Short Answer:

In isometric projection, a circle appears as an ellipse due to the 3D viewing angle. To draw an ellipse, we first draw a square (or rhombus) on the isometric plane that represents the circular boundary. Then, using special methods like the four-centre method, we draw curved arcs inside that square to form an ellipse.

This method helps us to represent circular features like holes, cylinders, and round parts correctly in isometric view. Since circles become stretched when viewed at angles, ellipses are used to maintain a natural and realistic look in the 3D drawing.

Detailed Explanation:

Drawing an ellipse in an isometric projection

In mechanical drawing, isometric projections are used to give a 3D view of objects. When a circle is viewed in isometric projection, it no longer appears as a perfect round shape. Because of the angle (30°) used in isometric drawings, the circle gets stretched and appears as an ellipse. To show circles properly in isometric drawings—such as on cylindrical surfaces or round holes—we must draw ellipses using a correct and simple technique.

Let’s understand the step-by-step method to draw an ellipse in isometric projection in simple words.

1. Identify the plane of the circle
   Before drawing, first check which surface the circle lies on:

• Top plane (horizontal)
• Front plane (vertical)
• Side plane (vertical side)

The method is same for all, only the direction of the ellipse changes based on the plane.

2. Draw an isometric square (rhombus)
   To begin, draw a square in isometric form that represents the circle’s boundary. For example, if the diameter of the circle is 40 mm, draw a 40 mm × 40 mm rhombus on the selected isometric surface.

• On top plane: use 30° left and right lines
• On front/side plane: use vertical and 30° line accordingly

This square is the base in which the ellipse will be drawn.

3. Mark midpoints of all sides
   Find and mark the midpoints of all four sides of the rhombus. These points help guide the curves while drawing the ellipse.
4. Draw centerlines
   Join the opposite midpoints to form two diagonals (centerlines) inside the rhombus. These intersecting lines divide the rhombus into equal sections and give the center of the ellipse.
5. Use the four-centre method to draw the ellipse
   The four-centre method is the most common and simple way to draw an ellipse manually. It uses four different arc segments to form the ellipse:

• Use a compass or free hand to draw four smooth arcs joining the important points—corners and midpoints.
• The arcs are drawn using carefully placed centers to make sure the shape stays balanced and curved.

This method is widely used in technical drawing because it is easy and gives good visual results.

6. Smooth the ellipse
   After drawing the arcs, use a pencil or template to smoothen the shape. The result should look like an oval or ellipse inside the rhombus.
7. Finalize the ellipse
   Once you are satisfied with the shape, darken the ellipse with a pencil or pen. This ellipse now represents the original circle in isometric view.

Where ellipses are used in isometric drawing:

• Circular holes on flat surfaces
• Pipes or cylinders viewed from angle
• Round features on mechanical components
• Gear bases and bearings in 3D view

Tips for accuracy:

• Use sharp pencil for neat drawing
• Draw lightly at first and darken later
• Practice drawing rhombus and midpoints carefully
• Use ellipse templates for faster and more accurate curves (optional)

Conclusion:

To draw an ellipse in isometric projection, we use a square (rhombus) to represent the circular boundary and apply the four-centre method to draw curved arcs. This method helps to show circular shapes accurately in 3D views. Since a circle becomes distorted in isometric drawings, using an ellipse maintains the correct appearance and visual balance. Mastering this technique is important for making professional and easy-to-read technical drawings in mechanical engineering.