Question 3 of 5: Interpreting the GD&T annotations

Question 3: Interpreting the GD&T annotations (10 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

What is printed. The drawing is an L-bracket whose upright carries a cylindrical pin projecting horizontally.

• In the front view, a datum-feature symbol (filled triangle to a box “A”) is attached to an extension line of the upright’s inner vertical face, the face the pin projects from.
• In the end view, the pin’s size dimension ⌀25 carries a feature control frame. The figure shows it clearly as ⊥ | .05 | A: perpendicularity, tolerance .05, datum A.

1. Datum feature A. The inner face of the upright is datum feature A. For inspection, the real (imperfect) surface is placed against a true plane: a surface plate or fixture face that contacts its high points. That plane becomes datum A, the reference every A-referenced tolerance is measured from.
2. ⌀25 — size. The pin’s diameter is 25, toleranced by the title-block general tolerance. By Rule #1 (envelope principle), the pin must also have perfect form at its maximum material size.
3. ⊥ — perpendicularity (an orientation control). The frame is attached to the size dimension, so it controls the pin’s axis (its derived median line), not its surface. The axis must be perpendicular to datum A within the stated zone. It controls tilt only; it does not locate the pin. The pin’s height and side position come from other dimensions.
4. .05 — the tolerance zone. As printed there is no ⌀ symbol in front of .05, so the zone is two parallel planes .05 apart, both perpendicular to datum A, and the pin axis must lie between them. Over a pin of length ℓ, this allows an axis tilt of at most .05/ℓ radians in the direction the zone controls. For a round pin the designer usually intends ⊥ | ⌀.05 | A. That zone is a cylinder .05 in diameter with its axis exactly perpendicular to A, and it restricts tilt in every direction. The figure below contrasts the two readings.
5. A — datum reference. The zone is oriented relative to datum A only. No secondary datum is needed, because perpendicularity to a plane fixes only orientation.
6. No modifier (RFS). No Ⓝ appears after the tolerance, so it applies regardless of feature size: .05 is allowed whatever the pin’s actual diameter, with no bonus tolerance.
7. Units note. “.05” without a leading zero follows the inch convention, while ⌀25 reads as metric. In metric (ISO/CSA) practice the value is written 0.05. The tolerance is .05 in the drawing’s units.

In plain terms: measured from the upright’s inner face, the pin must stick out square to that face. Its axis may lean by no more than a .05 band (or a ⌀.05 cylinder, if that was intended) over its length, whatever its actual size within the size limits.

[Figure not reproduced: Fig. Q3 — perpendicularity zone as printed (two parallel planes .05 apart, ⊥ A) versus the cylindrical-zone reading (⌀.05). Zone widths exaggerated. See the official exam paper.]