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MULTI-VIEW DRAWINGS AND ORTHOGRAPHIC PROJECTION
OBJECTIVES:
The students are expected to:
- Understand the principles of orthographic projection
- Lay out multiview drawings
- Recognize basic lines and planes
- Project curved edges
- Locate intersections between various geometric forms
In industry, a complete and clear description of the shape and size of an object to be made is necessary; to make certain that the object will be manufactured exactly as intended by the designer. In order to provide this information clearly and accurately, a number of views, systematically arranged are used. This system of views is called multiview projection. Each view provides certain definite information if the view is taken in a direction perpendicular to a principal face or side of the object. The observer is theoretically at an infinite distance from the object.
An object has three principal dimensions: width, height, and depth. In technical drawing, these fixed terms are used for dimensions taken in these directions, regardless of the shape of the object. The terms “length” and “thickness” are not used because they cannot be applied in all cases. Any one view of a three dimensional object can show only two dimensions; the third dimension will be found in an adjacent view.
MULTIVIEW DRAWINGS are based on parallel projection techniques and are used when there is a need to represent the features of an object more accurately than is possible with a single (pictorial) view. A multiview drawing is a collection of flat 2-D drawings that work together to give you an accurate representation of the overall object. With a pictorial drawing, all three dimensions of the object are represented in a single view. The disadvantage of this approach is that not all features in all three dimensions can be shown with optimal clarity. In a multiview projection, however, each view concentrates on only two dimensions of the object, so particular features can be shown with a minimum of distortion. (Enough views are generated to capture all the important features of the object.
ORIENTING & SELECTING THE FRONT VIEW
When creating a multiview drawing of a design, the selection and orientation of the front view is an important first step. The front view is chosen as the most descriptive of
the object; for example, what would normally be thought of as the side of the car is chosen as the front view because it is the most descriptive. In addition, the object must be properly oriented in the view. The orientation of the object is based on its function.
6 principal views of a house
CHOOSING THE VIEWS FOR A MULTIVIEW DRAWING
Another way to understand the views of an object is to pick it up and turn it around. This may be hard to imagine with something like a house, but many of the objects you will be sketching are considerably smaller and can be held in the hand. Imagine picking up the object and holding it so that you are looking at the front. Now, rotate the object so that you are looking at its top. Rotate it back where you started and then rotate it back to where you started and then rotate it so you are looking at its right side. There are an infinite number of intermediate views of the object between the points of rotation at which you stopped; for right now, however, consider only those views that are rotated 900 from each other. These are considered regular or principal views, and each represents two primary dimensions of the object. If you continue rotating the object in 900 increments, you will see as many as six regular views.
A multiview drawing should have the minimum number of views necessary to describe an object completely. Normally, three views are all that are needed; however, the three views chosen must be the most descriptive ones. The most descriptive views are those that reveal the most information about the design, with the fewest features hidden from view.
ALPHABET OF LINES
VISIBLE OBJECT LINES are thick dark solid lines that are used on drawings to indicate the edges and details of an object. These lines should be most prominent on the drawing as they are drawn thicker than most other lines. Continuous and thick
HIDDEN LINES are thin dashed lines used to indicate a surface, edge contour of an object that cannot be seen, or is hidden from view.
CENTER LINES are thin lines made up of alternating long and short dashes. They are used to indicate the center of symmetrical features and also as an aid in dimensioning.
SECTION LINES are thin “cross hatching” Lines used in sectional views to symbolize different parts of an object or different materials from which an object is manufactured.
DIMENSIONS, EXTENSION AND LEADER LINES are thin lines used for dimensioning purposes. Dimension and Leader lines generally end with arrow heads and accompanied by a number indicating a size or location dimension. Extension lines aid dimension lines by extending the surface of the object so it can be dimensioned.
CUTTING PLANE OR VIEWING PLANE LINES are very thick lines that indicate where an imaginary cutting plane passes through an object, for viewing or sectioning purposes. Arrowheads on both ends of the line point to the surface to be viewed.
BREAK LINES are used to shorten a view, or to eliminate repetitive details when an
entire view is not necessary. Short break lines are thick wavy free hand lines. Long break
lines are thin straight zig-zags.
Rule 2: Hidden lines should always begin and end with a dash. An exception is when the hidden
line begins or ends at a parallel visible or hidden line.
Rule 3: Dashes should join at corners.
Using Center Lines
Center lines represent axes of symmetry.
They are important for interpreting cylindrical shapes.
They are also used to indicate circle of centers and paths of motion.
Creating Center Lines
Rule 1: Center lines should start and end with long dashes.
Rule 2: Center lines should intersect by crossing either the long dashes or the short dashes.
Rule 3: Center lines should extend a short distance beyond the object or feature.
Creating phantom lines
Rule 1: Phantom lines should start and end with a long dash.
Using Break Lines
Break lines are used to show imaginary
breaks in an object.
Creating Break Lines
There are two types of break lines. If the
distance to traverse is short, a series of connecting arcs is used. If the distance is long the thin
straight line with a jog is used.
Line Precedence
If two lines occur in the same place, the line that is considered to be the least important is omitted.
Lines in order of precedence/importance are as follows;
- Cutting Plane line
- Visible line
- Hidden line
- Center line
CUTTING PLANES
Imaginary cutting planes used to create section views are passed through the object to
reveal interior features.
TREATMENT OF HIDDEN LINES Normally hidden lines are omitted from section
views
OPTIONAL USE OF HIDDEN LINES - Hidden lines can be shown in section views
usually to eliminate the need for another view.
PLACEMENT OF CUTTING PLANE LINES The cutting plane line is placed in the view
where the cutting plane appears on edge.
STANDARD CUTTING PLANE LINESTYLES Standard cutting plane line styles are thick
lines terminated with arrows.
FULL SECTIONS
You may see different kinds of sections on prints. A full section is one where the cutting
plane line passes through the part in a straight line, cutting it in two. Half of the part is
then removed. (Caterpillar Training Institute)
Passing the cutting plane completely through an object results in a full section. If the cutting
plane is simple, it has the appearance of halving an object.
Full sections
REVOLVED SECTION
When a long part such as an "I" beam is shown with a section view, a revolved section
is used. This type of section is made by passing the cutting plane through the center of
the part and then turning the cross section 90˚ to face the viewer.
REMOVED SECTION
A removed section is drawn separately from the multi view drawing and placed in an
alternate location on the print.
BROKEN OUT SECTION
Often, only a small portion of a part must be shown. Here a broken-out section is used. A broken-out section is drawn in one of the views and shown with a freehand break line.
Oblique surfaces do not show as true shape in any of
the standard views. Die-cut object 4 has an oblique
surface "D." This surface does not show as true shape in the front, top or right side views.
An auxiliary view of an inclined surface will provide a true description of the surface. In the drawing, line "A" in the front view represents the edge of the inclined surface "E."
The auxiliary view must be a direct projection of the incline so two projection lines are drawn perpendicular to the inclined surface. One line must be at each end of line "A."
Line "A" in the auxiliary view is drawn parallel to the inclined line "B" in the front view.
This line represents the true height of the inclined surface.
The depth of the inclined surface (C and D) is taken from the right side view and drawn
on each of the projection lines.
Line "G," which completes the auxiliary view, is drawn parallel to line "A," connecting the
ends of lines "C" and "D."
The auxiliary view now shows the true size and shape of the inclined surface. Two auxiliary views are needed to show the true size and shape of an oblique surface. See
surface "D" below.
