# ORTHOGRAPHIC PROJECTION

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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. Compare using a 2D CAD program with sketching on a sheet of paper. 7. List the dimensions that transfer between top, front, and right-side views. 8. Transfer depth between the top and right-side views. 9. Label points where surfaces intersect.

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.

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.

The Orthographic Projection
The front, top, and right-side views of the object shown now without the folding lines.

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.

Two Views Many objects need only two views to clearly describe their shape. If an object requires only two views, and the left-side and right-side views show the object equally well, use the right-side view.

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.

Position of the Side View
Sometimes, drawing three views using the conventional arrangement wastes space.

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.

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.

VIEWS OF SURFACES There are terms used for describing a surface’s orientation to the plane of projection. The three orientations that a plane surface can have to the plane of projection are normal, inclined, and oblique. Note how a plane surface that is perpendicular to a plane of projection appears on edge as a straight line

ANGLES If an angle is in a normal plane (a plane parallel to a plane of projection) it will show true size on the plane of projection to which it is parallel.

SIMILAR SHAPES OF SURFACES
If a flat surface is viewed from several different positions, each view will show the same number of sides and a similar shape. This consistency of shapes is useful in analyzing views.

INTERPRETING VIEWS One method of interpreting sketches is to reverse the mental process used in projecting them.

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:

2D DRAWING REPRESENTATION
C H A P T E R S I X

OBJECTIVES 1. Represent curved surfaces in multiview drawings
2. Show intersections and tangencies of curved and planar surfaces 3. Represent common types of holes 4. Show fillets, rounds, and runouts in a 2D drawing 5. Use partial views 6. Apply revolution conventions when necessary for clarity 7. Draw removed views and projected views 8. Show right- and left-hand parts 9. Project curved surfaces by points 10. Show and label an enlarged detail 11. Show conventional breaks

Common Manufactured Features
Fillet Round Counterbore Countersink Spotface Boss Lug Flange Chamfer Neck Keyway/Keyseat Knurl Bushing

Conventional Representations
Standard orthographic projections don’t always show complex shapes as clearly and simply as you may wish, so certain alternative practices, referred to as conventions, are accepted. Conventions are like rules for breaking the rules. Note how these views are projected Orthographic Views of Intersecting and Tangent Surfaces. (Lockhart, Shawna D.; Johnson, Cindy M., Engineering Design Communication: Conveying DesignThrough Graphics, 1st, © Printed and Electronically reproduced by permission of Pearson Education, Inc., Upper Saddle River, New Jersey.)

VISUALIZING AND DRAWING COMPLEX CYLINDRICAL SHAPES
Steps

CYLINDERS WHEN SLICED Cylinders are often machined to form plane or other types of surfaces. Normal surfaces appear true shape in the view where the line of sight is perpendicular to the surface. In the two other views that normal surface appears on edge. The back half remains unchanged.

CYLINDERS AND ELLIPSES
If a cylinder is cut by an inclined plane, the inclined surface is bounded by an ellipse. This ellipse will appear as a circle in the top view, as a straight line in the front view, and as an ellipse in the side view. When a circular shape is shown inclined in another view and projected into the adjacent view it will appear as an ellipse, even though the shape is a circle.

INTERSECTIONS AND TANGENCIES
Where a curved surface is tangent to a plane surface no line is drawn, but when it intersects a plane surface, a definite edge is formed. When plane surfaces join a contoured surface, a line is shown if they are tangent, but not shown if they intersect.

Intersections of Cylinders
When the intersection is small, its curved shape is not plotted accurately because it adds little to the sketch or drawing for the time it takes. Instead it is shown as a straight line. When the intersection is larger, it can be approximated by drawing an arc with the radius the same as that of the large cylinder.

FILLETS AND ROUNDS A rounded interior corner is called a fillet. A rounded exterior corner is called a round. Rounds on a CAD Model of a Design for a Three-Hole Punch Fillets on a CAD Model. (Courtesy of Douglas Wintin.) (Courtesy of Ross Traeholt.)

RUNOUTS Small curves called runouts are used to represent fillets that connect with plane surfaces tangent to cylinders. Runouts from different filleted intersections will appear different owing to the shapes of the horizontal intersecting members.

CONVENTIONAL EDGES There is a conventional way of showing rounded and filleted edges for the sake of clarity. Added lines depicting rounded and filleted edges. Rounded and filleted intersections eliminate sharp edges and can make it difficult to present the shape clearly.

NECESSARY VIEWS What are the absolute minimum views
required to completely define an object? One-View Drawing Two-View Drawing Three-View Drawing

PARTIAL VIEWS You can use a break line to limit the partial view…
A view may not need to be complete but needs to show what is necessary to clearly describe the object. This is called a partial view and is used to save sketching time and make the drawing less confusing to read. OR You can use a break line to limit the partial view…

Showing Enlarged Details
When adding a detail, draw a circle around the features that will be included in the detail Place the detail view on the sheet as you would a removed view. Label successive details with the word DETAIL followed by a letter, as in DETAILA, DETAIL B,

Conventional Breaks To shorten the view of a long object, you can use break lines… Using a break to leave out a portion of the part, but allows the scale for the ends to be increased to show the details clearly.

ALIGNMENT OF VIEWS Always draw views in the “standard” arrangement...
Because CAD makes it easy to move whole views, it is tempting to place views where they fit on the screen or plotted sheet and not in the standard arrangement. This is not acceptable. 3D CAD software that generates 2D drawing views as projections of the 3D object usually has a setting to select from third-angle or first-angle projection. Check your software if you are unsure which projection methods are available.

REMOVED VIEWS A removed view is a complete or partial view removed to another place on the sheet so that it is no longer in direct projection with any other view. Removed View Using View Indicator Arrow Removed View Using Viewing-Plane Line

RIGHT-HAND AND LEFT-HAND PARTS
Often, parts function in pairs of similar opposite parts, but opposite parts can rarely be exactly alike. On sketches and drawings a left-hand part is noted as LH, and a right-hand part as RH.

REVOLUTION CONVENTIONS
Regular multiview projections are sometimes awkward, confusing, or actually misleading. Revolutions like these are frequently used in connection with sectioning. Revolved sectional views are called aligned sections.

Common Features Shown in Orthographic Views

Common Features Shown in Orthographic Views Continued…

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