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Practice MCQ

Unit 5 - Notes

MEC136

Unit 5: Isometric Views

1. Introduction

Isometric projection is a method of pictorial representation in engineering drawing. Unlike orthographic projection, which represents 3D objects using multiple 2D views (Front, Top, Side), isometric projection shows the object in a single view, providing a realistic impression of its three-dimensional shape.

  • Etymology: Derived from the Greek words "Iso" (equal) and "Metric" (measure), implying that the scale along the three axes is the same.
  • Principle: The object is placed such that its three mutually perpendicular edges (axes) make equal angles with the plane of projection.

1.1 Terminology

To construct an isometric view correctly, one must understand the fundamental components:

  • Isometric Axes: The three lines meeting at a point and making an angle of 120° with each other. In a standard orientation, one axis is vertical, and the other two are inclined at 30° to the horizontal.
  • Isometric Lines: Lines on the object that are parallel to the isometric axes. Dimensions can be measured directly on these lines.
  • Non-Isometric Lines: Lines that are not parallel to the isometric axes (e.g., the slant edge of a pyramid). Dimensions cannot be measured directly on these lines. They must be drawn using the "Box Method" or "Offset Method" by locating their endpoints using isometric coordinates.
  • Isometric Planes: The faces of the object that are parallel to the isometric axes (e.g., the top, front, and side faces of a cube).

2. Isometric Scale

In a true isometric projection, the object is tilted, causing the edges to appear foreshortened.

  • True Scale: The actual dimensions of the object.
  • Isometric Scale: Because the axes are inclined to the plane of projection, the lengths are foreshortened by approximately 18%.

Mathematical Derivation:
Ideally, lines are drawn at to the horizontal. However, the true length is projected from a angle in orthographic construction.

Distinction between View and Projection:

  1. Isometric View (or Drawing): Drawn using the True Scale (1:1). This is standard for most engineering shop drawings because it is easier to execute.
  2. Isometric Projection: Drawn using the Isometric Scale ().

3. Isometric Views of Solids

3.1 Isometric Views of Prisms

Prisms have a uniform cross-section and rectangular side faces.

  • Rectangular Prism (Box):
    1. Draw the three axes ().
    2. Mark length, width, and height on the respective axes.
    3. Complete the parallel lines to form the box.
  • Polygonal Prisms (Hexagonal/Pentagonal):
    1. Box Method: Enclose the polygon (base) in a rectangle in the orthographic view.
    2. Draw the isometric view of this enclosing rectangle (rhombus/parallelogram).
    3. Transfer the coordinates of the polygon corners onto the isometric rectangle.
    4. Draw vertical lines from these corners equal to the height of the prism.
    5. Join the top points to replicate the base shape.

3.2 Isometric Views of Pyramids

Pyramids have a polygonal base and triangular faces meeting at an apex.

  1. Draw the isometric view of the base (using the Box Method).
  2. Locate the center of the base.
    • For squares/rectangles: Intersection of diagonals.
    • For triangles/hexagons: Midpoint of the enclosing box or geometric centroid.
  3. Draw a vertical line from the center representing the Axis Height.
  4. Join the apex (top of the axis) to the corners of the base.

3.3 Composite Solids (One Object on Another)

This involves drawing two solids sharing a common vertical axis.

  • Procedure:
    1. Draw the bottom solid first.
    2. Locate the center of the top face of the bottom solid.
    3. Use this center point as the reference to start drawing the base of the top solid.
    4. Visibility: Only draw the visible lines. If the top object is larger or obscures the bottom object, erase the lines of the bottom object that fall "behind" or "under" the top object.

4. Dimensioning in Isometric Views

Dimensioning isometric drawings requires specific alignment to maintain the 3D illusion.

  • System: The Aligned System is mandatory. Unidirectional dimensioning breaks the 3D effect.
  • Extension Lines: Must be drawn parallel to the isometric axes (extensions of the object lines).
  • Dimension Lines: Must be parallel to the line being measured.
  • Text/Arrowheads: The text must be written upright relative to the dimension line so it appears to lie on the face of the object.

5. AutoCAD Commands for 3D Modeling

While traditional 2D AutoCAD uses "Isodraft," modern engineering drawing utilizes 3D modeling workspaces to generate isometric views.

Workspace: Switch to 3D Modeling.

5.1 3P UCS Rotation (User Coordinate System)

In 3D AutoCAD, the default drawing plane is XY. To draw on the face of a 3D object (e.g., a circle on the side of a cube), you must rotate the coordinate system.

  • Command: UCS
  • Option: 3 Point
  • Function: Defines the new orientation of the XY plane.
    1. Origin: Click the corner of the face you want to work on.
    2. X-Axis: Click a point along the desired X-direction (horizontal edge of the face).
    3. Y-Axis: Click a point along the desired Y-direction (vertical edge of the face).
  • Result: The grid aligns with the selected face, allowing you to draw 2D shapes (circles, polygons) directly on that plane.

5.2 Standard Shapes (Primitives)

AutoCAD provides pre-defined solid primitives under the "Modeling" tab.

  • Box: Creates a solid box (requires corner point, length, width, height).
  • Cylinder: Creates a cylinder (requires center point, radius, height).
  • Cone: Creates a pointed cone.
  • Sphere: Creates a solid ball.
  • Wedge: Creates a triangular prism.
  • Torus: Creates a donut shape.

5.3 Extrude (EXTRUDE)

Converts a 2D closed object into a 3D solid by stretching it along the Z-axis.

  • Prerequisite: The 2D shape must be a closed Polyline or Region.
  • Workflow:
    1. Draw a closed shape (e.g., Rectangle, Circle).
    2. Type EXT or select Extrude.
    3. Select the object.
    4. Specify the height of extrusion (or specify a path/taper angle).

5.4 Revolve (REVOLVE)

Creates a solid by sweeping a 2D open or closed profile around an axis. Ideal for cylindrical or symmetrical parts (e.g., pulleys, shafts, wheels).

  • Workflow:
    1. Draw the cross-section profile (half of the object).
    2. Type REV.
    3. Select the profile.
    4. Define the axis of revolution (select two points forming the centerline).
    5. Specify the angle of revolution (usually ).

5.5 Presspull (PRESSPULL)

A versatile tool that can create solids or subtract material (holes) dynamically.

  • Difference from Extrude: Presspull does not require a joined polyline; it detects "bounded areas."
  • Workflow:
    1. Type PRESSPULL.
    2. Hover inside a closed area (it will highlight).
    3. Click and drag the mouse.
      • Dragging outward adds volume (creates solid).
      • Dragging inward into an existing solid removes volume (creates a hole or cut).

6. Hands-on Practice on 3D Drawings

Practical Exercise: Modeling a Hexagonal Prism with a Central Hole

Objective: Create a Hexagonal Prism (Base side 30mm, Height 80mm) with a cylindrical hole (Radius 15mm) through the center.

Step-by-Step AutoCAD Workflow:

  1. Setup:

    • Open AutoCAD.
    • Switch workspace to 3D Modeling.
    • Set View to SE Isometric (South-East Isometric).
    • Set Visual Style to 2D Wireframe (easier to snap points).

  2. Create the Base:

    • Command: POL (Polygon).
    • Number of sides: 6.
    • Center: 0,0,0.
    • Option: Inscribed in circle.
    • Radius: 30.

  3. Create the Prism (Extrude):

    • Command: EXT (Extrude).
    • Select the hexagon.
    • Height: 80.

  4. Create the Hole Profile:

    • The hole is on the top face. We do not strictly need to move the UCS if using object snaps, but let's do it for practice.
    • Command: UCS -> Face -> Select the top face of the prism.
    • Command: C (Circle).
    • Center: Move mouse to center of hexagon top face until Center snap appears.
    • Radius: 15.

  5. Subtract the Material (Presspull method):

    • Command: PRESSPULL.
    • Click inside the Circle.
    • Drag the mouse downwards (through the prism).
    • Click to confirm.

  6. Final Visualization:

    • Command: VS (Visual Styles).
    • Select Concept or Shades of Gray to see the solid model with the hole clearly visible.

  7. Generate 2D Views (Optional Layout):

    • Go to Layout Tab.
    • Command: VIEWBASE -> From Model Space.
    • Place the Front, Top, and Isometric views automatically.