Circular shapes in isometric drawing (Ellipse)

1. Ellipse
When a cone is cut by a section plane B-B at an angle, a more than half of the apex angle i.e., e and less than 90°, the curve of the section is an ellipse. Its size depends on the angle and the distance of the section plane from the apex of the cone.

STEPS of DRAWING A FOUR-CENTER ELLIPSE:
Approximate ellipses such as these, constructed from four arcs, are accurate enough for most isometric drawings. The four-centre method can be used only for ellipses in isometric planes. Earlier versions of CAD software, such as AutoCAD Release 10, used this method to create the approximate elliptical shapes available in the software. Current releases use an accurate ellipse.
1. Draw or imagine a square enclosing the circle in the Multiview drawing. Draw the isometric view of the square (an equilateral parallelogram with sides equal to the diameter of the circle).
2. Mark the midpoint of each line and draw a perpendicular line from each.
3. Draw the two large arcs, with radius R, from the intersections of the perpendiculars in the two closest corners of the parallelogram.
4. Draw the two small arcs, with radius r, from the intersections of the perpendiculars within the parallelogram, to complete the ellipse.

Notes:
1. Here is a useful rule. The major axis of the ellipse is always at right angles to the centreline of the cylinder, and the minor axis is at right angles to the major axis and coincides with the centreline.
2. As a check on the accurate location of these centres, you can draw a long diagonal of the parallelogram as shown in Step 4. The midpoints of the sides of the parallelogram are points of tangency for the four arcs.