How does a circle lying in an isometric plane appear?
Answer: C
One diameter of the circle lies along an isometric line and foreshortens with it while the perpendicular diameter foreshortens differently, which is exactly the condition for an ellipse. It is why isometric drawings of holes and shafts are set out as ellipses throughout.
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On which line does the major axis of an isometric ellipse lie?
Answer: D
The rhombus that encloses the circle has unequal diagonals, and the ellipse touches the midpoints of its sides with its major axis along the longer one. Getting the two diagonals the wrong way round produces an ellipse visibly tilted against the face it should lie in.
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What does the four-centre method produce when applied to an isometric circle?
Answer: A
Two arcs of large radius and two of small are struck from the corners and the diagonal intersections of the enclosing rhombus. Being circular arcs it is not a true ellipse and departs slightly near the ends of the axes, which is accepted for the speed it offers.
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Why can angles not be set out in isometric with a protractor?
Answer: B
Only lengths along isometric lines survive the projection in a predictable ratio; an angle between two such lines opens or closes according to the plane it lies in. A 90 degree corner of the object appears as either 60 or 120 degrees on the drawing.
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A right angle on the object appears in isometric as
Answer: C
Which of the two it becomes depends on the orientation of the corner within its isometric plane. It is the clearest demonstration that angular measurement on an isometric drawing is meaningless without reference to the underlying object.
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How is an irregular curved outline transferred into isometric?
Answer: D
A grid of isometric lines is laid over the orthographic view and each point of the curve is located by its two offsets, then plotted in the isometric box. Enough points are taken for a smooth curve to be drawn through them by hand or with a template.
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How does a sphere appear in an isometric drawing?
Answer: A
Its silhouette is the same circle from every direction, so no foreshortening applies to the outline itself. In a true isometric projection that circle has the sphere's actual diameter, which is the one case where projection and drawing coincide in size.
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How does oblique projection differ from isometric in its treatment of one face?
Answer: B
Keeping the front face parallel to the plane of projection leaves it in true shape, with the depth drawn at an angle. It is a great convenience where that face carries most of the detail, particularly circles which stay circular rather than becoming ellipses.
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What distinguishes cabinet from cavalier oblique projection?
Answer: C
Halving the depth counteracts the exaggerated appearance that cavalier projection gives, since drawing the depth full size makes the object look unnaturally long. Both forms keep the front face true, which is the defining property of oblique projection.
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A 100 mm edge is drawn in true isometric projection as
Answer: D
Multiplying by the isometric ratio of 0.816 gives 81.6 millimetres. An isometric drawing would show the full 100 millimetres and be correspondingly larger, which is why the two must not be confused when absolute size matters.
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