Technical Sketching and Shape Description

1. Technical Sketching and Shape Description Technical Sketching

2. Projections • Projections • Behind every drawing of an object is a space • relationship involving four imaginary things: • The observer’s eye or the station point • The object • The plane of projection • The projectors or visual rays or lines of sight In the diagram, (efgh) is the projection of the object (ABCD) on the plane of projections (A) as viewed 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 of projection. The projectors for a “cone” of projectors resulting in a foreshortened image known as a perspective.

3. Projections If the observer’s eye is imagined as infinitely distant from the object and the plane of projection, the projectors will be parallel. This type of projection is know as a parallel projections. If the projectors are also perpendicular to the plane of projection the result is an orthographic or right-angle projections.

4. Projections • Classification of projections: • There are two main types of projection: • Perspective (central projection) • Parallel Projection

5. Projections In First Angle Projection the object is placed in the First Quadrant. This means that the object floats above and before the viewing planes and the Vertical Plane is behind the object and the Horizontal Plane is underneath the object. First angle projection is the standard throughout Europe and Asia (excluding Japan).

6. Projections Relative positions of six views in first angle projection

7. Projections In Third Angle Projection the Object is placed in the Third Quadrant. This means that the object is positioned below and behind the viewing planes and the Vertical Plane is in front of the object and the Horizontal Plane is above the object. Third angle projection is the standard in the U.S., Japan, Canada and Australia.

8. Projections Relative positions of six views in third angle projection

9. Multiview Projection The method of viewing an object to obtain a multiview projection is illustrated in figure a. Between the observer and the object a transparent plane is located parallel to the front view. The view is obtained by drawing perpendicular lines (projectors) from all points of the edges of the object to the plane of projection (figure b). The piercing points of these projectors form lines on the projection plane (figure c)

10. Multiview Projection A similar procedure can be used to obtain the top view (figure a) and the right-side view (figure b).

11. Multiview Projection If planes of projection are placed parallel to the principal faces of the object, they form a “glass box” as shown in figure a. Since the glass box has six sides, six views of the object can be obtained. To show the views on a flat sheet of paper it is necessary to unfold the planes so that they will all lie in the same plane. All planes except the rear plane are hinged to the frontal plane (figure b).

12. Multiview Projection The positions of the six planes after they have been revolved are shown. The front, top, and bottom views all line up vertically and are the same width. The rear, left-side, front, and right-side views all line up horizontally and are the same height.

13. Multiview Projection The 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 folding lines are omitted in industrial practice (figure b).

14. Multiview Projection Since all depth dimensions in the top and side views must correspond accurate methods of transferring these distances must be used. The depth dimension between the top and side views can be transferred either with dividers or a scale. A 45 degree miter line can also be used to project the depth dimension between the top and side views.

15. Views of an Object A pictorial drawing shows an object as it appears to the observer but cannot describe the object fully because it does not show the exact shapes and sizes no matter which direction it is viewed from. Industry requires a more complete and clear description of an object to make certain the object is manufactured exactly as intended by the designer or engineer. To accurately describe an object a number of systematically arranged views are used. This system is called multiview projection. To obtain a view the observer is looking perpendicularly toward one of the faces of the object to obtain a true view of the shape and size of that side.

16. Views of an Object Views of an object can be obtained by revolving the object. To obtain the top view, hold the object in the front view position. Revolve the object to bring the top of the object up and toward you. To obtain the right side view, hold the object in the front view position. Revolve the object to bring the right side toward you. The front, top, and right side views are arranged as shown and are called the three regular view because they are the views most frequently used.

17. Views of an Object Any object can be viewed from six mutually perpendicular directions. The six views are always arranged as shown. The three principal dimensions of an object are Height, Width, and Depth. Any one view can only show two dimensions. The third dimension is found in an adjacent view.

18. Views of an Object The front view of an object should show the object in its operating position. The front view should also show the best shape of the object and the most detail. In the example the side of the automobile was selected as the front view of the drawing rather than the actual front of the automobile. Machine parts are often drawn in the position that it occupies in the assembly drawing.

19. Views of an Object A production drawing should show only those views needed for a clear and complete shape description of the object. Often only two views are needed to clearly describe the shape of an object. In selecting the views, show only those that best show the essential contours or shapes and have 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.

20. Multiview Projection If three views of an object are drawn using the conventional arrangement of views a large wasted space is left on the paper (figure a). In such cases the profile plane may be considered hinged to the horizontal plane instead of the frontal plane which results in better spacing of the views (figure b).

21. Multiview Projection No line should be drawn where a curved surface is tangent to a plane surface. When a curved surface intersects a plane surface a definite edge is formed. Show are examples of intersections and tangencies.

22. Multiview Projection The correct method of representing fillets in connection with plane surfaces tangent to cylinders is shown in figure a and figure b. These small curves are called runouts. Runouts have a radius equal to that of the fillet and a curvature of one eighth of a circle (figure c).

23. Multiview Projection Examples of typical filleted intersections.