Parallel Projections

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Parallel Projections
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Parallel Projections

• There are 2 broad types of parallel projections
• Orthographic Projections ????
• Front, Side, Top
• Section ???
• Oblique Projections ????
• Axonometric ?????
• Isometric ???????

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Orthographic Projections

• 3D object is projected onto a plane orthogonally
  (at 90 degrees)

Parallel projection
Perpendicular
Projection plane
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Why Parallel Projection

• International Standard
• Objective, rule driven
• Unique interpretation, no misapprehension
• Dimension-able

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Plan View Sketches
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Orthographic Projections

• Six basic views
• You can project the object in six orthographic
  directions
• The resulting views are called basic views
• You may imagine the object is placed inside a
  transparent box

Transparent box
Six basic views
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Orthographic Projections

• As it is inconvenient to carry the box around,
  the box is unfolded onto a common plane drawing
  paper

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Alignment of Views

• The view must line up
• The views must be of the same scale
• The placement and orientation of the view must
  follow the box-unfolding principle
• Violation of the above rules might cause
  misinterpretation

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Sectional Drawings

• The object is imagined to be cut by a plane the
  portion nearer to observer is removed.
• The interior is exposed shown in continuous
  lines
• Hatching is applied at the plane of cutting

The object is cut with the front part removed
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Sectional Drawings

• Use a thin chain line w/ thick continuous lines
  at both ends to represent the imaginary cutting
  plane
• Label the sectional view
• Draw cutting line on a view adjacent to the
  sectional view
• Add label viewing direction
• Since sectional cutting is imaginary, all other
  views remain uncut

Normal top side views, plus a sectional front
view
SECTION A_A
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Layout of Views on Paper
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Constructing an Axonometric

• Draw a circle of any size and locate OA OB by
  the angle of rotation angle of tilt
  respectively
• From B project vertically to cut the horizontal
  axes at E and from E draw an arc to cut OA OC
  at F G
• Draw horizontal line from B to meet the vertical
  axis at B. Then draw horizontal lines from F G
  to meet the vertical projections of A C at A
  C.
• OA OB OC will be the locations of the
  objects 3 axes in axonometric projection.
• If OA OB OC are the natural lengths of the
  object, OA OB OC will be their lengths as
  appeared on the drawing.
• The length along the axes are foreshortened in
  the following ratio-
• axis OA OA/OA
• axis OB OB/OB
• axis OC OC/OC

90o
Angle of rotation
E
Angle of tilt
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Axonometric

• Axonometric projections have many variations
• Some have different levels of distortion among
  the different faces
• Some may have one of the sides with no distortion
  at all
• If the angles among X, Y and Z axes are the same,
  i.e. 120 degrees, emphasis is given to all the 3
  sets of planes. This type of projection is called
  Isometric

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Circles in Parallel Projections

• When drawing circles for different projection
  angles, we may first construct a square with
  divided quadrants for establishing 8 points along
  the circumference of the circle
• Then for the projection view, construct a similar
  grid for locating the points and drawing a circle
  that passes through all these points and the 4
  tangent points

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Limitation of Parallel Projections

• While parallel projection provides the least of
  amount of distortion in depicting an object, it
  does not do a very good job in terms of mimicking
  how we see the real world around us.
• As a result, it lacks the sense of reality that
  human eyes would adjust in perceiving the
  physical world.