Unit 3: Orthographic Projections

1. Introduction to Orthographic Projections

Definition

Orthographic projection is a method of representing three-dimensional objects on a two-dimensional surface. The term is derived from "Ortho" (straight/perpendicular) and "Graphy" (drawing). It is the universal language of engineering drawing.

The Principle of Projection

In orthographic projection, the following conditions must be met:

1. The Observer: Is assumed to be at an infinite distance.
2. The Projectors: The lines of sight (projectors) are parallel to each other.
3. The Plane of Projection: The projectors are perpendicular (at ) to the plane of projection.

Principal Planes and Views

To describe an object completely, it is usually projected onto three mutually perpendicular planes:

1. Vertical Plane (VP): The plane in front of the observer. The view obtained is the Front View (Elevation).
2. Horizontal Plane (HP): The plane perpendicular to the VP. The view obtained is the Top View (Plan).
3. Profile Plane (PP): The plane perpendicular to both VP and HP. The view obtained is the Side View (Profile View).

2. Projection Systems: First vs. Third Angle

The space is divided into four quadrants by the intersection of the Vertical and Horizontal planes. Engineering drawings primarily utilize the First and Third quadrants.

First Angle Projection

• Region: Used primarily in Europe, Asia (including India), and ISO standards.
• Position: The object is placed in the First Quadrant (above HP and in front of VP).
• Arrangement: Object is between the Observer and the Plane of Projection.
• Projection Concept: The plane is assumed to be non-transparent (opaque). The image is projected onto the wall behind the object.
• Layout on Paper:
  • Front View: Drawn above the XY line.
  • Top View: Drawn below the XY line (below the Front View).
  • Left Side View: Drawn on the Right of the Front View.
  • Right Side View: Drawn on the Left of the Front View.

Third Angle Projection

• Region: Used primarily in the USA, Canada, and ANSI standards.
• Position: The object is placed in the Third Quadrant (below HP and behind VP).
• Arrangement: The Plane of Projection is between the Observer and the Object.
• Projection Concept: The plane is assumed to be transparent (glass). The observer looks through the plane to see the object.
• Layout on Paper:
  • Top View: Drawn above the XY line (above the Front View).
  • Front View: Drawn below the XY line.
  • Right Side View: Drawn on the Right of the Front View.
  • Left Side View: Drawn on the Left of the Front View.

Comparison Summary

3. AutoCAD Commands: Linetype and Properties

In engineering drawing, different lines represent different features (edges, center lines, hidden parts).

Linetype Command (LINETYPE or LT)

This command loads and manages linetypes within the drawing.

• Continuous: Represents visible edges.
• Hidden (Dashed): Represents edges that are not visible from the current view.
• Center (Long-Short-Long): Represents axes of symmetry or centers of circles.

How to load a linetype:

1. Type LT and press Enter.
2. Click "Load".
3. Select the desired linetype (e.g., HIDDEN2, CENTER2) and click OK.

Linetype Scale (LTSCALE or LTS)

Controls the global scale factor of linetypes. If dashed lines look solid, the LTS is likely too small or too large relative to the drawing units.

• Command: LTS -> Enter value (e.g., 0.5 or 20, depending on zoom level).

Object Properties (PROPERTIES or CH)

Accessed via Ctrl+1 or typing PR. This palette allows modification of:

• Color: Changes visibility/layer color.
• Layer: Moves object to a different layer.
• Linetype: Overrides layer linetype.
• Linetype Scale: Changes scale for the selected object only.
• Line Weight: Changes the printed thickness of the line.

4. AutoCAD Modify Commands

These commands are essential for editing geometry after it is created. They are found in the "Modify" panel of the Ribbon.

Move (M)

Displaces objects a specified distance in a specified direction.

1. Type M -> Enter.
2. Select objects -> Enter.
3. Specify Base Point (point to move from).
4. Specify Second Point (point to move to).

Rotate (RO)

Rotates objects around a base point.

1. Type RO -> Enter.
2. Select objects -> Enter.
3. Specify Base Point (pivot point).
4. Specify Rotation Angle (positive is Counter-Clockwise, negative is Clockwise).
   • Option (Reference): Allows rotation based on an existing angle reference.

Trim (TR)

Trims objects to meet the edges of other objects.

1. Type TR -> Enter.
2. Modern Mode (Quick Mode): Simply click on the segment you want to delete. AutoCAD auto-detects boundaries.
3. Standard Mode: Select cutting edges -> Enter -> Select object to trim.
   • Shift + Select while in Trim command extends the object instead.

Copy (CO or CP)

Creates duplicates of objects.

1. Type CO -> Enter.
2. Select objects -> Enter.
3. Specify Base Point.
4. Specify Second Point. (Command repeats until Esc is pressed).

Erase (E)

Removes objects from the drawing.

1. Type E -> Enter.
2. Select objects (Click, Window, or Crossing) -> Enter.

Mirror (MI)

Creates a mirrored copy of selected objects across a specified axis. Essential for symmetrical orthographic views.

1. Type MI -> Enter.
2. Select objects -> Enter.
3. Specify first point of Mirror Line.
4. Specify second point of Mirror Line.
5. Prompt: "Erase source objects? [Yes/No]": Usually No.

Scale (SC)

Enlarges or reduces selected objects, keeping the proportions of the object the same.

1. Type SC -> Enter.
2. Select objects -> Enter.
3. Specify Base Point.
4. Specify Scale Factor:
   • : Enlarges (e.g., 2 doubles the size).
   • : Reduces (e.g., 0.5 halves the size).

Fillet (F)

Rounds and fillets the edges of objects.

1. Type F -> Enter.
2. Type R (Radius) -> Enter -> Input radius value.
3. Select first object.
4. Select second object.
   • Note: If Radius = 0, Fillet creates a sharp corner (trims excess).

Chamfer (CHA)

Bevels the edges of objects (creates an angled corner).

1. Type CHA -> Enter.
2. Select Method (Distance or Angle).
   • Distance: Type D -> Enter -> Input first distance -> Input second distance.
3. Select first line.
4. Select second line.

Array (AR)

Creates multiple copies of objects in a pattern.

1. Rectangular Array:
Creates a grid of rows and columns.

• Select object -> Type ARRAYRECT.
• Use the Ribbon or Properties to adjust:
  • Columns: Vertical lines of objects.
  • Rows: Horizontal lines of objects.
  • Levels: Z-axis (3D).
  • Spacing: Distance between items.

2. Polar Array:
Copies objects in a circular pattern around a center point.

• Select object -> Type ARRAYPOLAR.
• Specify Center Point.
• Adjust Items (total number) and Fill Angle (usually 360).

5. Hands-on Practice: Generating 2D Orthographic Drawings

Workflow Strategy

To successfully draw orthographic projections in AutoCAD, follow this standardized workflow:

Step 1: Setup

1. Units: Type UN -> Set to Millimeters (or Inches based on problem) -> Precision 0.00.
2. Limits (Optional): Type LIMITS -> 0,0 to 297,210 (A4 size). Z -> A (Zoom All).
3. Layers: Type LA and create the following:
   • Object Line: White/Black, Continuous, Weight 0.30mm.
   • Hidden Line: Yellow/Blue, Hidden linetype.
   • Center Line: Red, Center linetype.
   • Construction/Projection: Gray, Continuous, Default weight (Turn this off or freeze it later).

Step 2: Reference Lines

1. Draw the X-Y Line (Reference Line) horizontally.
2. If creating a side view, draw a vertical reference line intersecting the X-Y line.
3. For the Side View projection, draw a 45-degree miter line starting from the intersection of the vertical and horizontal reference lines.

Step 3: Front View Construction

1. Analyze the dimensions (Length and Height).
2. Using LINE or PLINE, draw the outline of the Front View on the correct side of the X-Y line (Above for 1st Angle).
3. Use OFFSET to establish internal features like slots or holes.

Step 4: Projecting to Top View

1. Switch to the "Construction" layer.
2. Use XLINE (Vertical) or RAY to drop projectors down from every corner/feature of the Front View.
3. Draw horizontal lines representing the Depth of the object.
4. Switch to "Object" layer and trace the visible boundaries using TRIM and LINE.

Step 5: Projecting to Side View

1. Project horizontal lines from the Front View to the Side View area.
2. Project horizontal lines from the Top View to the 45-degree miter line. Where they intersect, project them vertically into the Side View area.
3. The intersection of these projectors defines the Side View geometry.

Step 6: Finalizing

1. Hidden Details: Identify internal features (holes, slots) that are blocked by solid material. Draw these using the "Hidden" layer (dashed).
2. Center Lines: Add center lines to all circular features and axes of symmetry using the "Center" layer.
3. Clean up: Turn off the Construction layer.
4. Dimensions: Use DIMLINEAR, DIMRADIUS, etc., to annotate the drawing.