Page 73 - CITS - ED - Mechanical

ENGINEERING DRAWING - CITS

The front view and the top view of a sphere resting on flat surface are shown in Fig 17a.
O as its centre, D is the diameter and P is the point of contact with the surface.
Assume a vertical section the centre of the sphere. Its shape will be a circle of diameter D. The isometric projec-
tion of this circle are ellipses 1 & 2 Fig 17(b) drawn in two different vertical positions around the same centre O.
The major axis in each case is equal to D. The distance of the point P from the centre O is equal to the isometric
radius of the sphere.

Fig 16 Fig 17

Again, assume a horizontal section through the centre of the sphere.
The isometric projection of this circle is shown by the ellipse 3, drawn in a horizontal position around the same
centre O. In all the three cases 1,2 & 3 the outermost points on the ellipse from the centre O is equal to 1/2 D.
Thus, it can be seen that in an isometric projection, the distances of all the points on the surface of a sphere from
its centre are equal to the radius of the sphere. Hence, the isometric projection of a sphere is a circle whose
diameter is equal to the true diameter of the sphere.
Also the distance of the centre of the sphere from its point of contact with the flat surface is equal to the isometric
radius OP of the sphere.
It is therefore of the utmost importance to note that isometric scale must invariably be used while drawing isometric
projection of solids in conjunction with spheres or having spherical parts.

Fig 18

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CITS : Engineering Drawing (Mechanical) - Exercise 4