2OBJECTIVES1. 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 withnormal, 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 sheetof paper.7. List the dimensions that transfer between top, front, andright-side views.8. Transfer depth between the top and right-side views.9. Label points where surfaces intersect.
3UNDERSTANDING PROJECTIONS To make and interpret drawings you need to know how to createprojections 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.
4Views of ObjectsThe 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.
5Multiview ProjectionThe system of views is called multiview projection. Each view provides certain definite information.
6The Six Standard ViewsAny object can be viewed from six mutually perpendicular directions,
7Revolving the Object to Produce Views Revolving the Object to Produce Views. You can experience different views by revolving an object.
8Principal DimensionsThe 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 threeprincipal dimensions; the third is foundin an adjacent view. Height is shown inthe rear, left-side, front, and right-sideviews. Width is shown in the rear, top,front, and bottom views. Depth isshown in the left-side, top, right-side,and bottom views.
9Projection MethodThe 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
10Horizontal 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.
11The Glass BoxOne 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.
12Unfolding 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).
13The Glass Box UnfoldedLines 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.
14The Orthographic Projection The front, top, and right-side views of the object shown now without the folding lines.
15Transferring 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.
16Necessary ViewsThe 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.
17Two ViewsMany 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.
18One-ViewOften, a single view supplemented by a note or by lettered symbols isEnough.
19Choice of Front ViewThe view chosen for the front view in this case is the side, not the front, of the automobile.
20Third-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; thefour angles produced are called the first, second, third, and fourth angles (similar to naming quadrants on a graph.) If theobject 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.
21Position of the Side View Sometimes, drawing three views using the conventional arrangement wastes space.
22First-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.
23Hidden LinesThick, dark lines represent features of the object that are directly visible. Dashed lines represent features that would be hidden behind other surfaces.
24Centerlines 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 patternsThe centerline pattern is composed of three dashes: one long dash on each end with a short dash in the middle.
25PRECEDENCE OF LINESA 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).
26Centerlines continued… Centerlines (symbol: ) are used to indicate symmetrical axes of objects or features, bolt circles, and paths of motion.
27VIEWS OF SURFACESThere 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 projectionappears on edge as a straight line
28ANGLESIf 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.
29SIMILAR 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.
30INTERPRETING VIEWSOne method of interpreting sketches is to reverse the mental process used in projecting them.
31MODELSOne 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:
33OBJECTIVES 1. Represent curved surfaces in multiview drawings 2. Show intersections and tangencies of curved and planar surfaces3. Represent common types of holes4. Show fillets, rounds, and runouts in a 2D drawing5. Use partial views6. Apply revolution conventions when necessary for clarity7. Draw removed views and projected views8. Show right- and left-hand parts9. Project curved surfaces by points10. Show and label an enlarged detail11. Show conventional breaks
34Common Manufactured Features FilletRoundCounterboreCountersinkSpotfaceBossLugFlangeChamferNeckKeyway/KeyseatKnurlBushing
36VISUALIZING AND DRAWING COMPLEX CYLINDRICAL SHAPES Steps
37CYLINDERS WHEN SLICEDCylinders 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.
38CYLINDERS 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.
39INTERSECTIONS 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.
40Intersections 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 astraight 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.
41FILLETS AND ROUNDSA 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 PunchFillets on a CAD Model.(Courtesy of Douglas Wintin.)(Courtesy of Ross Traeholt.)
42RUNOUTSSmall 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 intersectingmembers.
43CONVENTIONAL EDGESThere 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.
44NECESSARY VIEWS What are the absolute minimum views required to completely define an object?One-View DrawingTwo-View DrawingThree-View Drawing
45PARTIAL 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 sketchingtime and make the drawing less confusing to read.ORYou can use a break line to limit the partial view…
46Showing Enlarged Details When adding a detail, draw a circlearound the features that will be included in the detailPlace 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,
47Conventional BreaksTo 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 endsto be increased to show the details clearly.
48ALIGNMENT OF VIEWS Always draw views in the “standard” arrangement... Because CAD makes it easy to move whole views, it istempting to place views where they fit on the screen orplotted sheet and not in the standard arrangement. This is not acceptable.3D CAD software that generates 2D drawing views asprojections 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.
49REMOVED VIEWSA 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 ArrowRemoved View Using Viewing-Plane Line
50RIGHT-HAND AND LEFT-HAND PARTS Often, parts function in pairs of similar opposite parts, but oppositeparts can rarely be exactly alike.On sketches and drawings a left-hand part is noted as LH, and a right-hand part as RH.
51REVOLUTION 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.