violate. In the previous Y14.5
standards it was stated that the position tolerance
must always have a datum reference. In spite of
this, in the lower segment of a composi te
position tolerance a datum reference was not
required. In other words, the concept of using a
position tolerance without a datum reference was
supported but discouraged. The 2009 revision
illustrates and supports this concept.
The 2009 revision has also added another way to
accomplish the same thing. The note CONTINUOUS
FEATURE or the continuous feature symbol is used to
identify a group of two or more features of size
where there is a requirement that they be treated
geometrically as a single feature of size. On the
next drawing the continuous feature has been applied
to the 2X Ø16 features. This drawing has the same
meaning as the previous drawing. In both cases the
two features must not violate an envelope of Ø16.02
and the local size of the features must be within
the limits of size at each cross section. On the
first drawing the envelope of Ø16.02 is required
because of the position tolerance of Ø0 at MMC. On
the second drawing the envelope is required because
of Rule #1.
This
month’s Tip to you is to be sure you tie your
interrupted features of size together. If they are
features of size which will serve as the primary
datum feature—use one of these methods.
|
