When
Inspecting Flatness, There Is No Datum.
Flatness
is another geometric tolerance that is challenging to inspect.
It requires isolating the feature from the rest of the part
since there can never be a datum referenced with flatness. In
this example, the flatness has been applied to datum feature
A. Three possible inspection methods are illustrated. In all
cases, the considered feature is isolated from the rest of the
part and aligned relative to the indicator. In the first case,
the part is leveled on the surface plate. In the second
illustration, the surface is leveled by placing it on three
equal height gage blocks. The indicator is then moved across
the surface. In the third illustration, the CMM will
mathematically "level" the points of the surface contacted by
the probe. In all cases the FIM (Full Indicator Movement) may
not exceed 0.2mm.
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| Problem: If the surface is convex,
the part will rock making it difficult to
determine the minimum indicator reading over the
entire surface. |
Problem: Ideally, the gage blocks
should be placed under the high points on the
surface. Otherwise, the indicator movement may not
be the lowest possible. |
| In Examples 1 and 2 the
problem may cause an acceptable surface to be
rejected. |
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Of course, a CMM will automatically align
the points to evaluate the flatness
error.
Problem: Often insufficient
points are taken to evaluate the flatness error.
As a result, an out of spec surface may be
accepted.
Bottom line, inspecting
flatness requires time and
patience. | | |
