**True
Ellipses in Isometric**

True ellipses can be drawn in isometric by the following methods,

**Offset Measurements****Conjugate diameters****Ellipse guides**

**Ellipses
by Offset Measurements**

If a circle lies on a plane that is not
perpendicular to the line of vision then it will project as an
ellipse. Lets take a square based prism and drill a hole down
through the centre of its top face. In isometric the hole in the
top of the prism will project as an ellipse. If we take the plan
of the object and locate points on the circle by means of offset
location measurements and then transfer these measurements to the
isometric drawing, the ellipse is easily drawn. Two lines are
required to find a single point on the circle. Start by drawing
horizontal lines through the circle. Where these lines and the
circle intersect draw vertical lines. Measure the distance from
each of the horizontal and vertical lines to a corner of the
prism. This will give the measurements required to draw the
ellipse in the isometric view. Instead of measuring from a corner
of the prism it is often convenient to use the centre lines of
the circle as a datum line. A datum line is a line from which all
measurements are taken from or with respect to. Follow the
procedure shown in the animation to find measurements 'a' and
'b'.

Transfer the horizontal 'a' measurements
onto the isometric drawing. Next transfer the 'b' measurements.
Where these lines intersect give points on the ellipse. The
ellipse on the top of the object is the same as that on the
bottom. Transfer points on the top ellipse parallel to the
vertical axes a distance equal to the heigth of the object i.e.
'c'. Follow the same procedure for each set of lines. Take as
many sets of lines as necessary to draw a smooth ellipse.

It is not the purpose of this site to explain, illustrate and
demonstrate the numerous methods of constructing true and
approximate ellipses. Some methods are briefly discussed below.
The offset location measurements method is the onle method shown
for generating true ellipses. This method can also be used to
draw ellipses in dimetric and trimetric.

**Angles,
Curves, Ellipses and Arcs in Isometric**

**Angles:** Angles only project as true size when the plane
of the angle is parallel to the plane of projection. As we saw
earlier the angle between the adjacent edges of a cube in
isometric project as 120º and not 90º. Thus, angles can project
larger or smaller than true size.

Take this object whos outline projects as an
equilateral triangle in plan. None of these angles will project
as 60º in the isometric drawing. Establish this fact for
yourself by drawing the object in isometric and then measure each
of the angles. The isometric drawing of this object is shown
below.

How about questions where we are not given all the dimensions and have to work them out from angles?

In the elevation view of the object we
do not know dimension 'X' so how do we find it? Draw the plan and
elevation and then draw the enclosing box for the isometric
drawing. If we draw a rectangle around the outline of the
elevation we can step off dimension 'X'. Simply step this
distance off parallel to an isometric line in the isometric
drawing. Complete the drawing in the usual way. Thus, the key to
solving angular measurements in isometric is to convert them to
linear measurements along isometric lines.

**Curves:**
Curves in isometric are drawn by means of offset location
measurements method (also known as offset measurements). Pick
points on the plan and elevation and locate these points by
offset measurements. These measurements are then transfered to
the isometric view in the usual way. Use an irregular curve to
connect all the points.

**Ellipses:**** **Circles will always
appear as ellipses in isometric and can be drawn using offset
measurements. Ellipses can be drawn in isometric using any off
the conventional ellipse constructions. The Four Centre Ellipse
is quite satisfactory in most cases. This involves circumscribing
a square about the circle in the orthographic view. This square
is then transferred to the isometric view where it will appear as
an isometric square or a rhombus. Mark in the mid-points on the
lines of the square where the ellipse is a tangent to the rhombus
in this view also. Draw in the perpendicular bisectors of the
sides of the rhombus. Where these intersect gives the centres for
the four arcs forming the ellipse. For more accurate ellipses the
Orth Four-Centre Ellipse can be used or ellipse guides.

**Arcs:**
Arcs in isometric are simply sections of ellipses so any of the
constructions used to draw ellipses can be used to draw arcs.

**The
Sphere In Isometric**

Shown above is a sphere which has been enclosed in
a cube. A vertical cut is taken through the centre of the sphere
and on a plane parallel to one face of the cube so that a great
circle results. This great circle appears as an ellipse on the
isometric drawing. The only points of this ellipse which appear
on the isometric drawing of the sphere, are the points on the
extremities of the major axis. Using one of these points and the
centre of the ellipse as radius the isometric drawing of the
sphere is completed.

The isometric drawing of the sphere is a circle whose diameter is Ö(3/2) times that of the actual diameter of the sphere. The isometric projection of the sphere is simply a circle whose diameter is equal to the true diameter of the sphere. It is important when drawing in an isometric drawing that the circle drawn is an isometric drawing and not an isometric projection of the sphere. If a isometric projection of a sphere is drawn in an isometric drawing then the drawing will be distorted as the sphere will appear smaller than it should.

In axonometric the sphere will always project as a circle. The radius of this circle will be the radius of the sphere.

**Hidden
lines, Centre lines**

**and
Dimensioning in Isometric**

Hidden lines are always omitted unless they provide information not readily available from the axonometric drawing or projection.

Centre lines are drawn if they are needed to indicate symmetry or if they are needed for dimensioning. Where possible they are omitted.

Two systems of dimensioning are approved by ANSI namely, the pictorial plane system (aligned) and the unidirectional system. The former being the preferred system. Inclined lettering is usually not recommended for pictorials (it has been used on this web site for demonstration purposes). Typically, simple vertical lettering is used for either system. In Ireland we use the British Standard 308 (BS 308) when dimensioning. Here is an example of good dimensioning in isometric.