Notes on practical mechanical drawing, written for the use of the students in general engineering drawing in the University of Illinois by Wilson Victor Tyson 1864-

Author:Wilson, Victor Tyson, 1864- [from old catalog]
Language: eng
Format: epub, pdf
Tags: Mechanical drawing
Publisher: State College, Pa., The author
Published: 1907-03-25T05:00:00+00:00

31. To draw an ellipse by the focii method.

To draw it by the principle, in other words, that the sum of the focal radii to any point on the c^lr^'e is a constant (see Fig. 17). The constant is equal to 2a. The focal radii are F P and FT, respectively.

When the major and minor axis are given, the focii may^be obtained as follows: With a radius equal to the

Irregular Curves and Geometrical Drawing

89

semi-major axis, and a center at either extremity of the minor axis, describe arcs to cut the major axis in points which will be the focii, for FY plus YF' = 2a. To find any point on the curve, as P, take any radius not less than XF or X'F^ and with center at F, describe an arc of a circle;

Fig. 17

XI

with a radius equal to the difference between 2a, and the radius just taken, and a center at F', describe an arc to intersect the first one at P, which will be a point on the curve. There are three other points on the curve in addition to P which are symmetrical with respect to the axis, hence the practical method of procedure is to arbitrarily divide the major axis between the focus and the center of the ellipse into several parts, each to give two radii for four symmetrical points on the curve. With each radius taken

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