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3.26 Ellipses for Oblique Drawings

Circular shapes that are parallel to the front plane of projection appear as circles in oblique view. In isometric views they appear elliptical. Figure 3.63 shows a comparison of a cylinder drawn in isometric and oblique views.

3.63

3.63 Oblique and Isometric Projections for a Cylinder

It is not always possible to orient the view of an object so that all its rounded shapes are parallel to the plane of projection. For example, the object shown in Figure 3.64a has two sets of circular contours in different planes. Both cannot be simultaneously placed parallel to the plane of projection, so in the oblique projection, one of them must be viewed as an ellipse.

3.64

3.64 Circles and Arcs Not Parallel to Plane of Projection

If you are sketching, you can just block in the enclosing rectangle and sketch the ellipse tangent to its sides. Using CAD, you can draw the ellipse by specifying its center and major and minor axes. In circumstances where a CAD system is not available, if you need an accurate ellipse, you can draw it by hand using a four-center arc approximation.

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