Lecture 3Monday, 15 December 2014 1

ENGINEERING GRAPHICS1E9

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 sidesequal to the diameter of the circle.•From the near corners of the box, draw two large arcs withradius R, using the two red points as centres.•Draw the two smaller arcs with radius r, using two green pointsas 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.