Orthographic Projections, ContinuedCircles, arcs,and ellipses(Continued)Ellipses have two axes, a major axis (long) and a minor axis (short). Whenthe horizontal and vertical centerlines of a circle drawn parallel to the planeof projection is drawn in isometric and each has parallel tangents, theybecome conjugate diametersrepresenting the major and minor axes,respectively. The two diameters of an ellipse are conjugate when each isparallel to the tangents at the ends of the other. One of a given pair of givenconjugate diameters is, as a rule, not perpendicular to the other. In general,here are three rules to remember when drawing ellipses in isometric, (1) themajor axis of an ellipse is equal to the diameter of the circle, (2) the majoraxis of the ellipse is always at right angles to the centerline of the circle, and(3) the minor axis is at right angles to the major axis, which coincides withthe centerline of the circle. ‘Another way of drawing ellipses in isometric isto use an ellipse template. Ellipse templates are available in many differentdegrees with the major and minor axes marked on the template. Base yourselection of the appropriate ellipse on the location and degree of the axes.Figure 6-18 shows the relationship of the conjugate diameters of a circle tothe major and minor axes of an ellipse.Figure 6-18.—Major and minor axes.OffsetOffset measurements are measurements used to locate features or edges withmeasurementsrespect to the features and edges on the main surface of the object. Featureand edges parallel to edges of the main surface remain parallel in isometricdrawings.Continued on next page6-23

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