Orthographic Projections
Lines
are used to construct a drawing. Various type of lines are used to construct
meaningful drawings. Each line in a drawing is used to convey some specific
information. The types of lines generally used in engineerign drawing is
shown in Table |
Types of lines generally used in drawings
All visible edges are to be represented by visible lines. This
includes the boundary of the object and intersection between two planes.
All hidden edges and features should be represented by dashed lines.
Figure shows the orthographic front view (line of sight in the
direction of arrow)of an object. The external boundary of the object is a
rectangle and is shown by visible lines. In Figure, the step part of
the object is hidden and hence shown as dashed lines while for the position of
the object shown in figure, the step part is directly visible and hence
shown by the two solid lines.
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Figure shows the front view (view along the direction
indicated by the arrow) of a solid and hollow cylindrical object. The
front view of the solid cylinder is seen as a rectangle figure. For the
hollow cylinder in addition to the rectangle representing the boundary of the
object, two dashed lines are shown to present the boundary of the hole, which
is a hidden feature in the object.
Next Figure shows the Front view of three objects. Figure is the view of
one part of a hollow cylinder which has been split in to two equal parts. The
wall thickness can be represented by the two visible lines. Figure is one part of solid cylinder which has been sectioned in to two
equal part. Where as figure is one part of a solid cylindrical
part which has been split in to two unequal parts. The edge formed by the
intersection of two surfaces are represented by solid lines. In case of
cylindrical objects or when holes are present in a component, the center of the
holes or center lines of cylinder will have to be represented in the drawing by
means of center lines as shown in figure . Figure 5 shows the FV, TV, and RHSV of an object showing visible edges, hidden edges (or holes), and center lines.
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| shows the pictorial view and front view of (a) a hollow cylindrical object and (b) solid cylindrical object. |
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shows the pictorial view and front view of sectioned part of
(a) a hollow cylindrical object (b) solid cylindrical object and
(c) solid cylinder split in to two unequal parts.
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| shows the centre lines for cylindrical objects |
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Showing TV, FV and RHSV of an object showing the three types of
lines mentioned above. The pictorial view of the object is shown at the top
hight hand side.
Conventions used for lines
In orthographic projections, many
times different types of lines may fall at the same regions. In such cases, the
following rules for precedence of lines are to be followed:
·
Visible lines take precedence over all other
lines
·
Hidden lines take precedence over center
lines
·
Center lines have lowest precedence
When a visible line and a
hidden line are to be drawn at the same area, It will be shown by the visible
line only and no hidden line will be shown. Similarly, in case of hidden
line and centre line, onlu hidden line will be shown. In such case, the centre
line will be shown only if it is extending beyond the length of the
hidden line.
Intersecting Lines in Orthographic
Projections
The conventions used when
different lines intersect is shown in figure
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The conventions practiced for
intersection lines.
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| The conventions practiced for intersection lines.
Some ortho graphic projections of
solids showing the different lines and their precedence are shown as
examples below. The 3-D view of the respective objects are also shown in the
figures with the direction of arrow representing the line of sight in the front
view.
A few examples of the projections showing the conventions in drawing are presented below.
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