Technical Sketching and ShapeDescription Technical Sketching
ProjectionsProjectionsBehind every drawing of an object is a spacerelationship involving four imaginary things:1. The observer’s eye or the station point2. The object3. The plane of projections4. The projectors or visual rays or lines of sightIn the diagram, (efgh) is the projection of the object (ABCD) on the plane of projections (A) asviewed by the observer whose eye is at the station point (O).The image on the plane is produced by the points at which the projectors pierce the plane ofprojection.The projectors for a “cone” of projectors resulting in a foreshortened image known as aperspective.
ProjectionsIf the observer’s eye is imagined as infinitelydistant from the object and the plane ofprojection, the projectors will be parallel.This type of projection is know as a parallelprojections.If the projectors are also perpendicular to theplane of projection the result is an orthographicor right-angle projections.
ProjectionsClassification of projections:There are two main types of projection:Perspective (central projection)Parallel Projection
Multiview ProjectionThe method of viewing an object to obtain a multiview projection is illustrated in figure a. Betweenthe observer and the object a transparent plane is located parallel to the front view. The view isobtained by drawing perpendicular lines (projectors) from all points of the edges of the object to theplane of projection (figure b). The piercing points of these projectors form lines on the projectionplane (figure c)
Multiview ProjectionA similar procedure can be used to obtain the top view (figure a) and theright-side view (figure b).
Multiview ProjectionIf planes of projection are placed parallel to the principal faces of the object, they form a “glassbox” as shown in figure a. Since the glass box has six sides, six views of the object can beobtained.To show the views on a flat sheet of paper it is necessary to unfold the planes so that they will alllie in the same plane. All planes except the rear plane are hinged to the frontal plane (figure b).
Multiview ProjectionThe positions of the sixplanes after they havebeen revolved are shown.The front, top, andbottom views all line upvertically and are thesame width.The rear, left-side, front,and right-side views allline up horizontally andare the same height.
Multiview ProjectionThe front, top, and right-side views of the object are shown with folding lines between the views.These folding lines correspond to the hinge lines of the glass box (figure a).The H/F folding line is between the top and front views.The F/P folding line is between the front and right-side views.Folding lines are useful in solving graphical problems in descriptive geometry. As a rule foldinglines are omitted in industrial practice (figure b).
Views of an ObjectA pictorial drawing shows an object as itappears to the observer but cannot describe theobject fully because it does not show the exactshapes and sizes no matter which direction it isviewed from.Industry requires a more complete and cleardescription of an object to make certain theobject is manufactured exactly as intended bythe designer or engineer.To accurately describe an object a number ofsystematically arranged views are used. Thissystem is called multiview projection.To obtain a view the observer is lookingperpendicularly toward one of the faces of theobject to obtain a true view of the shape andsize of that side.
Views of an ObjectViews of an object can beobtained by revolving the object.To obtain the top view, hold theobject in the front view position.Revolve the object to bring the topof the object up and toward you.To obtain the right side view, holdthe object in the front viewposition. Revolve the object tobring the right side toward you.The front, top, and right side viewsare arranged as shown and arecalled the three regular viewbecause they are the views mostfrequently used.
Views of an ObjectAny object can be viewedfrom six mutuallyperpendicular directions.The six views are alwaysarranged as shown.The three principaldimensions of an objectare Height, Width, andDepth. Any one view canonly show twodimensions. The thirddimension is found in anadjacent view.
Views of an ObjectThe front view of an object should show theobject in its operating position. The frontview should also show the best shape of theobject and the most detail.In the example the side of the automobilewas selected as the front view of thedrawing rather than the actual front of theautomobile.Machine parts are often drawn in theposition that it occupies in the assemblydrawing.
Views of an ObjectA production drawing should show only those views needed for a clear and complete shapedescription of the object. Often only two views are needed to clearly describe the shape of anobject.In selecting the views, show only those that best show the essential contours or shapes andhave the lease number of hidden lines.Unnecessary or duplicate views are eliminated or not shown. In the example, the left side,rear, and bottom views are eliminated.
Multiview ProjectionIf three views of an object are drawn using the conventional arrangement of views a largewasted space is left on the paper (figure a). In such cases the profile plane may be consideredhinged to the horizontal plane instead of the frontal plane which results in better spacing of theviews (figure b).
Technical SketchingFreehand sketches are of great value todesigners and engineers in organizing theirthoughts and recording their idea.The term “freehand sketch” does not mean acrude or sloppy freehand drawing.A freehand sketch should be made with careand with attention to proportion, clarity, andcorrect line technique.A freehand sketch only requires a pencil, paper,and eraser.
Technical SketchingSoft pencils, such as HB or F, should be usedfor freehand sketching.A sharp point is used to produce thin lines fordrawing center lines, hidden lines, anddimension and extension lines. These linesshould be thin and dark.A rounded point is used to produce visibleobject lines that are thick and dark.A sharp point is also used to draw constructionlines. Construction lines are drawn thin andlight.
Technical SketchingA good freehand line is not expected to be asrigidly straight or exactly uniform as a line drawnwith instruments.
Technical SketchingOne method of sketching circles is to lightlysketch the enclosing square, mark themidpoints of the sides, draw arcs tangent to thesides of the square, then heavy in the finalcircle.Another method is to sketch the two centerlines, add light radial lines, sketch light arcsacross the lines a the estimated radius distancefrom the center, then heavy in the final circle.
Sketching Three ViewsSteps in Making a SketchStep 1:block in the enclosing rectanglefor the three views usingconstruction lines.Step 2:Block in all details usingconstruction lines.Step 3:Sketch all arcs and circles usingconstruction lines.Step 4:Lighten all construction lines.Step 5:Darken in all final lines.
Sketching Three ViewsHidden linesHidden lines are used to show hidden features. They are made thin and dark (dense black).A hidden line is a dashed line consisting of 1/8” dashes with 1/32” spaces.Correct and incorrectpractices in drawinghidden lines.
Sketching Three ViewsCenter LinesCenter lines are used to indicate axes of symmetry, bolt circles, paths of motion and indimensioning. They are made thin and dark (dense black).A center line consists of a long line, short dash, and a long line.Center lines extend 1/4” past the feature for which they were drawn.Examples of center lineapplications.
Sketching Three ViewsPrecedence of lines:Visible object lines, hidden lines, and centerlines often coincide on a drawing. Thedrafter must determine which lines to showand which ones to eliminate.A visible object line always takesprecedence over hidden lines and centerlines (A) & (B).A hidden line always takes precedence overa center line (C).
Multiview ProjectionNo line should be drawn where a curved surface is tangent to a plane surface. When a curvedsurface intersects a plane surface a definite edge is formed. Show are examples ofintersections and tangencies.
Multiview ProjectionThe correct method of representing fillets in connection with plane surfaces tangent tocylinders is shown in figure a and figure b. These small curves are called runouts. Runoutshave a radius equal to that of the fillet and a curvature of one eighth of a circle (figure c).
The Glass Box Method of ProjectionThe Glass Box:If planes of projection are placedparallel to the principal faces of anobject, they form a “glass box” asshown.Notice that the observer is always onthe outside looking in so that theobject is seen through the planes ofprojection.Since the glass box has six sides, sixviews of the object are obtained.
The Glass Box Method of ProjectionThe Glass Box:Since it is required to show the viewsof a three-dimensional object on aflat sheet of paper, it is necessary tounfold the planes so that they will alllie in the same plane.All planes except the rear plane arehinged to the frontal plane, the rearplane being hinged to the side plane.Each plane revolves outwardly fromthe original box position until it lies inthe frontal plane.
The Glass Box Method of ProjectionThe Glass Box:Alignment of the six principal views
The Glass Box Method of ProjectionThe Glass Box: Planes of ProjectionFrontal plane of projection – the plane uponwhich the front view is projected.Horizontal plane of projection – the plane uponwhich the top view is projected.Profile plane of projection – the plane uponwhich the side view is projected.
The Glass Box Method of ProjectionThe Glass Box: Fold LinesFold lines are imaginary lines separating theplanes of projection corresponding to the hingedlines of the glass box.
The Glass Box Method of ProjectionThe Glass Box:Any one view of an object can only show twoprincipal dimensions, the third dimension mustbe found in an adjacent view.