The document discusses isometric projections, which show all three faces of an object simultaneously and equally foreshortened. It describes the process of creating isometric drawings, including defining isometric axes, drawing box constructions, and techniques for approximating circles and drawing non-circular curves. Key steps involve locating points through offset measurements from isometric and non-isometric lines.

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ENGINEERING GRAPHICS
1E9
Lecture 3: Isometric
Projections

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What is ISOMETRIC?
• It is a method of producing pictorial view
of an object showing all three faces of the
object simultaneously.
• It is a type of parallel projection
• It is a type of axonometric projection

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Axonometric Projections
• Observer at infinity
• Projectors parallel to each other and perpendicular
to projection plane
• Object is inclined with respect to projection plane

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Types of Axonometric
Projections
Isometric Projection
Dimetric Projection
Trimetric Projection

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Isometric Projections
• All angles between axonometric axes are
equal
• The three coordinate axes of the object
appear equally foreshortened (about 3/4th
of true length)
• The angles between any two of the
three coordinate axes is 120°

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Isometric Terminology
• The three coordinate axes are called
isometric axes
• Any line parallel to isometric axes is called
isometric line
• A non-isometric line is a line not parallel to
any one of the three isometric axis
• In isometric projection of cube, the faces of
the cube and any plane parallel to them is
called isometric planes

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Isometric Scale
• True lengths of the edges of the object are
equally foreshortened
• Correct isometric projection can be drawn
using an isometric scale (always smaller
than ordinary scale)

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Isometric Drawing
Isometric Projection:
Drawing prepared
with isometric scale on
isometric axes
Isometric Drawing:
Drawing prepared
with ordinary scale
on isometric axes

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Steps:
Step 1
Isometric sketches begin with defining
isometric axes, three lines, one vertical
and two drawn at 30° from the horizontal.

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Steps:
Step 2
Three lines of the isometric axes represent
the three primary dimensions of the
object: width, height, and depth

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Steps:
Step 3
Draw the font face of the isometric block.

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Steps:
Step 4
Draw the rest of the isometric block.

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Steps:
Step 5
Add details to the block starting from the
front face. Then add details to the other
faces.

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Steps:
Step 6
Darken all visible lines to complete the
isometric sketch. (make sure that
construction lines are light)

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• Axonometric projection shows all 3 dimensions,
length, width and height.
• The isometric lines are only drawn to scale. Objects
composed entirely of isometric lines can be drawn
by taking all measurements parallel to main edges
of the enclosing box.
• Non-isometric lines are drawn by transferring the
ordinates (which are on isometric lines) of the end
of the lines
• Inclined and oblique surfaces are drawn using end
coordinates. Box construction and offset
measurements are common methods
• In an isometric drawing, an angle never appears
in its true size. Angles, irregular curves require
special techniques

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Objects with Normal Surfaces
Make an Isometric Drawing with corner A
at the bottom

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Objects with Normal Surfaces

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Objects with Oblique Surfaces
• Make an Isometric Drawing with corner A
at the bottom
NON-ISOMETRIC LINE

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Objects with Oblique Surfaces

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Objects with Non-isometric
Lines
• Make an Isometric Drawing with apex A
facing front

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Objects with Non-isometric Lines
Non-isometric lines are drawn with box construction
and offset measurements
Non-isometric lines are not drawn in true length in
isometric drawing (BA is shorter than CA in this
drawing)

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Irregular Objects
• Make an Isometric Drawing of the following
irregular object (pyramid)

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Irregular Objects
• OA and OB offsets help to locate apex O
• Complete box construction may not be
needed in each case

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Objects with Circular Geometry
A circle in a orthographic projection will appear as
an ellipse in an isometric drawing.
Instead of actual ellipses often approximate ellipses
are drawn for isometric drawing.
Four-centre ellipses are used to approximate ellipses
on isometric planes.
How to draw four-centre ellipse???

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Approximate Ellipse
•Draw the isometric centre lines of the circle. Using the
centre lines, draw an isometric square with sides equal to the
diameter of the circle.
•From the near corners of the box, draw two large arcs with
radius R, using the two red points as centres.
•Draw the two smaller arcs with radius r, using two green points
as centres.

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Cylinder

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Objects with Circular Geometry

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Objects with Non-Circular Curved
Surfaces
• Make an Isometric Drawing of the following
curved object

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Objects with Non-Circular Curved Surfaces
•A line that appears as a noncircular curve in a normal
orthographic view of an object appears as a non-isometric line
in an isometric drawing.
•Curves may be drawn using a series of points by measuring
along the normal lines in the orthographic view (offset
measurements) and transferring these points on isometric
drawing. Accuracy increases with number of points.