ORTHOGRAPHIC PROJECTION C H A P T E R F I V E

OBJECTIVES 1. Recognize and sketch the symbol for third-angle projection. 2. List the six principal views of projection. 3. Sketch the top, front, and right-side views of an object with normal, inclined, and oblique surfaces. 4. Understand which views show depth in a drawing showing top, front, and right-side views. 5. Know the meaning of normal, inclined, and oblique surfaces. 6. List the dimensions that transfer between top, front, and right-side views.

UNDERSTANDING PROJECTIONS To make and interpret drawings you need to know how to create projections and understand the standard arrangement of views. You also need to be familiar with the geometry of solid objects and be able to visualize a 3D object that is represented in a 2D sketch or drawing.

Views of Objects The system of views is called multiview projection. Each view provides certain definite information. For example, a front view shows the true shape and size of surfaces that are parallel to the front of the object.

Multiview Projection The system of views is called multiview projection. Each view provides certain definite information.

Parallel Projection

The Six Standard Views Any object can be viewed from six mutually perpendicular directions,

Revolving the Object to Produce Views Revolving the Object to Produce Views. You can experience different views by revolving an object.

Principal Dimensions The three principal dimensions of an object are width, height, and depth. The front view shows only the height and width of the object and not the depth. In fact, any principal view of a 3D object shows only two of the three principal dimensions; the third is found in an adjacent view. Height is shown in the rear, left-side, front, and right-side views. Width is shown in the rear, top, front, and bottom views. Depth is shown in the left-side, top, right-side, and bottom views.

Projection Method The outline on the plane of projection shows how the object appears to the observer. In orthographic projection, rays (or projectors) from all points on the edges or contours of the object extend parallel to each other and perpendicular to the plane of projection. The word orthographic means “at right angles.” Projection of an Object

Horizontal and Profile Projection Planes Specific names are given to the planes of projection. The front view is projected to the frontal plane. The top view is projected to the horizontal plane. The side view is projected to the profile plane.

The Glass Box One way to understand the standard arrangement of views on the sheet of paper is to envision a glass box. If planes of projection were placed parallel to each principal face of the object, they would form a box.

Unfolding the Glass Box To organize the views of a 3D object on a flat sheet of paper, imagine the six planes of the glass box being unfolded to lie flat. Note the six standard views (front, rear, top, bottom, right side, left side).

The Glass Box Unfolded Lines extend around the glass box from one view to another on the planes of projection. These are the projectors from a point in one view to the same point in another view.

Which Views to Present? General Guidelines Pick a Front View that is most descriptive of object Normally the longest dimension is chosen as the width (or depth) Most common combination of views is to use: Front, Top, and Side View Views other than the Principal Views are called Auxiliary Views Pick the views which will help in describing the object with highest clarity. Explain what is an auxiliary view. Explain that they are drawn to show specific features that are not clear in the Principal views.

Conventional Orthographic Views Height Depth Width Front View Top View Right Side View Note that the views are placed and aligned in the manner shown in the diagram. Remind the students that they have to follow the above convention for all their home work problems and exam problems. It is very important to maintain the alignment and correct placement relative to each other. Means line for top (and bottom) is straight across for both front view and right side view for example. Same thing between front and top for sides. Note : The following can be seen from the slide: Top View and front view have the same width Front View and Right / Left side view have the same height. The depth of Top view is same as the width of right/ left side view.

Transferring Depth Dimensions The depth dimensions in the top and side views must correspond point-for-point. When using CAD or instruments, transfer these distances accurately. You may find it convenient to use a 45° miter line to project dimensions between top and side views. You can transfer dimensions between the top and side views either with dividers or with a scale.

Necessary Views The top, front, and right-side views, arranged together, are called the three regular views because they are the views most frequently used. A sketch or drawing should contain only the views needed to clearly and completely describe the object.

One-View Often, a single view supplemented by a note or by lettered symbols is Enough.

Choice of Front View The view chosen for the front view in this case is the side, not the front, of the automobile.

Third-Angle Projection To understand the two systems, think of the vertical and horizontal planes of projection, as indefinite in extent and intersecting at 90° with each other; the four angles produced are called the first, second, third, and fourth angles (similar to naming quadrants on a graph.) If the object to be drawn is placed below the horizontal plane and behind the vertical plane, as in the glass box you saw earlier, the object is said to be in the third angle. In third-angle projection, the views are produced as if the observer is outside, looking in.

First-Angle Projection If the object is placed above the horizontal plane and in front of the vertical plane, the object is in the first angle. The biggest difference between third-angle projection and first-angle projection is how the planes of the glass box are unfolded.

Hidden Lines Thick, dark lines represent features of the object that are directly visible. Dashed lines represent features that would be hidden behind other surfaces.

HIDDEN LINES Hidden lines are used to represent surfaces that are not directly visible in an orthographic view.

Centerlines The centerline pattern is used to: • show the axis of symmetry for a feature or part • indicate a path of motion • show the location for bolt circles and other circular patterns The centerline pattern is composed of three dashes: one long dash on each end with a short dash in the middle.

PRECEDENCE OF LINES A visible line always takes precedence over and covers up a centerline or a hidden line when they coincide in a view (A and B). A hidden line takes precedence over a centerline (C).

Centerlines continued… Centerlines (symbol: ) are used to indicate symmetrical axes of objects or features, bolt circles, and paths of motion.

MODELS One of the best aids to visualization is an actual model of the object. Models don’t necessarily need to be made accurately or to scale. They may be made of any convenient material, such as modeling clay, soap, wood, wire, or Styrofoam, or any material that can easily be shaped, carved, or cut. Try making a soap or clay model from projected views:

SLANTED SURFACES Slanted surfaces are surfaces that are not parallel to either the horizontal or vertical axis.

COMPOUND LINES A compound line is formed when two slanted surfaces intersect. The true length of a compound line is not shown in the front, top, or side views.

OBLIQUE SURFACES Oblique surfaces are surfaces that do not appear correctly shaped in the front, top, or side views

PROJECTION BETWEEN VIEWS

ROUNDED SURFACES Rounded surfaces are surfaces that have constant radii, such as arcs or circles. Surfaces that do not have constant radii are classified as irregular surfaces