Orthographic Projections, Continued
Circles, arcs,
and ellipses
Circles, arcs, and ellipses do not appear in true size or shape in an isometric
drawing or projection because all surfaces of the projected object are angled
toward the plane of projection. To draw circles, arcs, and ellipses in
isometric drawings and projections, you must use the offset measurement or
four-center method. Make sure to draw enough points to accurately fix the
path of the curve. The more points plotted, the greater the accuracy. Once
you plot the points, lightly freehand sketch the curve and darken it with the
aid of an irregular or french curve.
To draw circles, arcs, or ellipses by plotting a series of points, use this table:
Step
Action
1
Draw randomly spaced parallel lines across a circle projected in
true size and shape on the plane of projection. To increase
accuracy, plot more points using more parallel lines.
2
Transfer these lines from the circle to the isometric drawing or
projection with a pair of dividers. To locate points in objects with
some width, vertically extend the points a distance equal to the
width of the object. Some of these points will appear as hidden
points and lines.
3
Draw the final ellipse freehand and darken it with an irregular or
french curve.
Continued on next page
6-21
