2Projections Two main types of projections: parallel & perspective. Every drawing/projection involves the spatial relationship of the these four elements:Station Point or Observer’s EyeObjectPlane of ProjectionProjectors or Visual Rays or Lines of Sight
6Object ViewsTo provide a clear and complete description of the shape and size of an object a number of views, systematically arranged, are used. This system of object representation is called multi-view projection.Multi-view projection is necessary since all objects are 3-D and all drawings are 2-D. One 2-D drawing is insufficient to clearly and completely define an object.
7Object ViewsThe Front View is an orthographic parallel projection.
8Object Views There are six orthogonal directions from which a part may be viewed.
9Object ViewsThe observer can move around the part to “see” the six views.
10Object ViewsThe observer can rotate the part to “see” the six views.
12The Six Standard Views The standard arrangement of the views is critical to clearlydescribing the object:Top, front, and bottomviews align verticallyRear, left, front, and rightviews align horizontally
13Transferring Dimensions The height, width, and depth of the object must correspond point-for-point.
14Transferring Dimensions The height, width, and depth of the object must correspond point-for-point.
15Transferring Dimensions The transfer of dimensions can be used to project a 3rd view.
16Transferring Dimensions The transfer of dimensions can be used to project a 3rd view.
17Transferring Dimensions The transfer of dimensions can be used to project a 3rd view.
18Necessary ViewsA sketch or drawing should only contain the views needed to clearly and completely describe the object.The Three Regular(Primary) Views arethe top, front, andright-side views.Use these regular viewsfirst to define the object.
19Choosing the Necessary Views Orient the object so that the front view shows the features of interest most clearlyChose the front view such that it has a large number of normal surfacesShow the object in the usual or operating orientationNext, show the right-side and/or top views unless other views are better(fewer hidden lines)Show only the views need to fully define the object’s geometrySee Step-by-Step 5.9 & Hands-On 5.6 pg. 144.
20Elements of a Projection: points A corner or point is associated with the common intersection of 3 or more surfaces on the object. In a sketch a point can represent either of two features on the object:a vertexthe point view of an edge (e.g. when two vertices are lined up one behind the other).
21Elements of a Projection: lines A straight visible or hidden line in a sketch has 3 possible meanings:an edge(intersection) between 2 surfacesthe edge view of a surfacethe limiting element of a curved surface.
22Elements of a Projection: hidden lines Dashed hidden lines are used to represent features that would be hidden behind other surfaces.Remember to choose views that show features with visible lines when possible then use hidden lines to make the drawing clear.
23Elements of a Projection: hidden lines Use dashes 1/8” long with 1/16” long gaps.Should intersect neatly except where a line of a different line type would appear to be extended.Should “jump” visible lines that they crossDraw closely spaced parallel hidden lines with the staggered dashesSee Step-by-Step 5.5 pg. 132 for sketching guidelines
24Hidden Lines: Hidden lines should join neatly with visible lines except when itcauses a visible lineto be extended. Whentwo different lines jointo form a single line,leave a gap on the lessimportant line.Hidden lines shouldjoin neatly to form “T”or “L” shaped inter-sections.Hidden lines shouldjump visible lines thatthey do not intersect.It is permissible for ahidden line to cross avisible line.
25Hidden Lines:Stagger the dashes on closely spaced parallel hidden lines.Hidden line dashes should intersect neatly to from clear corners, as in the bottom of this drilled hole.Intersecting hidden lines should form neat corners, as in this countersunk hole.
26Hidden Lines:Curved hidden lines should not extend a visible curve in the same direction. Leave a gap on the hidden line so that you can easily see where the visible line ends.Curved hidden lines dashes should extend to the point of tangency. Don’t end with a gap at the point of tangency. It makes it hard to see the location.
27Elements of a Projection: fillets & rounds Sharp corners are almost always to be avoided when designing a part.A fillet is a rounded interior corner.A round is a rounded exterior corner.
28Center Lines:Center lines are used to indicate symmetrical axes of objects or features, bolt circles, and paths of motion.Centerlines are useful in dimensioning as the are used to locate the center of the feature.Centerlines are not needed on fillets, rounds, or other self-locating features.
30In-Class Practice Problem Draw a three-view drawing of the following object
31Extra Problems:Problem #18 in Figure 5.61 on page 152.
32Extra Problems:Problem #1 in Figure 5.64 on page 154.
33ReferencesChapter 5 of Modern Graphics Communication by Giesecke, Mitchell, Spencer, Hill, Dygdon, Novak, and Lockhard, 3rd edition. Prentice-Hall, 2004.Technical Drawing by Giesecke, Mitchell, Spencer, Hill, Dygdon, and Novak, 9th edition. Macmillan, 1991.