Objectives

• Orthographic Projections

• View Selection

• Glass Box Approach

• First and Third Angle Projections

• Line Precedence

• Two View Drawings

• Tips

Orthographic Projections

• Reference: Technical Graphics, Chap 3:

• Advantage – Represent features of an object more accurately

• Example Problem

Orthographic Projections

• Orthographic Projections are a collection of 2-D drawings that work together to give an accurate overall representation of an object.

Defining the Six Principal

Views or Orthographic

Views

Which Views to Present?General Guidelines

• Pick a Front View that is most descriptive of object

• Normally the longest dimension is chosen as the width (or depth)

• Most common combination of views is to use:

– Front, Top, and Side View

• Any other view different from the Principal Views is called an Auxiliary View (see 3.15 in the Technical Graphics text)

Glass Box Approach

• Most powerful technique to understand orthographic projections

• Suspend the object with transparent strings inside a glass box

• Freeze the view from each direction (each of the six sides of the box) and unfold the box

• Animation illustrates glass-box approach

Glass Box Approach

Glass Box Approach

Glass Box Approach

Glass Box Approach

Glass Box Approach

Glass Box Approach

Third-angle Projection

First-angle Projection

First and Third Angle Projections

• First Angle – International

• Third Angle – U.S.

Conventional Orthographic Views

Height

Depth

Width

Front View

Top View

Right Side View

Is The Orthographic View OK?

Orthographic Must Be In Projection

Hidden and Center Lines• Hidden Line – used to represent features that

cannot be seen in the current view

• Centerlines – used to represent symmetry and to mark the center of circles and the axes of cylinders, and the axes of symmetrical parts, such as cylinders and bolts

For Example:

1. Visible

2. Hidden3. Center

• Visible lines takes precedence over all other lines

• Hidden lines and cutting plane lines take precedence over center lines

• Center lines have lowest precedence

Precedence of Lines

0.70 mm

0.35 mm

0.35 mm

Example: Application of Precedence

Intersecting Lines in Orthographic Projections

Solid Line Intersections

Dashed Line Special Case Intersections

Gap

Two-View Drawings• Some objects can be fully described by

two views, look for:

– Symmetry or Bodies of Rotation

Front View Right Side View

Front View

Right Side

Other Two-View Examples

Review Questions• Rectilinear grids are used for sketching

isometric pictorials … True/False

• Based on the lines of sight, orthographic projection drawings are classified as ___________ projections

• There are ____ standard principal views of orthographic projections

• Each view in an orthographic projection concentrates on ____ dimensions of the object

Hints for Orthographic Projection Sketching

• Identify the major features and overall dimensions of the object

• Do not use any straight-edge devices as a pencil guide when sketching by hand

• Start by drawing bounding boxes with light construction lines.

• Keep views aligned while sketching

Hints for Orthographic Projection Sketching• Title Information is required – follow

conventions

• Usage of construction lines is encouraged.

– Mandatory for circle or ellipse

• Orthographic projection:

– Alignment of the views is important!

– Will not be graded, if not aligned

Hints for Orthographic Projection Sketching

• Map inclined and oblique faces to all three views

• Follow the precedence of lines

• Darken all visible, hidden, and center lines

Sketching a Circle• Draw a square whose

sides are the diameter of the circle.

• At the center of each side define the point of tangency for the circle.

• Draw the diagonals of the square.

• Orient the paper so you can draw equal arcs to construct the circle