A student is trying to determine the flatness and parallelism of the top surface of a part whose cross-section, assumed uniform in the Z direction, is shown in the figure.
Which ONE of the following options is CORRECT?
Show Hint
Ask which tolerance zone needs the Datum to be drawn. Flatness never references a datum; parallelism always does.
\( a + b \) is parallelism while \( b - a \) is flatness.
\( a + b \) is flatness while \( b - a \) is parallelism.
Show Solution
The Correct Option isA
Solution and Explanation
Step 1: Using one decisive test instead of definitions:
There is one quick test that separates every form tolerance from every orientation tolerance: does the tolerance zone need a Datum to be defined, or not?
Form tolerances like flatness never need a Datum. Orientation tolerances like parallelism always need one. We can answer the whole question just by applying this single test to zones $a$ and $b$.
Step 2: Applying the test to zone $a$:
Looking at the figure, the band for $a$ is tilted at an angle that matches the slope of the actual top surface, and it has no fixed relationship to the bottom Datum line.
Since this zone can be defined using only the surface itself, with zero reference to the Datum, it fails the needs a Datum test, which means it must be the form tolerance, flatness.
Step 3: Applying the test to zone $b$:
The band for $b$ is drawn as two lines running exactly parallel to the bottom Datum surface, at a constant vertical offset from it.
This zone cannot even be drawn without first knowing where the Datum is, so it passes the needs a Datum test, which means it must be the orientation tolerance, parallelism.
Step 4: Ruling out the combined-quantity options:
Options (C) and (D) propose that $a+b$ or $b-a$ represent these tolerances, but flatness and parallelism are each single, independently defined tolerance zones read directly off the drawing, not sums or differences of two zone widths, so these options do not fit how GD&T zones are actually defined.
Final Answer:
The Datum reference test confirms $a$ is flatness and $b$ is parallelism.
\[ \boxed{\text{Option (A)}} \]
