7082698 4 the theory of projection

8/14/2019 7082698 4 the Theory of Projection

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The theory of projectionKCEC1101 CAD


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Projection MethodPerspective or Central ProjectionsLinear Perspectives Aerial Perspectives


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Projection MethodsParallel ProjectionsOblique Projections Orthographic Projections Multiview Projections

Axonometric Projections


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The attributes of each projection method


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Projection theory

Line of Sight (LOS) A LOS is an imaginary ray of light between an observer's eyeand an object.


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Projection theoryIn perspective projection, all LOS start at a single point


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Projection theoryIn parallel projection, all LOS are parallel and no start point (infinitive viewpoint)


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Multiview projection planes1. The frontal plane of projection is the plane onto which the front view of multiview is projected.


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Multiview projection planes2. The top view is projected onto the horizontal plane of projection, which is aplane suspended above and parallel to the top object.


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Multiview projection planes3. The right side is projected onto the right profile plane of projection, whichis a plane that is parallel to the right side of the object.


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Advantage of multiview drawingIt produce the true dimensions !!!Distorted angle

Distorted dimension

Multiview drawing


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The six principal of views1. 2. 3. 4. 5. 6. The front view The top view The right side view The left sideview The rear view The bottom view


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The six perpendicular plane of views


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Conventional view placementConventionally, the standard views used in a three-view drawing are the top, front, and right side view. Because the other three principal views are mirror image and do not add to the knowledge about the object.


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Projection dimensionsThe width dimension is common to the front and top views. The height dimension is common to the front and side views. The depth dimension is common to the top and side views.


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Projection arrangement

The arrangement of views may vary as long as the dimension alignment is correct.


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First-Angle ProjectionFirst angle projection is the standard in Europe and Asia.


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Profile plane

The difference between first and third angle projection is the placement of theobject and the projection plane The principal projection planes and quadrants used to create first- and third- angle projection drawings


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Pictorial comparison between first- third angle projection techniques


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First angle projectionFirst angles projection box

Box unfolding


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First angle projection

Box unfolding

Box unfolded show relative position of view Placement of 1st angle of view


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Third angle projection

The dihedral angles

Third angles projection box


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Third angle projection3rd angles projection box

Box unfolding


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Third angle projectionBox unfolding

Placement of 3rd angle of view

Box unfolded show relative position of view


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The rule of orthographic projection principles1. 2. 3. 4. 5. 6. 7. Alignment of features Distances in related views True length and size Foreshortening Configuration of planes Parallel features Edge views


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Rule 1: Alignment of featuresEvery point or feature in one view must be aligned on a parallel projector For example, the hole in the block is an example of a feature shown in one view and aligned on parallel projectors in the adjacent view Adjacent views are twoorthographic view placed next to each other such that the dimension they sharein common is aligned, using parallel projectors.hole feature


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An inclined line is parallel to a plane of projection, but inclined to the adjacent planes, it appears foreshortened in the adjacent planes.

For example line 3-4 is inclined and foreshortened in the top and right side view, but true length in the front view, because it is parallel to the frontal plane of projection


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Rule 4: ForeshorteningFeatures are foreshortening when the line of sight are not perpendicular to thefeatures For example oblique line 1-2 is not parallel to any of the principal planes ofprojection of the glass box.

An oblique line is not parallel to any principal plane of projection


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Rule 5: Configuration of planesAreas that are the same feature will always be similar in configuration from oneview to the next, unless viewed on edge

Oblique surface

Incline surface

Surface B and C are an example of the Rule of Configuration of planes


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Example of normal face projectionA normal face projects on all three principle image planes. (follow the rule ofedges view)


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Creating a tree-view sketch


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Example of representing filleted and rounded corners


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Representing the intersection of two cylinders

Small cylinder

Large cylinder(same size)


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Representing the intersection btw a cylinder and a prism

Small prism

Large prism


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Center lines


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Good & Poor Orientation

The major surface are parallel or perpendicular to the sides of the box (projections planes)

The surface are not parallel to the sides of the glass box produces views with many hidden lines.


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End


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7082698 4 the theory of projection

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