Size and Form Are Individual Controls.
All of the
tolerances defined in the Y14.5 standard have a purpose. In
general, they control size, form, orientation and location.
Often people give various tolerances more credit than they
deserve. The key to proper application of GD&T is
understanding the relationships and limitations of each of the
controls. The standard is very clear on the limitations of
size and form controls. They are individual controls. They do
not relate features to one another. A designer may think that
by specifying a size and quantity symbol, the resulting part
will look like the model on the right. Unfortunately, since
size is an individual control and does not control the
interrelationship between features, the model shown below may
result. Even if the straightness and circularity controls are
added, the lower model may be produced and still meet the
drawing's requirements. This drawing is merely stating that
each of the 8 cylinders must meet size, straightness and
circularity individually. In order to assure that you get a
part closer to the model at the right, a coaxial control must
be used.
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What the designer wants. |
What the designer might get according to the
print. | |
